自己实现Promise

new MyPromise((resolve, reject) => {
		     reject('ERROR');
}).then(result => {console.log(result)})

最基本的Promise (没有数显then链)

class MyPromise {
	constructor(executor) {
		// 每一个PROMISE实例都有一个状态和结果属性
		this.status = 'pending';
		this.value = undefined;

		// 用来存储基于THEN指定的成功或者失败的方法
		this.resolveArr = [];
		this.rejectArr = [];

		// 定义RESOLVE/REJECT方法用来改变PROMISE实例的状态和结果
		let change = (status, value) => {
			// 状态一但改变过，再改变则无效
			if (this.status !== 'pending') return;
			this.value = value;
			this.status = status;
			// 改变完成状态后，把基于THEN指定的对应方法执行
			let fnArr = status === 'resolved' ? this.resolveArr : this.rejectArr;
			fnArr.forEach(item => {
				if (typeof item !== 'function') return;
				item(this.value);
			});
		};
		// 为了保证执行RESOLVE/REJECT的时候，已经通过THEN把需要执行的方法弄好了，我们判断处理（没有方法的时候，我们让改变状态的操作延迟进行）
		let resolve = result => {
			if (this.resolveArr.length > 0) {
				change('resolved', result);
				return;
			}
			let delayTimer = setTimeout(_ => {
				change('resolved', result);
				clearTimeout(delayTimer);
			}, 0);
		};
		let reject = reason => {
			if (this.rejectArr.length > 0) {
				change('rejected', reason);
				return;
			}
			let delayTimer = setTimeout(_ => {
				change('rejected', reason);
				clearTimeout(delayTimer);
			}, 0);
		};

		// 每一次NEW PROMISE都会立即执行EXECUTOR函数
		try {
			executor(resolve, reject);
		} catch (err) {
			reject(err.message);
		}
	}

	// MyPromise.prototype.then
	then(resolveFn, rejectFn) {
		this.resolveArr.push(resolveFn);
		this.rejectArr.push(rejectFn);
	}
}