-
Notifications
You must be signed in to change notification settings - Fork 2
/
Lecture 5 - CS50's Web Programming with Python and JavaScript.htm
1626 lines (1310 loc) · 114 KB
/
Lecture 5 - CS50's Web Programming with Python and JavaScript.htm
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html>
<html class="wf-ptsans-n4-active wf-ptsans-n7-active wf-active" lang="en-us"><head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<meta charset="utf-8">
<meta name="viewport" content="initial-scale=1, width=device-width"><meta property="og:description" content="This course picks up where Harvard University's CS50 leaves off, diving more deeply into the design and implementation of web apps with Python, JavaScript, and SQL using frameworks like Django, React, and Bootstrap. Topics include database design, scalability, security, and user experience. Through hands-on projects, students learn to write and use APIs, create interactive UIs, and leverage cloud services like GitHub and Heroku. By semester’s end, students emerge with knowledge and experience in principles, languages, and tools that empower them to design and deploy applications on the Internet.">
<meta property="og:image" content="https://img.youtube.com/vi/24Kf3v7kZyE/maxresdefault.jpg"><meta property="og:title" content="Lecture 5 - CS50's Web Programming with Python and JavaScript">
<link href="https://cs50.harvard.edu/web/2020/favicon.ico?1645555620" rel="icon">
<!-- https://fonts.google.com/specimen/PT+Sans?query=PT+Sans&selection.family=PT+Sans:ital,wght@0,400;0,700;1,400;1,700 -->
<script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/webfont.js"></script>
<!-- http://getbootstrap.com/docs/4.6/getting-started/introduction/ -->
<script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/jquery.js"></script>
<script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/bootstrap.js"></script>
<!-- https://bootstrap-table.com/docs/getting-started/introduction/ -->
<link href="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/bootstrap-table.css" rel="stylesheet">
<script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/bootstrap-table.js"></script>
<script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/bootstrap-table-mobile.js"></script>
<!-- https://fontawesome.com/how-to-use/on-the-web/referencing-icons/basic-use -->
<link href="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/all.css" rel="stylesheet">
<!-- https://moment.github.io/luxon/ -->
<script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/luxon.js"></script>
<!-- http://docs.mathjax.org/ -->
<!-- https://www.jsdelivr.com/package/npm/mathjax?path=es5 -->
<script crossorigin="anonymous" integrity="sha256-+nfSJ1LiSvANSK3wvi6FjM+oFwoIQvURU1frozp5z6o=" src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/tex-chtml.js"></script><style type="text/css">.CtxtMenu_InfoClose { top:.2em; right:.2em;}
.CtxtMenu_InfoContent { overflow:auto; text-align:left; font-size:80%; padding:.4em .6em; border:1px inset; margin:1em 0px; max-height:20em; max-width:30em; background-color:#EEEEEE; white-space:normal;}
.CtxtMenu_Info.CtxtMenu_MousePost {outline:none;}
.CtxtMenu_Info { position:fixed; left:50%; width:auto; text-align:center; border:3px outset; padding:1em 2em; background-color:#DDDDDD; color:black; cursor:default; font-family:message-box; font-size:120%; font-style:normal; text-indent:0; text-transform:none; line-height:normal; letter-spacing:normal; word-spacing:normal; word-wrap:normal; white-space:nowrap; float:none; z-index:201; border-radius: 15px; /* Opera 10.5 and IE9 */ -webkit-border-radius:15px; /* Safari and Chrome */ -moz-border-radius:15px; /* Firefox */ -khtml-border-radius:15px; /* Konqueror */ box-shadow:0px 10px 20px #808080; /* Opera 10.5 and IE9 */ -webkit-box-shadow:0px 10px 20px #808080; /* Safari 3 & Chrome */ -moz-box-shadow:0px 10px 20px #808080; /* Forefox 3.5 */ -khtml-box-shadow:0px 10px 20px #808080; /* Konqueror */ filter:progid:DXImageTransform.Microsoft.dropshadow(OffX=2, OffY=2, Color="gray", Positive="true"); /* IE */}
</style><style type="text/css">.CtxtMenu_MenuClose { position:absolute; cursor:pointer; display:inline-block; border:2px solid #AAA; border-radius:18px; -webkit-border-radius: 18px; /* Safari and Chrome */ -moz-border-radius: 18px; /* Firefox */ -khtml-border-radius: 18px; /* Konqueror */ font-family: "Courier New", Courier; font-size:24px; color:#F0F0F0}
.CtxtMenu_MenuClose span { display:block; background-color:#AAA; border:1.5px solid; border-radius:18px; -webkit-border-radius: 18px; /* Safari and Chrome */ -moz-border-radius: 18px; /* Firefox */ -khtml-border-radius: 18px; /* Konqueror */ line-height:0; padding:8px 0 6px /* may need to be browser-specific */}
.CtxtMenu_MenuClose:hover { color:white!important; border:2px solid #CCC!important}
.CtxtMenu_MenuClose:hover span { background-color:#CCC!important}
