深度理解Promise(03)--手寫實現promise

上一篇介紹了
promise的概念和方法01
如何使用promise實現ajax02
這一篇介紹使用純原生js實現promise

class NewPromise {
 	constructor(executor) {
	 	if (typeof executor !== 'function') {
	 	 	throw new Error('Executor must be a function');
 		}
	 	this.state = 'pending';
	 	this.statesSave = [];
	 	const resolve = res => {
		 	if (this.state !== 'pending') {
		 		return;
	 		}
 			this.state = 'resolved';
		 	this.internalValue = res;
		 	for (const { onResolved } of this.statesSave) {
		 		onResolved(res);
		 	}
		 };
	 	const reject = err => {
		 	if (this.state !== 'pending') {
		 		return;
		 	}
		 	this.state = 'rejected';
		 	this.internalValue = err;
		 	for (const { onRejected } of this.statesSave) {
		 		onRejected(err);
		 	}
	 	};
	 	try {
	 	 executor(resolve, reject);
	 	} catch (err) {
	 	 reject(err);
	 	}
 	}
  
	then(onResolved, onRejected) {
		if (this.state === 'resolved') {
	 		onResolved(this.internalValue);
		} else if (this.$state === 'rejected') {
	 		onRejected(this.internalValue);
		} 
		else {
	 	    this.statesSave.push({ onResolved, onRejected });
	 	}
	}
}
 
let p = new NewPromise(resolve => {
	// resolve('Hello');
 	setTimeout(() => resolve('Hello'), 3000);
});
p.then(res => console.log(res));//Hello

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