一步步实现一个 Promise如何一步步实现promise。 1、三种状态； 2、状态流转； 3、then方法； 4、不

class实现:

class MPromise {
    constructor() {

    }
}

定义三种状态类型

const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';

设置初始状态

class MPromise {
    constructor() {
        // 初始状态为pending
        this.status = PENDING;
        this.value = null;
        this.reason = null;
    }
}

resolve 和 reject 方法

根据刚才的规范, 这两个方法是要更改status的, 从pending改到fulfilled/rejected.
注意两个函数的入参分别是value 和 reason.

class MPromise {
    constructor() {
        // 初始状态为pending
        this.status = PENDING;
        this.value = null;
        this.reason = null;
    }

    resolve(value) {
        if (this.status === PENDING) {
            this.status = FULFILLED;
            this.value = value;
        }
    }

    reject(reason) {
        if (this.status === PENDING) {
            this.status = REJECTED;
            this.reason = reason;
        }
    }
}

是不是发现咱们的promise少了入参, 咱们来加一下

入参是一个函数, 函数接收resolve和reject两个参数.
注意在初始化promise的时候, 就要执行这个函数, 并且有任何报错都要通过reject抛出去

class MPromise {
    constructor(fn) {
        // 初始状态为pending
        this.status = PENDING;
        this.value = null;
        this.reason = null;

        try {
            fn(this.resolve.bind(this), this.reject.bind(this));
        } catch (e) {
            this.reject(e);
        }
    }

    resolve(value) {
        if (this.status === PENDING) {
            this.status = FULFILLED;
            this.value = value;
        }
    }

    reject(reason) {
        if (this.status === PENDING) {
            this.status = REJECTED;
            this.reason = reason;
        }
    }
}

接下来来实现一下关键的then方法

then接收两个参数, onFulfilled 和 onRejected

then(onFulfilled, onRejected) {}

检查并处理参数, 之前提到的如果不是function, 就忽略. 这个忽略指的是原样返回value或者reason.

isFunction(param) {
    return typeof param === 'function';
}

then(onFulfilled, onRejected) {
    const fulFilledFn = this.isFunction(onFulfilled) ? onFulfilled : (value) => {
        return value;
    }
    const rejectedFn = this.isFunction(onRejected) ? onRejected : (reason) => {
        throw reason
    };
}

根据当前promise的状态, 调用不同的函数

then(onFulfilled, onRejected) {
    const fulFilledFn = this.isFunction(onFulfilled) ? onFulfilled : (value) => {
        return value;
    }
    const rejectedFn = this.isFunction(onRejected) ? onRejected : (reason) => {
        throw reason;
    };

    switch (this.status) {
        case FULFILLED: {
            fulFilledFn(this.value);
            break;
        }
        case REJECTED: {
            rejectedFn(this.reason);
            break;
        }
    }
}

这个时候有的同学要问了, 你这样写, 是在then函数被调用的瞬间就会执行. 那这时候如果status还没变成fulfilled或者rejected怎么办, 很有可能还是pending的.

那么我们首先要拿到所有的回调, 然后才能在某个时机去执行他. 新建两个数组, 来分别存储成功和失败的回调, 调用then的时候, 如果还是pending就存入数组.

then(onFulfilled, onRejected) {
    const fulFilledFn = this.isFunction(onFulfilled) ? onFulfilled : (value) => {
        return value;
    }
    const rejectedFn = this.isFunction(onRejected) ? onRejected : (reason) => {
        throw reason;
    };

    switch (this.status) {
        case FULFILLED: {
            fulFilledFn(this.value);
            break;
        }
        case REJECTED: {
            rejectedFn(this.reason);
            break;
        }
        case PENDING: {
            this.FULFILLED_CALLBACK_LIST.push(realOnFulfilled);
            this.REJECTED_CALLBACK_LIST.push(realOnRejected);
            break;
        }
    }
}

在status发生变化的时候, 就执行所有的回调. 这里咱们用一下es6的getter和setter. 这样更符合语义, 当status改变时, 去做什么事情. (当然也可以顺序执行, 在给status赋值后, 下面再加一行forEach)

get status() {
    return this._status;
}

set status(newStatus) {
    switch (newStatus) {
        case FULFILLED: {
            this.FULFILLED_CALLBACK_LIST.forEach(callback => {
                callback(this.value);
            });
            break;
        }
        case REJECTED: {
            this.REJECTED_CALLBACK_LIST.forEach(callback => {
                callback(this.reason);
            });
            break;
        }
    }
}

then的返回值这块内容比较多, 所以单拿出来了

7.1 如果 onFulfilled 或者 onRejected 抛出一个异常 e ，则 promise2 必须拒绝执行，并返回拒因 e。(这样的话, 我们就需要手动catch代码，遇到报错就reject)

then(onFulfilled, onRejected) {
    const fulFilledFn = this.isFunction(onFulfilled) ? onFulfilled : (value) => {
        return value;
    }
    const rejectedFn = this.isFunction(onRejected) ? onRejected : (reason) => {
        throw reason;
    };