.CtxtMenu_MenuClose:hover:focus { outline:none}
</style><style type="text/css">.CtxtMenu_Menu { position:absolute; background-color:white; color:black; width:auto; padding:5px 0px; border:1px solid #CCCCCC; margin:0; cursor:default; font: menu; text-align:left; text-indent:0; text-transform:none; line-height:normal; letter-spacing:normal; word-spacing:normal; word-wrap:normal; white-space:nowrap; float:none; z-index:201; border-radius: 5px; /* Opera 10.5 and IE9 */ -webkit-border-radius: 5px; /* Safari and Chrome */ -moz-border-radius: 5px; /* Firefox */ -khtml-border-radius: 5px; /* Konqueror */ box-shadow:0px 10px 20px #808080; /* Opera 10.5 and IE9 */ -webkit-box-shadow:0px 10px 20px #808080; /* Safari 3 & Chrome */ -moz-box-shadow:0px 10px 20px #808080; /* Forefox 3.5 */ -khtml-box-shadow:0px 10px 20px #808080; /* Konqueror */}
.CtxtMenu_MenuItem { padding: 1px 2em; background:transparent;}
.CtxtMenu_MenuArrow { position:absolute; right:.5em; padding-top:.25em; color:#666666; font-family: null; font-size: .75em}
.CtxtMenu_MenuActive .CtxtMenu_MenuArrow {color:white}
.CtxtMenu_MenuArrow.CtxtMenu_RTL {left:.5em; right:auto}
.CtxtMenu_MenuCheck { position:absolute; left:.7em; font-family: null}
.CtxtMenu_MenuCheck.CtxtMenu_RTL { right:.7em; left:auto }
.CtxtMenu_MenuRadioCheck { position:absolute; left: .7em;}
.CtxtMenu_MenuRadioCheck.CtxtMenu_RTL { right: .7em; left:auto}
.CtxtMenu_MenuInputBox { padding-left: 1em; right:.5em; color:#666666; font-family: null;}
.CtxtMenu_MenuInputBox.CtxtMenu_RTL { left: .1em;}
.CtxtMenu_MenuComboBox { left:.1em; padding-bottom:.5em;}
.CtxtMenu_MenuSlider { left: .1em;}
.CtxtMenu_SliderValue { position:absolute; right:.1em; padding-top:.25em; color:#333333; font-size: .75em}
.CtxtMenu_SliderBar { outline: none; background: #d3d3d3}
.CtxtMenu_MenuLabel { padding: 1px 2em 3px 1.33em; font-style:italic}
.CtxtMenu_MenuRule { border-top: 1px solid #DDDDDD; margin: 4px 3px;}
.CtxtMenu_MenuDisabled { color:GrayText}
.CtxtMenu_MenuActive { background-color: #606872; color: white;}
.CtxtMenu_MenuDisabled:focus { background-color: #E8E8E8}
.CtxtMenu_MenuLabel:focus { background-color: #E8E8E8}
.CtxtMenu_ContextMenu:focus { outline:none}
.CtxtMenu_ContextMenu .CtxtMenu_MenuItem:focus { outline:none}
.CtxtMenu_SelectionMenu { position:relative; float:left; border-bottom: none; -webkit-box-shadow:none; -webkit-border-radius:0px; }
.CtxtMenu_SelectionItem { padding-right: 1em;}
.CtxtMenu_Selection { right: 40%; width:50%; }
.CtxtMenu_SelectionBox { padding: 0em; max-height:20em; max-width: none; background-color:#FFFFFF;}
.CtxtMenu_SelectionDivider { clear: both; border-top: 2px solid #000000;}
.CtxtMenu_Menu .CtxtMenu_MenuClose { top:-10px; left:-10px}
</style>
<!-- https://github.com/verlok/vanilla-lazyload -->
<script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/intersection-observer.js"></script>
<script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/lazyload.js"></script>
<!-- https://github.com/davidjbradshaw/iframe-resizer -->
<!-- https://www.jsdelivr.com/package/npm/iframe-resizer?path=js -->
<script crossorigin="anonymous" integrity="sha256-S8f0Q/V9VcfrqYgWo9EFS9zuDMdPTBMC+CBW0RjxQbs=" src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/iframeResizer.js"></script>
<!-- https://github.com/scratchblocks/scratchblocks/releases -->
<script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/scratchblocks.js"></script><style><![CDATA[ .sb-label {
font-family: Lucida Grande, Verdana, Arial, DejaVu Sans, sans-serif;
font-weight: bold;
fill: #fff;
font-size: 10px;
word-spacing: +1px;
}
.sb-obsolete { fill: #d42828; }
.sb-motion { fill: #4a6cd4; }
.sb-looks { fill: #8a55d7; }
.sb-sound { fill: #bb42c3; }
.sb-pen { fill: #0e9a6c; }
.sb-events { fill: #c88330; }
.sb-control { fill: #e1a91a; }
.sb-sensing { fill: #2ca5e2; }
.sb-operators { fill: #5cb712; }
.sb-variables { fill: #ee7d16; }
.sb-list { fill: #cc5b22 }
.sb-custom { fill: #632d99; }
.sb-custom-arg { fill: #5947b1; }
.sb-extension { fill: #4b4a60; }
.sb-grey { fill: #969696; }
.sb-bevel {
filter: url(#bevelFilter);
}
.sb-input {
filter: url(#inputBevelFilter);
}
.sb-input-number,
.sb-input-string,
.sb-input-number-dropdown {
fill: #fff;
}
.sb-literal-number,
.sb-literal-string,
.sb-literal-number-dropdown,
.sb-literal-dropdown {
font-weight: normal;
font-size: 9px;
word-spacing: 0;
}
.sb-literal-number,
.sb-literal-string,
.sb-literal-number-dropdown {
fill: #000;
}
.sb-darker {
filter: url(#inputDarkFilter);
}
.sb-outline {
stroke: #fff;
stroke-opacity: 0.2;
stroke-width: 2;
fill: none;
}
.sb-define-hat-cap {
stroke: #632d99;
stroke-width: 1;
fill: #8e2ec2;
}
.sb-comment {
fill: #ffffa5;
stroke: #d0d1d2;
stroke-width: 1;
}
.sb-comment-line {
fill: #ffff80;
}
.sb-comment-label {
font-family: Helevetica, Arial, DejaVu Sans, sans-serif;
font-weight: bold;
fill: #5c5d5f;
word-spacing: 0;
