关于promise

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promise实现

以下为手写promise，实现的比较粗略
promise作为构造函数实现

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<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>promise的实现</title>
</head>
<body>
<script>

    // let a = new Promise((resolve, reject) => {
    //     console.log(123);
    //     setTimeout(function () {
    //         resolve('hhh')
    //     }, 0)
    //
    // });
    // a.then(res => {
    //     console.log(1233);
    //     console.log(res)
    // }, rej => {
    //     console.log(rej)
    // })
    // Promise.all([1,2,3]).then(val => {
    //     console.log(val)
    // })
    // Promise.race([1,2,3]).then(val => {
    //     console.log(val)
    // })

    console.log('----------------------');
    function promise(executor) {
        let self = this;
        self.status = 'pending';
        self.data = null;
        self.callbacks = {};

        function resolve(data) {
            if (self.status !== 'pending') return;
            self.status = 'resolved';
            self.data = data;
            if (self.callbacks.onResolved) {
                self.callbacks.onResolved(data)
            }
        }
        function reject(data) {
            if (self.status !== 'pending') return;
            self.status = 'rejected';
            self.data = data;
            if (self.callbacks.onRejected) {
                self.callbacks.onRejected(data)
            }
        }
        executor(resolve, reject)
    }
    promise.prototype.then = function(onResolved, onRejected) {
        let self = this;
        onResolved = typeof onResolved === 'function' ? onResolved : value => value;
        onRejected = typeof onRejected === 'function' ? onRejected : reason => {throw reason};
        if (self.status === 'resolved') {
            queueMicrotask(() => {
                handle(onResolved)
            })
        } else if (self.status === 'rejected') {
            queueMicrotask(() => {
                handle(onRejected)
            })
        } else if (self.status === 'pending') {
            self.callbacks = {
                onResolved: () => {
                    queueMicrotask(() => {
                        handle(onResolved)
                    })
                },
                onRejected: () => {
                    queueMicrotask(() => {
                        handle(onRejected)
                    })
                }
            }
        }
        function handle(callback) {
            return new promise((resolve, reject) => {
                resolve(callback(self.data))
            })
        }
    };
    promise.resolve = function(val) {
        return new promise(resolve => {
            resolve(val)
        })
    }

    promise.race = function (args) {
        return new promise((resolve, reject) => {
            args.forEach(item => {
                promise.resolve(item).then(val => {
                    resolve(val)
                }, val => {
                    reject(val)
                })
            })
        })

    };
    promise.all = function (args) {
        let results = [];
        let num = 0;
        let total = args.length
        return new promise((resolve, reject) => {
            args.forEach((item, index) => {
                promise.resolve(item).then(val => {
                    results[index] = val;
                    num++;
                    if (num === total) {
                        resolve(results)
                    }
                }, val => {
                    reject(val)
                })
            })
        })
    };

    let b = new promise((resolve, reject) => {
        console.log(123)
        setTimeout(function () {
            resolve('hhh')
        }, 0)
    });
    b.then(res => {
        console.log(222);
        console.log(res)
    });

    promise.race(['r','a','c', 'e']).then(val => {
        console.log(val)
    })

    promise.all(['a','l','l']).then(val => {
        console.log(val)
    })
</script>
</body>
</html>

参考实现方式

promise A+ 规范

promise面试题