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promise自定义封装

初始结构搭建：

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Promise-封装 | 1 - 初始结构搭建</title>
    <script src="./promise.js"></script>
</head>
<body>
    <script>
        let p = new Promise((resolve, reject) => {
            resolve('OK');
        });

        p.then(value => {
            console.log(value);
        }, reason=>{
            console.warn(reason);
        })
    </script>
</body>
</html>

promise.js:

function Promise(executor){

}

//添加 then 方法
Promise.prototype.then = function(onResolved, onRejected){

}

resolve与reject结构搭建:

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Promise-封装 | 2 - resolve 与 reject </title>
    <script src="./promise.js"></script>
</head>
<body>
    <script>
        let p = new Promise((resolve, reject) => {
            resolve('OK');
        });

        p.then(value => {
            console.log(value);
        }, reason=>{
            console.warn(reason);
        })
    </script>
</body>
</html>

promise.js:

//声明构造函数
function Promise(executor){
    //resolve 函数
    function resolve(data){

    }
    //reject 函数
    function reject(data){

    }

    //同步调用『执行器函数』
    executor(resolve, reject);
}

//添加 then 方法
Promise.prototype.then = function(onResolved, onRejected){

}

3-resolve与reject函数实现

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Promise-封装 | 2 - resolve 与 reject </title>
    <script src="./promise.js"></script>
</head>
<body>
    <script>
        let p = new Promise((resolve, reject) => {
            // resolve('OK');
            reject("error");
        });

        console.log(p);

        // p.then(value => {
        //     console.log(value);
        // }, reason=>{
        //     console.warn(reason);
        // })
    </script>
</body>
</html>

promise.js:

//声明构造函数
function Promise(executor){
    //添加属性
    this.PromiseState = 'pending';
    this.PromiseResult = null;
    //保存实例对象的 this 的值
    const self = this;// self _this that
    //resolve 函数
    function resolve(data){
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'fulfilled';// resolved
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
    }
    //reject 函数
    function reject(data){
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'rejected';// 
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
    }

    //同步调用『执行器函数』
    executor(resolve, reject);
}

//添加 then 方法
Promise.prototype.then = function(onResolved, onRejected){

}

throw抛出错误改变状态

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Promise-封装 | 4 - throw 抛出异常改变状态 </title>
    <!-- <script src="./promise.js"></script> -->
</head>
<body>
    <script>
        let p = new Promise((resolve, reject) => {
            // resolve('OK');
            // reject("error");
            //抛出异常
            throw "error";
        });

        console.log(p);

        // p.then(value => {
        //     console.log(value);
        // }, reason=>{
        //     console.warn(reason);
        // })
    </script>
</body>
</html>

promise.js:

//声明构造函数
function Promise(executor){
    //添加属性
    this.PromiseState = 'pending';
    this.PromiseResult = null;
    //保存实例对象的 this 的值
    const self = this;// self _this that
    //resolve 函数
    function resolve(data){
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'fulfilled';// resolved
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
    }
    //reject 函数
    function reject(data){
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'rejected';// 
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
    }
    try{
        //同步调用『执行器函数』
        executor(resolve, reject);
    }catch(e){
        //修改 promise 对象状态为『失败』
        reject(e);
    }
}

//添加 then 方法
Promise.prototype.then = function(onResolved, onRejected){

}

状态只能修改一次

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Promise-封装 | 5 - 状态只能修改一次 </title>
    <script src="./promise.js"></script>
</head>
<body>
    <script>
        let p = new Promise((resolve, reject) => {
            reject("error");
            resolve('OK');
            //抛出异常
            // throw "error";
        });

        console.log(p);
    </script>
</body>
</html>

promise.js:

//声明构造函数
function Promise(executor){
    //添加属性
    this.PromiseState = 'pending';
    this.PromiseResult = null;
    //保存实例对象的 this 的值
    const self = this;// self _this that
    //resolve 函数
    function resolve(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'fulfilled';// resolved
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
    }
    //reject 函数
    function reject(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'rejected';// 
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
    }
    try{
        //同步调用『执行器函数』
        executor(resolve, reject);
    }catch(e){
        //修改 promise 对象状态为『失败』
        reject(e);
    }
}