font-size: 12px;
}
.sb-diff {
fill: none;
stroke: #000;
}
.sb-diff-ins {
stroke-width: 2px;
}
.sb-diff-del {
stroke-width: 3px;
}
]]></style><style><![CDATA[ .sb3-label {
font: 500 12pt Helevetica Neue, Helvetica, sans-serif;
fill: #fff;
word-spacing: +1pt;
}
.sb3-motion { fill: #4c97ff; stroke: #3373cc; }
.sb3-motion-alt { fill: #4280d7; }
.sb3-motion-dark { fill: #4c97ff; }
.sb3-looks { fill: #9966ff; stroke: #774dcb; }
.sb3-looks-alt { fill: #855cd6; }
.sb3-looks-dark { fill: #bd42bd; }
.sb3-sound { fill: #cf63cf; stroke: #bd42bd; }
.sb3-sound-alt { fill: #c94fc9; }
.sb3-sound-dark { fill: #bd42bd; }
.sb3-control { fill: #ffab19; stroke: #cf8b17; }
.sb3-control-alt { fill: #ec9c13; }
.sb3-control-dark { fill: #cf8b17; }
.sb3-events { fill: #ffbf00; stroke: #cc9900; }
.sb3-events-alt { fill: #e6ac00; }
.sb3-events-dark { fill: #cc9900; }
.sb3-sensing { fill: #5cb1d6; stroke: #2e8eb8; }
.sb3-sensing-alt { fill: #47a8d1; }
.sb3-sensing-dark { fill: #2e8eb8; }
.sb3-operators { fill: #59c059; stroke: #389438; }
.sb3-operators-alt { fill: #46b946; }
.sb3-operators-dark { fill: #389438; }
.sb3-variables { fill: #ff8c1a; stroke: #db6e00; }
.sb3-variables-alt { fill: #ff8000; }
.sb3-variables-dark { fill: #db6e00; }
.sb3-list { fill: #ff661a; stroke: #e64d00; }
.sb3-list-alt { fill: #ff5500; }
.sb3-list-dark { fill: #e64d00; }
.sb3-custom { fill: #ff6680; stroke: #ff3355; }
.sb3-custom-alt { fill: #ff4d6a; }
.sb3-custom-dark { fill: #ff3355; }
.sb3-custom-arg { fill: #ff6680; stroke: #ff3355; }
/* extension blocks, e.g. pen */
.sb3-extension { fill: #0fbd8c; stroke: #0b8e69; }
.sb3-extension-alt { fill: #0da57a; }
.sb3-extension-line { stroke: #0da57a; }
.sb3-extension-dark { fill: #0b8e69; }
/* obsolete colors: chosen by hand, indicates invalid blocks */
.sb3-obsolete { fill: #ed4242; stroke: #ca2b2b; }
.sb3-obsolete-alt { fill: #db3333; }
.sb3-obsolete-dark { fill: #ca2b2b; }
/* grey: special color from the Scratch 3.0 design mockups */
.sb3-grey { fill: #bfbfbf; stroke: #909090; }
.sb3-grey-alt { fill: #b2b2b2; }
.sb3-grey-dark { fill: #909090; }
.sb3-input-color {
stroke: #fff;
}
.sb3-input-number,
.sb3-input-string {
fill: #fff;
}
.sb3-literal-number,
.sb3-literal-string,
.sb3-literal-number-dropdown,
.sb3-literal-dropdown {
word-spacing: 0;
}
.sb3-literal-number,
.sb3-literal-string {
fill: #575e75;
}
.sb3-comment {
fill: #ffffa5;
stroke: #d0d1d2;
stroke-width: 1;
}
.sb3-comment-line {
fill: #ffff80;
}
.sb3-comment-label {
font: 400 12pt Helevetica Neue, Helvetica, sans-serif;
fill: #000;
word-spacing: 0;
}
.sb3-diff {
fill: none;
stroke: #000;
}
.sb3-diff-ins {
stroke-width: 2px;
}
.sb3-diff-del {
stroke-width: 3px;
}
]]></style>
<link href="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/page.css" rel="stylesheet">
<link rel="stylesheet" href="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/css.css" media="all"><script src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/jekyll-theme-cs50.js"></script>
<script>
window.CS50 = {
local: {"day":"numeric","hour":"numeric","minute":"numeric","month":"long","timeZoneName":"short","weekday":"long","year":"numeric"},
locale: "en",
tz: "America/New_York"
};
</script>
<title>Lecture 5 - CS50's Web Programming with Python and JavaScript</title>
<style id="MJX-CHTML-styles">
mjx-container[jax="CHTML"] {
line-height: 0;
}
mjx-container [space="1"] {
margin-left: .111em;
}
mjx-container [space="2"] {
margin-left: .167em;
}
mjx-container [space="3"] {
margin-left: .222em;
}
mjx-container [space="4"] {
margin-left: .278em;
}
mjx-container [space="5"] {
margin-left: .333em;
}
mjx-container [rspace="1"] {
margin-right: .111em;
}
mjx-container [rspace="2"] {
margin-right: .167em;
}
mjx-container [rspace="3"] {
margin-right: .222em;
}
mjx-container [rspace="4"] {
margin-right: .278em;
}
mjx-container [rspace="5"] {
margin-right: .333em;
}
mjx-container [size="s"] {
font-size: 70.7%;
}
mjx-container [size="ss"] {
font-size: 50%;
}
mjx-container [size="Tn"] {
font-size: 60%;
}
mjx-container [size="sm"] {
font-size: 85%;
}
mjx-container [size="lg"] {
font-size: 120%;
}
mjx-container [size="Lg"] {
font-size: 144%;
}
mjx-container [size="LG"] {
font-size: 173%;
}
mjx-container [size="hg"] {
font-size: 207%;
}
mjx-container [size="HG"] {
font-size: 249%;
}
mjx-container [width="full"] {
width: 100%;
}
mjx-box {
display: inline-block;
}
mjx-block {
display: block;
}
mjx-itable {
display: inline-table;
}
mjx-row {
display: table-row;
}
mjx-row > * {
display: table-cell;
}
mjx-mtext {
display: inline-block;
}
mjx-mstyle {
display: inline-block;
}
mjx-merror {
display: inline-block;
color: red;
background-color: yellow;
}
mjx-mphantom {
visibility: hidden;
}
mjx-assistive-mml {
position: absolute !important;
top: 0px;
left: 0px;
clip: rect(1px, 1px, 1px, 1px);
padding: 1px 0px 0px 0px !important;
border: 0px !important;
display: block !important;
width: auto !important;
