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MyPromise.js
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MyPromise.js
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const STATE = {
// states of the promise
FULFILLED: "fulfilled",
REJECTED: "rejected",
PENDING: "pending",
};
class MyPromise {
// we define a constructor and pass a calback func to it which contain our resolve and reject states.
#thencbs = []; // empty array to store the callbacks passed to then.
#catchcbs = []; // empty array to store the callbacks passed to catch.
#state = STATE.PENDING; // info about the current state of the promise (pending, rejected, fulfilled).
#value; // the value passed into the function onSuccess and onFail.
#onSuccessBind = this.#onSuccess.bind(this); // to properly bind the this variable to amke sure its hooked up.
#onFailBind = this.#onFail.bind(this);
constructor(cb) {
// our callback as discussed above will have two states: on Success state and on Failure state.
try {
cb(this.#onSuccessBind, this.#onFailBind); // work as resolve, reject
} catch (error) {
this.#onFail(error);
}
}
#runCallbacks() {
if (this.#state === STATE.FULFILLED) {
this.#thencbs.forEach((cb) => {
cb(this.#value); // run all then cbs for success state
});
}
this.#thencbs = []; // remove all the cbs after execution to avoid re-running them.
if (this.#state === STATE.REJECTED) {
this.#catchcbs.forEach((cb) => {
cb(this.#value); // run all catch cbs for fail state
});
}
this.#catchcbs = []; // remove all the cbs after execution to avoid re-running them.
}
// defining the two states (private methods)
#onSuccess(value) {
queueMicrotask(() => {
if (this.#state !== STATE.PENDING) return; // as we want resolve and reject to run only once in a promise.
// promise can contain more promises inside them so to handle them we call then after checking that it is indeed a instance of a promise.
if (value instanceof MyPromise) {
value.then(this.#onSuccessBind, this.#onFailBind);
return;
}
this.#value = value;
this.#state = STATE.FULFILLED; // changing states as per success or fail
this.#runCallbacks();
});
}
#onFail(value) {
queueMicrotask(() => {
if (this.#state !== STATE.PENDING) return; // as we want resolve and reject to run only once in a promise.
// promise can contain more promises inside them so to handle them we call then after checking that it is indeed a instance of a promise.
if (value instanceof MyPromise) {
value.then(this.#onSuccessBind, this.#onFailBind);
return;
}
// when the code gives a error but no catch is provided.
if (this.#catchcbs.length === 0) {
throw new UncaughtPromiseError(value);
}
this.#value = value;
this.#state = STATE.REJECTED; // changing states as per success or fail
this.#runCallbacks();
});
}
// Promises contain public methods like then, catch, finally...
then(thencb, catchcb) {
// contains a callback with next set of actions to be performed.
// we define an array of callbacks because a promise can have multiple callback uppon successful execution.
return new MyPromise((resolve, reject) => {
// push callback on to the array for execution on success state.
this.#thencbs.push(result => {
// in case we dont care avobt the successtate, and only care about fail state, or in other words our thencb is null we directly resolve the promise.
if (thencb == null) {
resolve(result);
return;
}
// but if thencb is not null.
try {
resolve(thencb(result));
} catch (error) {
reject(error);
}
});
// push callback on to the array for execution on fail state.
this.#catchcbs.push(result => {
// in case we dont care avobt the rejectstate, and only care about success state, or in other words our catchcb is null we directly reject the promise.
if (catchcb == null) {
reject(result);
return;
}
// but if catchcb is not null.
try {
resolve(catchcb(result));
} catch (error) {
reject(error);
}
});
this.#runCallbacks(); // when there are still callbacks left to run even after the state is changed to fullfilled.
});
}
catch(cb) {
// calling the then method with thencb as undefined and catchcb as given cb.
return this.then(undefined, cb);
}
finally(cb) {
return this.then(
(result) => {
cb();
return result;
},
(result) => {
cb();
throw result;
}
);
}
static resolve(value) {
// resolve the promise
return new Promise((resolve, reject) => {
resolve(value);
});
}
static reject(value) {
// reject the promise
return new Promise((resolve, reject) => {
reject(value);
});
}
// static func to resolve all promises in the passed promises array and return the array of results.
// if any one promise fails we immediately reject.
static all(promises) {
const results = [];
let completedPromises = 0;
return new MyPromise((resolve, reject) => {
for (let i = 0; i < promises.length; i++) {
const promise = promises[i];
promise
.then((value) => {
completedPromises++;
results[i] = value;
if (completedPromises === promises.length) {
resolve(results);
}
})
.catch(reject);
}
});
}
// static func to resolve all promises in the passed promises array and return the array of results.
// but here if we resolve everyPromise and store value for success and reason for failure.
static allSettled(promises) {
const results = [];
let completedPromises = 0;
return new MyPromise((resolve) => {
for (let i = 0; i < promises.length; i++) {
const promise = promises[i];
promise
.then((value) => {
results[i] = { status: STATE.FULFILLED, value };
})
.catch((reason) => {
results[i] = { status: STATE.REJECTED, reason };
})
.finally(() => {
completedPromises++;
if (completedPromises === promises.length) {
resolve(results);
}
});
}
});
}
// returns the first promise that runs in success state.
static race(promises) {
return new MyPromise((resolve, reject) => {
promises.forEach((promise) => {
promise.then(resolve).catch(reject);
});
});
}
// returns the set of all promises that resturn a fail state.
static any(promises) {
const errors = [];
let rejectedPromises = 0;
return new MyPromise((resolve, reject) => {
for (let i = 0; i < promises.length; i++) {
const promise = promises[i];
promise.then(resolve).catch((value) => {
rejectedPromises++;
errors[i] = value;
if (rejectedPromises === promises.length) {
reject(new AggregateError(errors, 'All promises were rejected'));
}
});
}
});
}
}
// Custom error class to throw error in case any promise gives error.
class UncaughtPromiseError extends Error {
constructor(error) {
super(error);
this.stack = `(in promise) ${error.stack}`;
}
}
module.exports = MyPromise;