Promise + gì

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Code viết tay: Promise +

const PENDING = 'PENDING';
const RESOLVED = 'RESOLVED';
const REJECTED = 'REJECTED';

class Promise {
	constructor(executor) {
		this.status = PENDING; 
		this.value = undefined; 
		this.reason = undefined; 
		let resolve = (value) => {
			if (this.status === PENDING) {
				this.value = value;
				this.status = RESOLVED;
			}
		};
		let reject = (reason) => {
			if (this.status === PENDING) {
				this.reason = reason;
				this.status = REJECTED;
			}
		};
		
		try {
			executor(resolve, reject);
		} catch(e) {
			console.log('catch::::', e);
			reject(e); 
		}
	}
	then(onfulfilled, onrejected) {
		if (this.status === RESOLVED) {
			onfulfilled(this.value);
		}
		if (this.status === REJECTED) {
			onrejected(this.reason);
		}
	}
}
module.exports = Promise;

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