overflow: hidden !important;
-webkit-touch-callout: none;
-webkit-user-select: none;
-khtml-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
user-select: none;
}
mjx-assistive-mml[display="block"] {
width: 100% !important;
}
mjx-c::before {
display: block;
width: 0;
}
.MJX-TEX {
font-family: MJXZERO, MJXTEX;
}
.TEX-B {
font-family: MJXZERO, MJXTEX-B;
}
.TEX-I {
font-family: MJXZERO, MJXTEX-I;
}
.TEX-MI {
font-family: MJXZERO, MJXTEX-MI;
}
.TEX-BI {
font-family: MJXZERO, MJXTEX-BI;
}
.TEX-S1 {
font-family: MJXZERO, MJXTEX-S1;
}
.TEX-S2 {
font-family: MJXZERO, MJXTEX-S2;
}
.TEX-S3 {
font-family: MJXZERO, MJXTEX-S3;
}
.TEX-S4 {
font-family: MJXZERO, MJXTEX-S4;
}
.TEX-A {
font-family: MJXZERO, MJXTEX-A;
}
.TEX-C {
font-family: MJXZERO, MJXTEX-C;
}
.TEX-CB {
font-family: MJXZERO, MJXTEX-CB;
}
.TEX-FR {
font-family: MJXZERO, MJXTEX-FR;
}
.TEX-FRB {
font-family: MJXZERO, MJXTEX-FRB;
}
.TEX-SS {
font-family: MJXZERO, MJXTEX-SS;
}
.TEX-SSB {
font-family: MJXZERO, MJXTEX-SSB;
}
.TEX-SSI {
font-family: MJXZERO, MJXTEX-SSI;
}
.TEX-SC {
font-family: MJXZERO, MJXTEX-SC;
}
.TEX-T {
font-family: MJXZERO, MJXTEX-T;
}
.TEX-V {
font-family: MJXZERO, MJXTEX-V;
}
.TEX-VB {
font-family: MJXZERO, MJXTEX-VB;
}
mjx-stretchy-v mjx-c, mjx-stretchy-h mjx-c {
font-family: MJXZERO, MJXTEX-S1, MJXTEX-S4, MJXTEX, MJXTEX-A ! important;
}
@font-face /* 0 */ {
font-family: MJXZERO;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Zero.woff") format("woff");
}
@font-face /* 1 */ {
font-family: MJXTEX;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Main-Regular.woff") format("woff");
}
@font-face /* 2 */ {
font-family: MJXTEX-B;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Main-Bold.woff") format("woff");
}
@font-face /* 3 */ {
font-family: MJXTEX-I;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Math-Italic.woff") format("woff");
}
@font-face /* 4 */ {
font-family: MJXTEX-MI;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Main-Italic.woff") format("woff");
}
@font-face /* 5 */ {
font-family: MJXTEX-BI;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Math-BoldItalic.woff") format("woff");
}
@font-face /* 6 */ {
font-family: MJXTEX-S1;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Size1-Regular.woff") format("woff");
}
@font-face /* 7 */ {
font-family: MJXTEX-S2;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Size2-Regular.woff") format("woff");
}
@font-face /* 8 */ {
font-family: MJXTEX-S3;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Size3-Regular.woff") format("woff");
}
@font-face /* 9 */ {
font-family: MJXTEX-S4;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Size4-Regular.woff") format("woff");
}
@font-face /* 10 */ {
font-family: MJXTEX-A;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_AMS-Regular.woff") format("woff");
}
@font-face /* 11 */ {
font-family: MJXTEX-C;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Calligraphic-Regular.woff") format("woff");
}
@font-face /* 12 */ {
font-family: MJXTEX-CB;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Calligraphic-Bold.woff") format("woff");
}
@font-face /* 13 */ {
font-family: MJXTEX-FR;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Fraktur-Regular.woff") format("woff");
}
@font-face /* 14 */ {
font-family: MJXTEX-FRB;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Fraktur-Bold.woff") format("woff");
}
@font-face /* 15 */ {
font-family: MJXTEX-SS;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_SansSerif-Regular.woff") format("woff");
}
@font-face /* 16 */ {
font-family: MJXTEX-SSB;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_SansSerif-Bold.woff") format("woff");
}
@font-face /* 17 */ {
font-family: MJXTEX-SSI;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_SansSerif-Italic.woff") format("woff");
}
@font-face /* 18 */ {
font-family: MJXTEX-SC;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Script-Regular.woff") format("woff");
}
@font-face /* 19 */ {
font-family: MJXTEX-T;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Typewriter-Regular.woff") format("woff");
}
@font-face /* 20 */ {
font-family: MJXTEX-V;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Vector-Regular.woff") format("woff");
}
@font-face /* 21 */ {
font-family: MJXTEX-VB;
src: url("https://cdn.jsdelivr.net/npm/mathjax@3.1.2/es5/output/chtml/fonts/woff-v2/MathJax_Vector-Bold.woff") format("woff");
}
</style></head>
<body class="">
<div class="alert fixed-top mb-0 rounded-0 shadow alert-success alert-dismissible" data-alert="success dismissible" data-hash="edc24fffed7a3a9ad8c189ef5e2aca41" id="alert" role="alert">
<button aria-label="Close" class="btn-close" data-bs-dismiss="alert" type="button"></button><p>Interested in <a href="https://cs50.harvard.edu/web/2020/#how-to-take-this-course" class="alert-link">a verified certificate, a professional certificate, or transfer credit and accreditation</a>?