//添加 then 方法
Promise.prototype.then = function(onResolved, onRejected){

}

then方法执行回调

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Promise-封装 | 7 - 异步任务 then 方法实现 </title>
    <script src="./promise.js"></script>
</head>
<body>
    <script>
        //实例化对象
        let p = new Promise((resolve, reject) => {
            setTimeout(() => {
                // resolve('OK');
                reject("error");
            }, 1000);
        });

        p.then(value => {
            console.log(value);
        }, reason=>{
            console.warn(reason);
        });

        console.log(p);
    </script>
</body>
</html>

promise.js:

//声明构造函数
function Promise(executor){
    //添加属性
    this.PromiseState = 'pending';
    this.PromiseResult = null;
    //声明属性
    this.callback = {};
    //保存实例对象的 this 的值
    const self = this;// self _this that
    //resolve 函数
    function resolve(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'fulfilled';// resolved
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
        //调用成功的回调函数
        if(self.callback.onResolved){
            self.callback.onResolved(data);
        }
    }
    //reject 函数
    function reject(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'rejected';// 
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
        //执行回调
        if(self.callback.onResolved){
            self.callback.onResolved(data);
        }
    }
    try{
        //同步调用『执行器函数』
        executor(resolve, reject);
    }catch(e){
        //修改 promise 对象状态为『失败』
        reject(e);
    }
}

//添加 then 方法
Promise.prototype.then = function(onResolved, onRejected){
    //调用回调函数  PromiseState
    if(this.PromiseState === 'fulfilled'){
        onResolved(this.PromiseResult);
    }
    if(this.PromiseState === 'rejected'){
        onRejected(this.PromiseResult);
    }
    //判断 pending 状态
    if(this.PromiseState === 'pending'){
        //保存回调函数
        this.callback = {
            onResolved: onResolved,
            onRejected: onRejected
        }
    }
}

7-异步任务下then方法执行回调

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Promise-封装 | 7 - 异步任务 then 方法实现 </title>
    <script src="./promise.js"></script>
</head>
<body>
    <script>
        //实例化对象
        let p = new Promise((resolve, reject) => {
            setTimeout(() => {
                // resolve('OK');
                reject("error");
            }, 1000);
        });

        p.then(value => {
            console.log(value);
        }, reason=>{
            console.warn(reason);
        });

        console.log(p);
    </script>
</body>
</html>

promise.js:

//声明构造函数
function Promise(executor){
    //添加属性
    this.PromiseState = 'pending';
    this.PromiseResult = null;
    //声明属性
    this.callback = {};
    //保存实例对象的 this 的值
    const self = this;// self _this that
    //resolve 函数
    function resolve(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'fulfilled';// resolved
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
        //调用成功的回调函数
        if(self.callback.onResolved){
            self.callback.onResolved(data);
        }
    }
    //reject 函数
    function reject(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'rejected';// 
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
        //执行回调
        if(self.callback.onResolved){
            self.callback.onResolved(data);
        }
    }
    try{
        //同步调用『执行器函数』
        executor(resolve, reject);
    }catch(e){
        //修改 promise 对象状态为『失败』
        reject(e);
    }
}

//添加 then 方法
Promise.prototype.then = function(onResolved, onRejected){
    //调用回调函数  PromiseState
    if(this.PromiseState === 'fulfilled'){
        onResolved(this.PromiseResult);
    }
    if(this.PromiseState === 'rejected'){
        onRejected(this.PromiseResult);
    }
    //判断 pending 状态
    if(this.PromiseState === 'pending'){
        //保存回调函数
        this.callback = {
            onResolved: onResolved,
            onRejected: onRejected
        }
    }
}

8-指定多个回调:

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Promise-封装 | 8 - 指定多个回调 </title>
    <script src="./promise.js"></script>
</head>
<body>
    <script>
        //实例化对象
        let p = new Promise((resolve, reject) => {
            setTimeout(() => {
                // resolve('OK');
                reject('No');
            }, 1000);
        });

        p.then(value => {
            console.log(value);
        }, reason=>{
            console.warn(reason);
        });

        p.then(value => {
            alert(value);
        }, reason=>{
            alert(reason);
        });

        console.log(p);


    </script>
</body>
</html>

promise.js:

//声明构造函数
function Promise(executor){
    //添加属性
    this.PromiseState = 'pending';
    this.PromiseResult = null;
    //声明属性
    this.callbacks = [];
    //保存实例对象的 this 的值
    const self = this;// self _this that
    //resolve 函数
    function resolve(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'fulfilled';// resolved
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
        //调用成功的回调函数
        self.callbacks.forEach(item => {
            item.onResolved(data);
        });
    }
    //reject 函数
    function reject(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'rejected';// 
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
        //执行失败的回调
        self.callbacks.forEach(item => {
            item.onRejected(data);
        });
    }
    try{
        //同步调用『执行器函数』
        executor(resolve, reject);
    }catch(e){
        //修改 promise 对象状态为『失败』
        reject(e);
    }
}

//添加 then 方法
Promise.prototype.then = function(onResolved, onRejected){
    //调用回调函数  PromiseState
    if(this.PromiseState === 'fulfilled'){
        onResolved(this.PromiseResult);
    }
    if(this.PromiseState === 'rejected'){
        onRejected(this.PromiseResult);
    }
    //判断 pending 状态
    if(this.PromiseState === 'pending'){
        //保存回调函数
        this.callbacks.push({
            onResolved: onResolved,
            onRejected: onRejected
        });
    }
}

异步修改状态then方法返回结果

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Promise-封装 | 10 - 异步任务 then 返回结果</title>
    <script src="./promise.js"></script>
</head>
<body>
    <script>
        //实例化对象
        let p = new Promise((resolve, reject) => {
            setTimeout(() => {
                // resolve('OK');
                reject("Error");
            }, 1000)
        });

        //执行 then 方法
        const res = p.then(value => {
            // return 'oh Yeah';
            throw 'error';
        }, reason=>{
            // console.warn(reason);
            throw 'error';
        });

        console.log(res);
    </script>
</body>
</html>

promise.js:

//声明构造函数
function Promise(executor){
    //添加属性
    this.PromiseState = 'pending';
    this.PromiseResult = null;
    //声明属性
    this.callbacks = [];
    //保存实例对象的 this 的值
    const self = this;// self _this that
    //resolve 函数
    function resolve(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'fulfilled';// resolved
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
        //调用成功的回调函数
        self.callbacks.forEach(item => {
            item.onResolved(data);
        });
    }
    //reject 函数
    function reject(data){
        //判断状态
        if(self.PromiseState !== 'pending') return;
        //1. 修改对象的状态 (promiseState)
        self.PromiseState = 'rejected';// 
        //2. 设置对象结果值 (promiseResult)
        self.PromiseResult = data;
        //执行失败的回调
        self.callbacks.forEach(item => {
            item.onRejected(data);
        });
    }
    try{
        //同步调用『执行器函数』
        executor(resolve, reject);
    }catch(e){
        //修改 promise 对象状态为『失败』
        reject(e);
    }
}

//添加 then 方法
Promise.prototype.then = function(onResolved, onRejected){
    const self = this;
    return new Promise((resolve, reject) => {
        //调用回调函数  PromiseState
        if(this.PromiseState === 'fulfilled'){
            try{
                //获取回调函数的执行结果
                let result = onResolved(this.PromiseResult);
                //判断
                if(result instanceof Promise){
                    //如果是 Promise 类型的对象
                    result.then(v => {
                        resolve(v);
                    }, r=>{
                        reject(r);
                    })
                }else{
                    //结果的对象状态为『成功』
                    resolve(result);
                }
            }catch(e){
                reject(e);
            }
        }
        if(this.PromiseState === 'rejected'){
            onRejected(this.PromiseResult);
        }
        //判断 pending 状态
        if(this.PromiseState === 'pending'){
            //保存回调函数
            this.callbacks.push({
                onResolved: function(){
                    try{
                        //执行成功回调函数
                        let result = onResolved(self.PromiseResult);
                        //判断
                        if(result instanceof Promise){
                            result.then(v => {
                                resolve(v);
                            }, r=>{
                                reject(r);
                            })
                        }else{
                            resolve(result);
                        }
                    }catch(e){
                        reject(e);
                    }
                },
                onRejected: function(){
                    try{
                        //执行成功回调函数
                        let result = onRejected(self.PromiseResult);
                        //判断
                        if(result instanceof Promise){
                            result.then(v => {
                                resolve(v);
                            }, r=>{
                                reject(r);
                            })
                        }else{
                            resolve(result);
                        }
                    }catch(e){
                        reject(e);
                    }
                }
            });
        }
    })
}

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原文地址：https://www.cnblogs.com/juham/p/14906494.html