class MyPromise {
    constructor(fn) {
        if (typeof fn !== 'function') {
            throw new TypeError(`MyPromise fn ${fn} is not a function`)
        }
        this.state = 'pending';
        this.value = void 0;
        fn(this.resolve.bind(this), this.reject.bind(this))
    }

    resolve(value) {
        if (this.state !== 'pending') return;
        this.state = 'fulfilled';
        this.value = value
    }

    reject(reason) {
        if (this.state !== 'pending') return;
        this.state = 'rejected';
        this.value = reason
    }

    then(fulfilled, rejected) {
        if (typeof fulfilled !== 'function' && typeof rejected !== 'function') {
            return this;
        }
        if (typeof fulfilled !== 'function' && this.state === 'fulfilled' ||
            typeof rejected !== 'function' && this.state === 'rejected') {
            return this;
        }
        return new MyPromise((resolve, reject) => {
            if (fulfilled && typeof fulfilled === 'function' && this.state === 'fulfilled') {
                let result = fulfilled(this.value);
                if (result && typeof result.then === 'function') {
                    return result.then(resolve, reject)
                } else {
                    resolve(result)
                }
            }
            if (rejected && typeof rejected === 'function' && this.state === 'rejected') {
                let result = rejected(this.value);
                if (result && typeof result.then === 'function') {
                    return result.then(resolve, reject)
                } else {
                    resolve(result)
                }
            }
        })
    }

    catch(rejected) {
        return this.then(null, rejected)
    }
}

new MyPromise((resolve, reject) => {
    console.log(1);
    //reject(2)
    resolve(2)
    console.log(3)
    setTimeout(() => {
        console.log(4)
    }, 0)
}).then(res => {
    console.log(res)
    return new MyPromise((resolve, reject) => {
        resolve(5)
    }).then(res => {
        return res
    })
}).then(res => {
    console.log(res)
}).catch(e => {
    console.log('e', e)
})

/**
 * 原生实现
 */
new Promise((resolve, reject) => {
    console.log(1);
    //reject(2)
    resolve(2)
    console.log(3)
    setTimeout(() => {
        console.log(4)
    }, 0)
}).then(res => {
    console.log(res)
    return new Promise((resolve, reject) => {
        resolve(5)
    }).then(res => {
        return res
    })
}).then(res => {
    console.log(res)
}).catch(e => {
    console.log('e', e)
})