And get vaccinated (as soon as it’s available to you). 💉 <a href="https://www.who.int/news-room/feature-stories/detail/getting-the-covid-19-vaccine" class="alert-link">Here’s why</a>. <a href="https://www.google.com/search?q=covid-19+vaccine+finder" class="alert-link">Here’s how</a>.</p></div>
<div class="container-fluid">
<div class="row">
<aside class="col-md" style="height: 513.667px; top: 81.3333px;">
<header><h1 data-id="cs50s-web-programming-with-python-and-javascript"><a href="https://cs50.harvard.edu/web/2020/">CS50’s Web Programming with Python and JavaScript</a></h1>
<p>OpenCourseWare</p>
<p><a class="pr-1 small" href="https://cs50.harvard.edu/donate">Donate<i aria-hidden="true" class="fas fa-external-link-alt ps-2"></i></a></p>
<p><a href="https://brianyu.me/">Brian Yu</a><br>
<a href="mailto:brian@cs.harvard.edu">brian@cs.harvard.edu</a></p>
<p><a href="https://cs.harvard.edu/malan/">David J. Malan</a>
<br>
<a href="mailto:malan@harvard.edu">malan@harvard.edu</a>
<br>
<a class="mr-1" href="https://www.facebook.com/dmalan"><i aria-hidden="true" class="fab fa-facebook-f" title="Facebook"></i><span class="sr-only">Facebook</span></a>
<a class="mr-1" href="https://github.com/dmalan"><i aria-hidden="true" class="fab fa-github" title="GitHub"></i><span class="sr-only">GitHub</span></a>
<a class="mr-1" href="https://www.instagram.com/davidjmalan/"><i aria-hidden="true" class="fab fa-instagram" title="Instagram"></i><span class="sr-only">Instagram</span></a>
<a class="mr-1" href="https://www.linkedin.com/in/malan/"><i aria-hidden="true" class="fab fa-linkedin" title="LinkedIn"></i><span class="sr-only">LinkedIn</span></a>
<a class="mr-1" href="https://orcid.org/0000-0001-5338-2522"><i aria-hidden="true" class="fab fa-orcid" title="ORCID"></i><span class="sr-only">ORCID</span></a>
<a class="mr-1" href="https://www.quora.com/profile/David-J-Malan"><i aria-hidden="true" class="fab fa-quora" title="Quora"></i><span class="sr-only">Quora</span></a>
<a class="mr-1" href="https://www.reddit.com/user/davidjmalan"><i aria-hidden="true" class="fab fa-reddit-alien" title="Reddit"></i><span class="sr-only">Reddit</span></a>
<a class="mr-1" href="https://www.tiktok.com/@davidjmalan"><i aria-hidden="true" class="fab fa-tiktok" title="TikTok"></i><span class="sr-only">Twitter</span></a>
<a class="mr-1" href="https://twitter.com/davidjmalan"><i aria-hidden="true" class="fab fa-twitter" title="Twitter"></i><span class="sr-only">Twitter</span></a></p></header>
<button aria-controls="nav" aria-expanded="false" class="btn btn-sm collapsed d-md-none" data-bs-target="aside > nav" data-bs-toggle="collapse">
Menu
</button>
<nav class="collapse d-md-block" id="nav"><hr>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/x/movie/">🍿 CS50x Movie Night 2022</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/x/puzzles/"><i class="fas fa-puzzle-piece pe-2"></i>CS50x Puzzle Day 2022</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/x/prepare/"><i class="fas fa-book pe-2"></i>How to Prepare for Technical Interviews</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.ly/zoom"><i class="fas fa-video pe-2"></i>Zoom Meetings</a></li>
</ul>
<hr>
<ol start="0">
<li><a href="https://cs50.harvard.edu/web/2020/weeks/0/">HTML, CSS</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/1/">Git</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/2/">Python</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/3/">Django</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/4/">SQL, Models, and Migrations</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/5/">JavaScript</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/6/">User Interfaces</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/7/">Testing, CI/CD</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/8/">Scalability and Security</a></li>
</ol>
<hr>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/web/2020/honesty/">Academic Honesty</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/web/2020/certificate/">CS50 Certificate</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/web/2020/faqs/">FAQs</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.me/cs50w">Gradebook</a></li>
</ul>
<hr>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.edx.org/ed">Ed Discussion</a> for Q&A</li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://us.edstem.org/quickstart/ed-discussion.pdf">Quick Start Guide</a></li>
</ul>
<hr>
<ul class="small fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.edx.org/web">edX</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://itunes.apple.com/us/course/id1505432709">iTunes U</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.youtube.com/playlist?list=PLhQjrBD2T380xvFSUmToMMzERZ3qB5Ueu">YouTube</a></li>
</ul>
<hr>
<ul class="small fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.noticeable.news/">Changelog</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.statuspage.io/">Status Page</a></li>
</ul>