    const fulFilledFnWithCatch = (resolve, reject) => {
        try {
            fulFilledFn(this.value);
        } catch (e) {
            reject(e)
        }
    };

    const rejectedFnWithCatch = (resolve, reject) => {
        try {
            rejectedFn(this.reason);
        } catch (e) {
            reject(e);
        }
    }

    switch (this.status) {
        case FULFILLED: {
            return new MPromise(fulFilledFnWithCatch);
        }
        case REJECTED: {
            return new MPromise(rejectedFnWithCatch);
        }
        case PENDING: {
            return new MPromise((resolve, reject) => {
                this.FULFILLED_CALLBACK_LIST.push(() => fulFilledFnWithCatch(resolve, reject));
                this.REJECTED_CALLBACK_LIST.push(() => rejectedFnWithCatch(resolve, reject));
            });
        }
    }
}

7.2 如果 onFulfilled 不是函数且 promise1 成功执行， promise2 必须成功执行并返回相同的值

const fulFilledFnWithCatch = (resolve, reject) => {
        try {
            fulFilledFn(this.value);
            resolve(this.value);
        } catch (e) {
            reject(e)
        }
    };

7.3 如果 onRejected 不是函数且 promise1 拒绝执行， promise2 必须拒绝执行并返回相同的据因。

需要注意的是，如果promise1的onRejected执行成功了，promise2应该被resolve

const rejectedFnWithCatch = (resolve, reject) => {
    try {
        rejectedFn(this.reason);
        if (this.isFunction(onRejected)) {
            resolve();
        }
    } catch (e) {
        reject(e);
    }
}

7.4 如果 onFulfilled 或者 onRejected 返回一个值 x ，则运行resolvePromise方法

then(onFulfilled, onRejected) {
    const fulFilledFn = this.isFunction(onFulfilled) ? onFulfilled : (value) => {
        return value;
    }
    const rejectedFn = this.isFunction(onRejected) ? onRejected : (reason) => {
        throw reason;
    };

    const fulFilledFnWithCatch = (resolve, reject, newPromise) => {
        try {
            if (!this.isFunction(onFulfilled)) {
                resolve(this.value);
            } else {
                const x = fulFilledFn(this.value);
                this.resolvePromise(newPromise, x, resolve, reject);
            }
        } catch (e) {
            reject(e)
        }
    };

    const rejectedFnWithCatch = (resolve, reject, newPromise) => {
        try {
            if (!this.isFunction(onRejected)) {
                reject(this.reason);
            } else {
                const x = rejectedFn(this.reason);
                this.resolvePromise(newPromise, x, resolve, reject);
            }
        } catch (e) {
            reject(e);
        }
    }

    switch (this.status) {
        case FULFILLED: {
            const newPromise = new MPromise((resolve, reject) => fulFilledFnWithCatch(resolve, reject, newPromise));
            return newPromise;
        }
        case REJECTED: {
            const newPromise = new MPromise((resolve, reject) => rejectedFnWithCatch(resolve, reject, newPromise));
            return newPromise;
        }
        case PENDING: {
            const newPromise = new MPromise((resolve, reject) => {
                this.FULFILLED_CALLBACK_LIST.push(() => fulFilledFnWithCatch(resolve, reject, newPromise));
                this.REJECTED_CALLBACK_LIST.push(() => rejectedFnWithCatch(resolve, reject, newPromise));
            });
            return newPromise;
        }
    }
}

resolvePromise

resolvePromise(newPromise, x, resolve, reject) {
    // 如果 newPromise 和 x 指向同一对象，以 TypeError 为据因拒绝执行 newPromise
    // 这是为了防止死循环
    if (newPromise === x) {
        return reject(new TypeError('The promise and the return value are the same'));
    }

    if (x instanceof MPromise) {
        // 如果 x 为 Promise ，则使 newPromise 接受 x 的状态
        // 也就是继续执行x，如果执行的时候拿到一个y，还要继续解析y
        // 这个if跟下面判断then然后拿到执行其实重复了，可有可无
        x.then((y) => {
            resolvePromise(newPromise, y, resolve, reject);
        }, reject);
    } else if (typeof x === 'object' || this.isFunction(x)) {
        // 如果 x 为对象或者函数
        // 这个坑是跑测试的时候发现的，如果x是null，应该直接resolve
        if (x === null) {
            return resolve(x);
        }

        let then = null;

        try {
            // 把 x.then 赋值给 then 
            then = x.then;
        } catch (error) {
            // 如果取 x.then 的值时抛出错误 e ，则以 e 为据因拒绝 promise
            return reject(error);
        }

