SOURCE

class Promise1 {
    constructor(excutor) {
        this.status = 'PENDING'
        this.value = undefined
        this.onFulfilledCallback = []
        this.onRejectedCallback = []

        const resolve = value => {
            setTimeout(() => {
                if (this.status === 'PENDING') {
                    this.status = 'FULFILLED'
                    this.value = value
                    this.onFulfilledCallback.forEach(fn => fn())
                }
            })
        }

        const reject = err => {
            setTimeout(() => {

                if (this.status === 'PENDING') {
                    this.status = 'REJECTED'
                    this.value = err
                    this.onRejectedCallback.forEach(fn => fn())
                }
            })
        }

        excutor(resolve, reject)
    }

    then(onFulfilled, onRejected) {
        const self = this
        const status = this.status

        return new Promise1((resolve, reject) => {
            if (status === 'FULFILLED') {
                const value = onFulfilled(self.value)
                if (value === null) return null
                if (typeof value === 'object' || typeof value === 'function') {
                    if (typeof value.then === 'function') {
                        value.then(resolve, reject)
                    } else {
                        resolve(value)
                    }
                } else { // 普通值
                    resolve(value)
                }
            } else if (status === 'REJECTED') {
                const value = onRejected(self.value)
                resolve(rejectedVal)
            } else if (status === 'PENDING') {
                self.onFulfilledCallback.push(() => {
                    const fulfilledVal = onFulfilled(self.value)
                    resolve(fulfilledVal)
                })
                self.onRejectedCallback.push(() => {
                    const rejectedVal = onRejected(self.value)
                    resolve(rejectedVal)
                })
            }
        })
    }
}

console.log('start')
setTimeout(() => {
    console.log('setTimeout')
})
const p = new Promise1((resolve, reject) => {
    resolve(1)
})

p.then(res => {
    console.log('第一个then:', res)
    return new Promise1((resolve, reject) => {
        resolve('return Promise')
    })
}).then(res => {
    console.log('第二个then:', JSON.stringify(res))
    return {
        then(resolve, reject) {
            resolve(22)
        }
    }
}).then(res => {
    console.log('第三个then:', res)
})

console.log('end')