<hr>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/web/2020/communities/"><strong>Communities</strong></a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.clubhouse.com/club/cs50">Clubhouse</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://discord.gg/cs50">Discord</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.edx.org/ed">Ed</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.facebook.com/groups/cs50/">Facebook Group</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.facebook.com/cs50/">Facebook Page</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://github.com/cs50">GitHub</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://gitter.im/cs50/x">Gitter</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://instagram.com/cs50">Instagram</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.linkedin.com/groups/7437240/">LinkedIn Group</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.linkedin.com/school/CS50/">LinkedIn Page</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.medium.com/">Medium</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.quora.com/topic/CS50">Quora</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.reddit.com/r/cs50/">Reddit</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.edx.org/slack">Slack</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.snapchat.com/add/cs50">Snapchat</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://soundcloud.com/cs50">SoundCloud</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.stackexchange.com/">Stack Exchange</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.tiktok.com/@cs50">TikTok</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://twitter.com/cs50">Twitter</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="http://www.youtube.com/subscription_center?add_user=cs50tv">YouTube</a></li>
</ul>
<hr>
<p><a href="https://cs50.harvardshop.com/"><img src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/GGMdcKt.png" alt="Harvard Shop"></a></p>
<hr>
<p><a href="https://cs50.harvard.edu/web/2020/license/" class="small"><i class="fab fa-creative-commons me-1"></i>License</a></p></nav>
<footer></footer>
</aside>
<main class="col-md" style="margin-bottom: 0px; margin-top: 81.3333px;">
<a data-id="" id="lecture-5" style="top: -81.3333px;"></a><h1 class="no_toc"><a data-id="" href="#lecture-5">Lecture 5</a></h1>
<ul id="markdown-toc">
<li><a href="#introduction" id="markdown-toc-introduction">Introduction</a></li>
<li><a href="#javascript" id="markdown-toc-javascript">JavaScript</a></li>
<li><a href="#events" id="markdown-toc-events">Events</a></li>
<li><a href="#variables" id="markdown-toc-variables">Variables</a></li>
<li><a href="#queryselector" id="markdown-toc-queryselector"><code class="language-plaintext highlighter-rouge">querySelector</code></a></li>
<li><a href="#dom-manipulation" id="markdown-toc-dom-manipulation">DOM Manipulation</a> <ul>
<li><a href="#javascript-console" id="markdown-toc-javascript-console">JavaScript Console</a></li>
<li><a href="#arrow-functions" id="markdown-toc-arrow-functions">Arrow Functions</a></li>
<li><a href="#todo-list" id="markdown-toc-todo-list">TODO List</a></li>
</ul>
</li>
<li><a href="#intervals" id="markdown-toc-intervals">Intervals</a></li>
<li><a href="#local-storage" id="markdown-toc-local-storage">Local Storage</a></li>
<li><a href="#apis" id="markdown-toc-apis">APIs</a> <ul>
<li><a href="#javascript-objects" id="markdown-toc-javascript-objects">JavaScript Objects</a></li>
<li><a href="#currency-exchange" id="markdown-toc-currency-exchange">Currency Exchange</a></li>
</ul>
</li>
</ul>
<a data-id="" id="introduction" style="top: -81.3333px;"></a><h2><a data-id="" href="#introduction">Introduction</a></h2>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>So
far, we’ve discussed how to build simple web pages using HTML and CSS,
and how to use Git and GitHub in order to keep track of changes to our
code and collaborate with others. We also familiarized ourselves with
the Python programming language, started using Django to create web
applications, and learned how to use Django models to store information
in our sites.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Today, we’ll introduce a new programming language: JavaScript.</li>
</ul>
<a data-id="" id="javascript" style="top: -81.3333px;"></a><h2><a data-id="" href="#javascript">JavaScript</a></h2>
<p>Let’s begin by revisiting a diagram from a couple of lectures ago:</p>
<p><img src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/client_server.png" alt="Client Server Diagram"></p>
<p>Recall that in most online interactions, we have a client/user that
sends an HTTP Request to a server, which sends back an HTTP Response.
All of the Python code we’ve written so far using Django has been
running on a server. JavaScript will allow us to run code on the client
side, meaning no interaction with the server is necessary while it’s
running, allowing our websites to become much more interactive.</p>
<p>In order to add some JavaScript to our page, we can add a pair of <code class="language-plaintext highlighter-rouge"><script></code> tags somewhere in our HTML page. We use <code class="language-plaintext highlighter-rouge"><script></code>
tags to signal to the browser that anything we write in between the two
tags is JavaScript code we wish to execute when a user visits our site.
