手撸promise【二、Promise源码】【代码详细注释/测试案例完整】

> 手撸Promise即完成promise的主要功能>- 声明构造函数>-resolve与reject>- throw抛出异常改变状态>-Promise的对象状态只能修改一次>-then方法执行回调>- 同步任务回调的执行>- 异步任务回调的执行>- 指定多个回调的实现>- 同步修改状态then方法结果返回>-异步修改状态then方法结果返回>- Promise的API>- 构造函数：

翼遥bingo

266人浏览 · 2022-06-22 17:01:51

翼遥bingo · 2022-06-22 17:01:51 发布

手撸Promise即完成promise的主要功能

声明构造函数
resolve与reject
throw抛出异常改变状态
Promise的对象状态只能修改一次

then方法执行回调
同步任务回调的执行
异步任务回调的执行
指定多个回调的实现
同步修改状态then方法结果返回
异步修改状态then方法结果返回

Promise的API
构造函数： then，catch
函数对象： resolve，reject，all，race方法

一、 index.html

测试手撸的promise.js

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="./promise.js"></script>
</head>

<body>
    <script>
        let p = new Promise((resolve, reject) => {
            resolve('OK');
            // reject("error");
            // throw "err";

            setTimeout(() => {
                // resolve('OK');
                // reject('err');
                // ............................抛出错误该咋办。。。。。。。。。。。。。。
                // throw "err";
            }, 1000);
        });
        console.log(p);
        // p.then(value => {
        //     console.log(value);

        // }, reason => {
        //     console.warn(reason);
        // })


        const res = p.then(value => {
            // alert(value);
            return 'hello';
            // return new Promise((resolve, reject) => {
            //     resolve("success");
            //     // reject("error");
            // })
            // throw "fail";
        }, reason => {
            alert(reason);
        })

        console.log(res);

        let res2 = p.catch(reason => {
            console.warn(reason);
        })
        console.log(res2);

        // 异常穿透，值传递
        p.then(value => {
            // console.log(111);
            // throw '失败了'
        }).then(value => {
            console.log(222);
        }).then(value => {
            console.log(333);
        }).catch(reason => {
            console.warn(reason);
        })


        // resolve方法测试用例
        const p1 = Promise.resolve('OK');
        const p2 = Promise.resolve(new Promise((resolve, reject) => {
            reject("errpr");
        }));
        const p3 = Promise.resolve(Promise.resolve('Oh Yeah!'));

        console.log(p1);

        // reject测试用例
        const r1 = Promise.reject('Error');
        const r2 = Promise.reject(new Promise((resolve, reject) => {
            resolve('ok')
        }))
        console.log(r1);
        console.log(r2);

        // 调用all方法
        let a1 = new Promise((resolve, reject) => {
            resolve('OK');
        })
        let a2 = Promise.resolve('Success');
        let a3 = Promise.resolve('Oh Yeah');
        let result3 = Promise.all([a1, a2, a3]);
        console.log(result3);

        // 调用race方法
        let ra1 = new Promise((resolve, reject) => {
            setTimeout(() => {
                resolve('OK');
            })
        })
        let ra2 = Promise.reject('Successs');
        let ra3 = Promise.resolve('Oh Yeah!')
        let result4 = Promise.race([ra1, ra2, ra3]);
        console.log(result4);

        // then方法回调的异步执行
        let then1 = new Promise((resolve, reject) => {
            resolve('OK');
            console.log(111);
        })
        then1.then(value => {
            console.log(222);
        })
        console.log(333);
    </script>
</body>

</html>

二、 promise.js

function Promise(executor) {
    // 添加属性
    this.PromiseState = 'pending';
    this.PromiseResult = null;
    // 生命属性
    this.callbacks = [];
    const self = this;

    // resolve函数
    function resolve(data) {
        // Promise对象状态只能修改一次
        if (self.PromiseState !== 'pending') return;
        // 1. 修改对象的状态（promiseState）
        self.PromiseState = 'fulfilled';
        // 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);
        // }
        setTimeout(() => {
            self.callbacks.forEach(item => {
                item.onResolved(item);
            })
        })
    }
    // reject函数
    function reject(data) {
        if (self.PromiseState !== 'pending') return;
        self.PromiseState = 'rejected';
        self.PromiseResult = data;
        // 调用失败的回调函数
        // if (self.callback.onRejected) {
        //     self.callback.onRejected(data);
        // }
        setTimeout(() => {
            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;
    // 判断回调函数参数->异常穿透
    if (typeof onRejected !== 'function') {
        onRejected = reason => {
            throw reason;
        }
    }
    if (typeof onResolved !== 'function') {
        onResolved = value => value;
        // value => { return value; }
    }
    return new Promise((resolve, reject) => {
        // 封装函数
        function callback(type) {
            try {
                // 获取回调函数的执行结果
                let result = type(self.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 === 'fulfilled') {
            setTimeout(() => {
                callback(onResolved);
            })
        }
        if (this.PromiseState === 'rejected') {
            setTimeout(() => {
                callback(onRejected);
            })
        }
        // 判断pending状态
        if (this.PromiseState === 'pending') {
            // 保存回调函数
            this.callbacks.push({
                onResolved: function() {
                    callback(onResolved);
                },
                onRejected: function() {
                    callback(onRejected);
                }
            })
        }
    })

}

// 添加 catch方法
Promise.prototype.catch = function(onRejected) {
    return this.then(undefined, onRejected);
}

// 添加resolve方法
Promise.resolve = function(value) {
    // 返回promise对象
    return new Promise((resolve, reject) => {
        if (value instanceof Promise) {
            value.then(v => {
                resolve(v);
            }, r => {
                reject(r);
            })
        } else {
            resolve(value);
        }
    })
}

// 添加reject方法
Promise.reject = function(reason) {
    return new Promise((resolve, reject) => {
        reject(reason);
    })
}

// 添加all方法
Promise.all = function(promises) {
    // 返回结果为promise对象
    return new Promise((resolve, reject) => {
        // 声明变量
        let count = 0;
        let arr = [];
        // 遍历
        for (let i = 0; i < promises.length; i++) {
            promises[i].then(v => {
                count++;
                arr[i] = v;
                if (count === promises.length) {
                    // 修改状态
                    resolve(arr);
                }
            }), r => {
                reject(r);
            }
        }
    })
}

// 添加 race 方法
Promise.race = function(promises) {
    return new Promise((resolve, reject) => {
        for (let i = 0; i < promises.length; i++) {
            promises[i].then(v => {
                // 修改返回对象的状态为[成功]
                resolve(v);
            }, r => {
                reject(r);
            })
        }
    })
}