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Javascript Interview Questions

Date and Time:

Date and Time Basics:

The Date() object is used to deal with both date and time. There are different ways to use it with different arguments. There are four ways to create a date object instance:

  • Date(): Returns current date and time along with the local time zone
const dt = new Date();
console.log(dt);    // Sat Jul 29 2023 19:12:03 GMT+0330 (Iran Standard Time)
  • Date(year, month, day, hours, minutes, seconds, milliseconds): This way is more preferable and better
const dt = new Date(1996, 8, 4, 9, 25, 40, 27); // month is 0 based - 0: january, 11: december
console.log(dt);    // Wed Sep 04 1996 09:25:40 GMT+0430 (Iran Daylight Time)
  • Date(milliseconds): In javascript this millisecond count starts from 1st january 1970
const dt = new Date(0);
console.log(dt);    // Thu Jan 01 1970 03:30:00 GMT+0330 (Iran Standard Time)

const nowInMS = new Date().getTime();
console.log(nowInMS);   // 1690647131793

console.log(new Date(1690647131793));   // Sat Jul 29 2023 19:42:11 GMT+0330 (Iran Standard Time)
  • Date("year-month-day"): Other parameters are optional
const dt1 = new Date("2020-1-20");   // month is 1 based - 1: january, 12: december
console.log(dt1);    // Mon Jan 20 2020 00:00:00 GMT+0330 (Iran Standard Time)

const dt2 = new Date("2020-08-23T12:32:00.493Z"); // If you don't use T or Z the result may vary from browser to browser
console.log(dt2);   // Sun Aug 23 2020 17:02:00 GMT+0430 (Iran Daylight Time)

The format shown above is the ISO 8601 extended format(yyyy-mm-ddThh:mm:ss.sssZ).

Relative Questions:

Explain the different ways of create date/time object?

// First:
new Date();

// Second:
new Date(year, month, day, hours, minutes, seconds, milliseconds);

// Third:
new Date(milliseconds);

// Fourth:
new Date("String");

What will be the output of the below code?

const dt = new Date(2020, 8, 23);
console.log(dt);    // Wed Sep 23 2020 00:00:00 GMT+0330 (Iran Standard Time)

Explain the various formats of ISO standard followed by javascript?
YYYY-MM-DDTHH:mm:ss.sssZ

Date Methods:

The Date() object helps you to create the dates and also the time. We don't have a separate object for time, we use the Date() object only. The Date() object has various methods for date and time related operations. In this section, we will go through a few date related set and get methods. Let's see a couple of examples:

let dt = new Date();
console.log(dt);    // Mon Jul 31 2023 18:21:00 GMT+0330 (Iran Standard Time)
// Get methods:
console.log(dt.getFullYear());  // 2023
console.log(dt.getMonth());     // 6  - 0 Based
console.log(dt.getDate());      // 31
console.log(dt.getDay());       // 1 - Day of the week(0 - 6)
console.log(dt.getUTCFullYear());   // 2023
console.log(dt.getUTCDay());    // 1

// Set methods:
dt.setFullYear(1996);   // year, month, day - month and day are optional
console.log(dt);    // Wed Jul 31 1996 18:28:50 GMT+0430 (Iran Daylight Time)
dt.setMonth(9);     // month, day - day is optional and month is 0 based
console.log(dt);    // Thu Oct 31 1996 18:34:35 GMT+0330 (Iran Standard Time)
dt.setDate(4);      // day=4
console.log(dt);    // Fri Oct 04 1996 18:36:29 GMT+0330 (Iran Standard Time)

Another useful method is Date.parse() which parses a string of date and returns the milliseconds:

// Syntax:
Date.parse(dateString);
// Or
new Date(dateString);

Let's see an example:

const dt = Date.parse("1996-09-04");
console.log(dt);    // 841795200000

Relative Questions:

Get a character month?

const dt = new Date();
const arrMonths = ["January", "February", "March", "april", "May", "June", "July", "August",
                   "September", "October", "November", "December"];
console.log(arrMonths[dt.getMonth()]);  // July

Find the date before 50 days of the given date?

const dt = new Date();
console.log(dt);    // Mon Jul 31 2023 18:54:07 GMT+0330 (Iran Standard Time)
dt.setDate(dt.getDate() - 50);
console.log(dt);    // Sun Jun 11 2023 18:54:07 GMT+0330 (Iran Standard Time)

What will be the output if you add35 as date in Date() constructor?

const dt = new Date(2020, 04, 35);
console.log(dt);    // Thu Jun 04 2020 00:00:00 GMT+0430 (Iran Daylight Time)

The autocorrection feature of the date object will validate the date automatically even if it is leap year.

Time Methods:

For time, javascript gives the very same Date object only. There is a unique static method called Date.now() which returns a timestamp in milliseconds elapsed since 1 Jan 1970 UTC. Let's see a couple of examples:

console.log(Date.now());    // 1691402471667
// Time get methods:
const dt = new Date();
console.log(dt.getTime());              // 1691402656744
console.log(dt.getHours());             // 13
console.log(dt.getMinutes());           // 34
console.log(dt.getMinutes());           // 34
console.log(dt.getMilliseconds());      // 744

There are also set methods. Let's see an example:

/*
By giving the milliseonds to setTime() method, it will set the time and the value
is started since 1 Jan 1970.
 */
const dt = new Date();
console.log(dt);            // Mon Aug 07 2023 13:41:56 GMT+0330 (Iran Standard Time)
dt.setTime(999999999);
console.log(dt);            // Mon Jan 12 1970 17:16:39 GMT+0330 (Iran Standard Time)

Relative Questions:

Calculate the date difference in days?

const dt1 = new Date(2023, 8, 7);
const dt2 = new Date(2023, 9, 4);
const n = dt2.getTime() - dt1.getTime();
const diff = n / 1000 / 3600 / 24;
console.log(diff);  // 27

How can you change hours or minutes in time?
Using setHours() and setMinutes() methods.