Our first program might look something like this:</p>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nx">alert</span><span class="p">(</span><span class="dl">'</span><span class="s1">Hello, world!</span><span class="dl">'</span><span class="p">);</span>
</code></pre></div></div>
<p>The <code class="language-plaintext highlighter-rouge">alert</code>
function in JavaScript displays a message to the user which they can
then dismiss. To show where this would fit into an actual HTML document,
here’s an example of a simple page with some JavaScript:</p>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o"><!</span><span class="nx">DOCTYPE</span> <span class="nx">html</span><span class="o">></span>
<span class="o"><</span><span class="nx">html</span> <span class="nx">lang</span><span class="o">=</span><span class="dl">"</span><span class="s2">en</span><span class="dl">"</span><span class="o">></span>
<span class="o"><</span><span class="nx">head</span><span class="o">></span>
<span class="o"><</span><span class="nx">title</span><span class="o">></span><span class="nx">Hello</span><span class="o"><</span><span class="sr">/title</span><span class="err">>
</span> <span class="o"><</span><span class="nx">script</span><span class="o">></span>
<span class="nx">alert</span><span class="p">(</span><span class="dl">'</span><span class="s1">Hello, world!</span><span class="dl">'</span><span class="p">);</span>
<span class="o"><</span><span class="sr">/script</span><span class="err">>
</span> <span class="o"><</span><span class="sr">/head</span><span class="err">>
</span> <span class="o"><</span><span class="nx">body</span><span class="o">></span>
<span class="o"><</span><span class="nx">h1</span><span class="o">></span><span class="nx">Hello</span><span class="o">!<</span><span class="sr">/h1</span><span class="err">>
</span> <span class="o"><</span><span class="sr">/body</span><span class="err">>
</span><span class="o"><</span><span class="sr">/html</span><span class="err">>
</span></code></pre></div></div>
<p><img src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/alert0.png" alt="alert"></p>
<a data-id="" id="events" style="top: -81.3333px;"></a><h2><a data-id="" href="#events">Events</a></h2>
<dl>
<dt>One feature of JavaScript that makes it helpful for web programming is that it supports <a href="https://medium.com/@vsvaibhav2016/introduction-to-event-driven-programming-28161b79c223">Event-Driven Programming</a>.</dt>
<dd>Event-Driven Programming is a programming paradigm that centers
around the detection of events, and actions that should be taken when an
event is detected.</dd>
</dl>
<p>An event can be almost anything including a button being clicked, the
cursor being moved, a response being typed, or a page being loaded.
Just about everything a user does to interact with a web page can be
thought of as an event. In JavaScript, we use <a href="https://www.w3schools.com/js/js_htmldom_eventlistener.asp">Event Listeners</a> that wait for certain events to occur, and then execute some code.</p>
<p>Let’s begin by turning our JavaScript from above into a <a href="https://www.w3schools.com/js/js_functions.asp">function</a> called <code class="language-plaintext highlighter-rouge">hello</code>:</p>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">function</span> <span class="nx">hello</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">alert</span><span class="p">(</span><span class="dl">'</span><span class="s1">Hello, world!</span><span class="dl">'</span><span class="p">)</span>
<span class="p">}</span>
</code></pre></div></div>
<p>Now, let’s work on running this function whenever a button is
clicked. To do this, we’ll create an HTML button in our page with an <code class="language-plaintext highlighter-rouge">onclick</code> attribute, which gives the browser instructions for what should happen when the button is clicked:</p>
<div class="language-html highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt"><button</span> <span class="na">onclick=</span><span class="s">"hello()"</span><span class="nt">></span>Click Here<span class="nt"></button></span>
</code></pre></div></div>
<p>These changes allow us to wait to run parts of our JavaScript code until a certain event occurs.</p>
<a data-id="" id="variables" style="top: -81.3333px;"></a><h2><a data-id="" href="#variables">Variables</a></h2>
<p>JavaScript is a programming language just like Python, C, or any
other language you’ve worked with before, meaning it has many of the
same features as other languages including variables. There are three
keywords we can use to assign values in JavaScript:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">var</code>: used to define a variable globally</li>
</ul>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">var</span> <span class="nx">age</span> <span class="o">=</span> <span class="mi">20</span><span class="p">;</span>
</code></pre></div></div>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">let</code>: used to define a variable that is limited in scope to the current block such as a function or loop</li>
</ul>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">let</span> <span class="nx">counter</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
</code></pre></div></div>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><code class="language-plaintext highlighter-rouge">const</code>: used to define a value that will not change</li>
</ul>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">const</span> <span class="nx">PI</span> <span class="o">=</span> <span class="mf">3.14</span><span class="p">;</span>
</code></pre></div></div>
<p>For an example of how we can use a variable, let’s take a look at a page that keeps track of a counter:</p>
<div class="language-html highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp"><!DOCTYPE html></span>
<span class="nt"><html</span> <span class="na">lang=</span><span class="s">"en"</span><span class="nt">></span>
<span class="nt"><head></span>
<span class="nt"><title></span>Count<span class="nt"></title></span>
<span class="nt"><script></span>
<span class="kd">let</span> <span class="nx">counter</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<span class="kd">function</span> <span class="nx">count</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">counter</span><span class="o">++</span><span class="p">;</span>
<span class="nx">alert</span><span class="p">(</span><span class="nx">counter</span><span class="p">);</span>
<span class="p">}</span>
<span class="nt"></script></span>
<span class="nt"></head></span>
<span class="nt"><body></span>