        // 如果 then 是函数
        if (this.isFunction(then)) {
            let called = false;
            // 将 x 作为函数的作用域 this 调用之
            // 传递两个回调函数作为参数，第一个参数叫做 resolvePromise ，第二个参数叫做 rejectPromise
            // 名字重名了，我直接用匿名函数了
            try {
                then.call(
                    x,
                    // 如果 resolvePromise 以值 y 为参数被调用，则运行 resolvePromise
                    (y) => {
                        // 如果 resolvePromise 和 rejectPromise 均被调用，
                        // 或者被同一参数调用了多次，则优先采用首次调用并忽略剩下的调用
                        // 实现这条需要前面加一个变量called
                        if (called) return;
                        called = true;
                        resolvePromise(promise, y, resolve, reject);
                    },
                    // 如果 rejectPromise 以据因 r 为参数被调用，则以据因 r 拒绝 promise
                    (r) => {
                        if (called) return;
                        called = true;
                        reject(r);
                    });
            } catch (error) {
                // 如果调用 then 方法抛出了异常 e：
                // 如果 resolvePromise 或 rejectPromise 已经被调用，则忽略之
                if (called) return;

                // 否则以 e 为据因拒绝 promise
                reject(error);
            }
        } else {
            // 如果 then 不是函数，以 x 为参数执行 promise
            resolve(x);
        }
    } else {
        // 如果 x 不为对象或者函数，以 x 为参数执行 promise
        resolve(x);
    }
}

onFulfilled 和 onRejected 是微任务

咱们可以用queueMicrotask包裹执行函数

then(onFulfilled, onRejected) {
    const fulFilledFn = this.isFunction(onFulfilled) ? onFulfilled : (value) => {
        return value;
    }
    const rejectedFn = this.isFunction(onRejected) ? onRejected : (reason) => {
        throw reason;
    };

    const fulFilledFnWithCatch = (resolve, reject, newPromise) => {
        queueMicrotask(() => {
            try {
                if (!this.isFunction(onFulfilled)) {
                    resolve(this.value);
                } else {
                    const x = fulFilledFn(this.value);
                    this.resolvePromise(newPromise, x, resolve, reject);
                }
            } catch (e) {
                reject(e)
            }
        })
    };

    const rejectedFnWithCatch = (resolve, reject, newPromise) => {
        queueMicrotask(() => {
            try {
                if (!this.isFunction(onRejected)) {
                    reject(this.reason);
                } else {
                    const x = rejectedFn(this.reason);
                    this.resolvePromise(newPromise, x, resolve, reject);
                }
            } catch (e) {
                reject(e);
            }
        })
    }

    switch (this.status) {
        case FULFILLED: {
            const newPromise = new MPromise((resolve, reject) => fulFilledFnWithCatch(resolve, reject, newPromise));
            return newPromise;
        }
        case REJECTED: {
            const newPromise = new MPromise((resolve, reject) => rejectedFnWithCatch(resolve, reject, newPromise));
            return newPromise;
        }
        case PENDING: {
            const newPromise = new MPromise((resolve, reject) => {
                this.FULFILLED_CALLBACK_LIST.push(() => fulFilledFnWithCatch(resolve, reject, newPromise));
                this.REJECTED_CALLBACK_LIST.push(() => rejectedFnWithCatch(resolve, reject, newPromise));
            });
            return newPromise;
        }
    }
}

简单写点代码测试一下

const test = new MPromise((resolve, reject) => {
    setTimeout(() => {
        resolve(111);
    }, 1000);
}).then(console.log);

console.log(test);

setTimeout(() => {
    console.log(test);

}, 2000)

这个时候会发现, 为什么我可以调用.then, 不可以调用.catch呢? 因为我们并没有在类里面声明catch方法

catch方法

catch (onRejected) {
    return this.then(null, onRejected);
}

promise.resolve

将现有对象转为Promise对象，如果 Promise.resolve 方法的参数，不是具有 then 方法的对象（又称 thenable 对象），则返回一个新的 Promise 对象，且它的状态为fulfilled。
```
static resolve(param) {
    if (param instanceof MyPromise) {
        return param;
    }

    return new MyPromise(function (resolve) {
        resolve(param);
    });
}
```
promise.reject

返回一个新的Promise实例，该实例的状态为rejected。Promise.reject方法的参数reason，会被传递给实例的回调函数。
```
static reject(reason) {
    return new MPromise((resolve, reject) => {
        reject(reason);
    });
}
```

promise.race

const p = Promise.race([p1, p2, p3]);

该方法是将多个 Promise 实例，包装成一个新的 Promise 实例。只要p1、p2、p3之中有一个实例率先改变状态，p的状态就跟着改变。那个率先改变的 Promise 实例的返回值，就传递给p的回调函数。

static race(promiseList) {
    return new MPromise((resolve, reject) => {
        const length = promiseList.length;

        if (length === 0) {
            return resolve();
        } else {
            for (let i = 0; i < length; i++) {
                MPromise.resolve(promiseList[i]).then(
                    (value) => {
                        return resolve(value);
                    },
                    (reason) => {
                        return reject(reason);
                    });
            }
        }
    });

}

写段测试代码

const test = new MPromise((resolve, reject) => {
    setTimeout(() => {
        resolve(111);
    }, 1000);
});

const test2 = new MPromise((resolve, reject) => {
    setTimeout(() => {
        resolve(222);
    }, 2000);
});

const test3 = new MPromise((resolve, reject) => {
    setTimeout(() => {
        resolve(333);
    }, 3000);
});

MPromise.race([test, test2, test3]).then(console.log);