Promise

MyPromise.js

const PENDING = Symbol('pending');
const FULFILLED = Symbol('fulfilled');
const REJECTED = Symbol('rejected');

class MyPromise {
    constructor(excutor) {
        this.state = PENDING;
        this.value = undefined;
        this.err = undefined;
        this.onFulfill = [];
        this.onReject = [];

        const resolve = (value) => {
            if(this.state === PENDING) {
                this.value = value;
                this.state = FULFILLED;
                this.onFulfill.map(fn => fn(value));
            }
        };
        const reject = (err) => {
            if(this.state === PENDING) {
                this.state = REJECTED;
                this.err = err;
                this.onReject.map(fn => fn(err));
            }
        };
        try {
            excutor(resolve, reject)
        } catch (err) {
            reject(err)
        }
    }

    then(onFulfilled = () => {}, onReject = () => {}) {
        // console.log(this.state)
        switch(this.state) {
            case FULFILLED:
                onFulfilled(this.value);
                break;
            case REJECTED:
                onReject(this.err);
                break;
            case PENDING:
                this.onFulfill.push(onFulfilled);
                this.onReject.push(onReject);
        }
    }
}

function timeoutPromise(ms) {
    return new MyPromise((resolve, reject) => {
        setTimeout(function() {
            resolve(123);
        }, ms)
    });
}

const p = timeoutPromise(3000);

p.then((value) => {
    console.log('time out 3000')
    console.log(value)
}, (err) => {
    console.log(err)
});