<span class="nt"><h1></span>Hello!<span class="nt"></h1></span>
<span class="nt"><button</span> <span class="na">onclick=</span><span class="s">"count()"</span><span class="nt">></span>Count<span class="nt"></button></span>
<span class="nt"></body></span>
<span class="nt"></html></span>
</code></pre></div></div>
<p><img src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/count.gif" alt="counting"></p>
<a data-id="" id="queryselector" style="top: -81.3333px;"></a><h2><a data-id="" href="#queryselector"><code class="language-plaintext highlighter-rouge">querySelector</code></a></h2>
<p>In addition to allowing us to display messages through alerts,
JavaScript also allows us to change elements on the page. In order to do
this, we must first introduce a function called <code class="language-plaintext highlighter-rouge">document.querySelector</code>. This function searches for and returns elements of the DOM. For example, we would use:</p>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">let</span> <span class="nx">heading</span> <span class="o">=</span> <span class="nb">document</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="dl">'</span><span class="s1">h1</span><span class="dl">'</span><span class="p">);</span>
</code></pre></div></div>
<p>to extract a heading. Then, to manipulate the element we’ve recently found, we can change its <code class="language-plaintext highlighter-rouge">innerHTML</code> property:</p>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nx">heading</span><span class="p">.</span><span class="nx">innerHTML</span> <span class="o">=</span> <span class="s2">`Goodbye!`</span><span class="p">;</span>
</code></pre></div></div>
<p>Just as in Python, we can also take advantage of <a href="https://www.w3schools.com/js/js_if_else.asp">conditions</a> in JavaScript. For example, let’s say rather than always changing our header to <code class="language-plaintext highlighter-rouge">Goodbye!</code>, we wish to toggle back and forth between <code class="language-plaintext highlighter-rouge">Hello!</code> and <code class="language-plaintext highlighter-rouge">Goodbye!</code>. Our page might then look something like the one below. Notice that in JavaScript, we use <code class="language-plaintext highlighter-rouge">===</code> as a stronger comparison between two items which also checks that the objects are of the same type. We typically want to use <code class="language-plaintext highlighter-rouge">===</code> whenever possible.</p>
<div class="language-html highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp"><!DOCTYPE html></span>
<span class="nt"><html</span> <span class="na">lang=</span><span class="s">"en"</span><span class="nt">></span>
<span class="nt"><head></span>
<span class="nt"><title></span>Count<span class="nt"></title></span>
<span class="nt"><script></span>
<span class="kd">function</span> <span class="nx">hello</span><span class="p">()</span> <span class="p">{</span>
<span class="kd">const</span> <span class="nx">header</span> <span class="o">=</span> <span class="nb">document</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="dl">'</span><span class="s1">h1</span><span class="dl">'</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="nx">header</span><span class="p">.</span><span class="nx">innerHTML</span> <span class="o">===</span> <span class="dl">'</span><span class="s1">Hello!</span><span class="dl">'</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">header</span><span class="p">.</span><span class="nx">innerHTML</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">Goodbye!</span><span class="dl">'</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">else</span> <span class="p">{</span>
<span class="nx">header</span><span class="p">.</span><span class="nx">innerHTML</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">Hello!</span><span class="dl">'</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nt"></script></span>
<span class="nt"></head></span>
<span class="nt"><body></span>
<span class="nt"><h1></span>Hello!<span class="nt"></h1></span>
<span class="nt"><button</span> <span class="na">onclick=</span><span class="s">"hello()"</span><span class="nt">></span>Click Here<span class="nt"></button></span>
<span class="nt"></body></span>
<span class="nt"></html></span>
</code></pre></div></div>
<p><img src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/toggle.gif" alt="toggle"></p>
<a data-id="" id="dom-manipulation" style="top: -81.3333px;"></a><h2><a data-id="" href="#dom-manipulation">DOM Manipulation</a></h2>
<p>Let’s use this idea of DOM manipulation to improve our counter page:</p>
<div class="language-html highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp"><!DOCTYPE html></span>
<span class="nt"><html</span> <span class="na">lang=</span><span class="s">"en"</span><span class="nt">></span>
<span class="nt"><head></span>
<span class="nt"><title></span>Count<span class="nt"></title></span>
<span class="nt"><script></span>
<span class="kd">let</span> <span class="nx">counter</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<span class="kd">function</span> <span class="nx">count</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">counter</span><span class="o">++</span><span class="p">;</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="dl">'</span><span class="s1">h1</span><span class="dl">'</span><span class="p">).</span><span class="nx">innerHTML</span> <span class="o">=</span> <span class="nx">counter</span><span class="p">;</span>
<span class="p">}</span>
<span class="nt"></script></span>
<span class="nt"></head></span>
<span class="nt"><body></span>
<span class="nt"><h1></span>0<span class="nt"></h1></span>
<span class="nt"><button</span> <span class="na">onclick=</span><span class="s">"count()"</span><span class="nt">></span>Count<span class="nt"></button></span>
<span class="nt"></body></span>
<span class="nt"></html></span>
</code></pre></div></div>
<p><img src="Lecture%205%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/count2.gif" alt="count 2"></p>
<p>We can make this page even more interesting by displaying an alert
every time the counter gets to a multiple of ten. In this alert, we’ll
want to format a string to customize the message, which in JavaScript we
can do using <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals">template literals</a>. Template literals requre that there are backticks (<code class="language-plaintext highlighter-rouge">`</code>) around the entire expression and a $ and curly braces around any substitutions. For example, let’s change our count function</p>