Promiseを手書きで書く(面接で十分)

//  
new Promise((resolve, reject) => {
   setTimeout(() => resolve(new Promise(res => res(333))), 2000)
}).then(res => {
    console.log(res)
    return 222
}).then().then(res => {
    console.log(res);
    return new Promise(res => {
        setTimeout(() => { res(666) }, 1000)
    }).then(res => {
        console.log(res);
        return 999
    })
}).then(res => {
    console.log(res);
})
/*
 *  ：
 * 333
 * 222
 * 666
 * 999
*/

//  
;(function (global) {
	global.Promise = Promise
	//  promise 
	const PENDING = 'pending'
	const FULFILLED = 'fulfilled'
	const REJECTED = 'rejected'

	function Promise(fn) {
		this.state = PENDING
		this.fulfilledQueue = []
		this.rejectedQueue = []
		try {
			fn(
				//  this 
				(res) => {
					this._resolve(res)
				},
				(err) => {
					this._reject(err)
				}
				//  this.resolve.bind(this), this.reject.bind(this))
			)
		} catch (e) {
			reject(e)
		}
	}

	Promise.prototype = {
		constructor: Promise,
		_resolve: function (value) {
			if (value instanceof Promise) {
				return value.then(
					//  this 
					(res) => {
						this._resolve(res)
					},
					(err) => {
						this._reject(err)
					}
				)
			}
			setTimeout(() => {
				if (this.state === PENDING) {
					this.state = FULFILLED
					this.value = value
					this.fulfilledQueue.forEach((fn) => fn(this.value))
				}
			})
		},
		_reject: function (value) {
			setTimeout(() => {
				if (this.state === PENDING) {
					this.state = REJECTED
					this.value = value
					this.rejectedQueue.forEach((fn) => fn(this.value))
				}
			})
		},
		then: function (fulfilled, rejected) {
			this.fulfilled =
				typeof fulfilled === 'function' ? fulfilled : (v) => v
			this.rejected =
				typeof rejected === 'function'
					? rejected
					: (err) => {
							throw err
					  }
			if (this.state === PENDING) {
				return new Promise((resolve, reject) => {
					this.fulfilledQueue.push(() => {
						try {
							this.value = this.fulfilled(this.value)
							resolve(this.value)
						} catch (error) {
							reject(error)
						}
					})
					this.rejectedQueue.push(() => {
						try {
							this.value = this.rejected(this.value)
							resolve(this.value)
						} catch (error) {
							reject(error)
						}
					})
				})
			}
			if (this.state === FULFILLED) {
				return new Promise((resolve, reject) => {
					setTimeout(() => {
						try {
							this.value = this.fulfilled(this.value)
							resolve(this.value)
						} catch (error) {
							reject(error)
						}
					})
				})
			}
			if (this.state === REJECTED) {
				return new Promise((resolve, reject) => {
					setTimeout(() => {
						try {
							this.value = this.rejected(this.value)
							resolve(this.value)
						} catch (error) {
							reject(error)
						}
					})
				})
			}
		},
	}
})(window)