SOURCE

const PENDING = 'PENDING';
const FULFILLED = 'FULFILLED';
const REJECTED = 'REJECTED';

class myPromise {
    constructor(executor) {
        this.status = PENDING;

        this.value = undefined;
        this.reason = undefined;

        this.onFulfillCallbacks = [];
        this.onRejectCallbacks = [];


        let resolve = (value) => {
            this.status = FULFILLED;
            this.value = value;

            this.onFulfillCallbacks.forEach((fn) => fn());
        };

        let reject = (reason) => {
            this.status = REJECTED;
            this.reason = reason;

            this.onRejectCallbacks.forEach((fn) => fn());
        }

        try {
            executor(resolve, reject);
        } catch (error) {
            reject(error);
        };
    };

    then(onFulfill, onReject) {
        onFulfill = typeof onFulfill === 'function' ? onFulfill : (value) => value;
        onReject = typeof onReject === 'function' ? onReject : (error) => {
            throw error;
        };

        let promise2 = new myPromise((resolve, reject) => {
            if (this.status === FULFILLED) {
                setTimeout(() => {
                    try {
                        let x = onFulfill(this.value);
                        this.resolvePromise(promise2, x, resolve, reject);
                    } catch (e) {
                        reject(e);
                    }
                }, 0);
            };

            if (this.status === REJECTED) {
                setTimeout(() => {
                    try {
                        let x = onReject(this.reason);
                        this.resolvePromise(promise2, x, resolve, reject);
                    } catch (e) {
                        reject(e);
                    }
                }, 0);
            };

            if (this.status === PENDING) {
                this.onFulfillCallbacks.push(() => {
                    try {
                        setTimeout(() => {
                            let x = onFulfill(this.value);
                            this.resolvePromise(promise2, x, resolve, reject);
                        }, 0);
                    } catch (e) {
                        reject(e)
                    }
                })

                this.onRejectCallbacks.push(() => {
                    try {
                        setTimeout(() => {
                            let x = onReject(this.reason);
                            this.resolvePromise(promise2, x, resolve, reject);
                        }, 0)
                    } catch (e) {
                        reject(e);
                    };
                });
            };
        });


        return promise2;
    };

    resolvePromise(promise2, x, resolve, reject) {
        if (promise2 === x) {
            return reject(new TypeError('chaining cycle detected for promise'));
        }

        let called = false;

        if (x !== null && (typeof x === 'function') || typeof x === 'object') {
            try {
                let then = x.then;
                if (typeof then === 'function') {
                    then.call(x, (y) => {
                        if (called) return;
                        called = true;
                        this.resolvePromise(promise2, y, resolve, reject);
                    },(error) => {
                        if(called) return;
                        called = true;
                        reject(error);
                    })
                }else{
                    resolve(x);
                }
            } catch (e) {
                reject(e);
            }
        }else{
            resolve(x);
        }
    };
};

const test3 = new myPromise((res,rej) => {
    res(3);
});


test3.then((value) => {
    console.log('value',value);
})