手写一个promise

[wuxianqiang]

先了解一下Promise，Promise是通过构造函数的方式来定义，Promise有三种状态，Promise支持then方法，那么知道了这些，就简单的实现一下。

function Promise(executer) {
    // 默认的状态
    this.status = "pending";
    // 成功时的参数
    this.value = undefined;
    // 失败时的参数
    this.reason = undefined;

    let resolve = value => {
        // 用箭头函数保留this指向
        if (this.status === "pending") {
            this.status = "resolved"
            this.value = value;
        }
    }
    let reject = reason => {
        if (this.status === "pending") {
            this.status = "rejected";
            this.reason = reason;
        }
    }

    try {
        // 代码没错时执行
        executer(resolve, reject);
    } catch (error) {
        // 代码有错时执行
        reject(error);
    }
}
Promise.prototype.then = function (onFulfilled, onRejected) {
    if (this.status === "resolved") {
        // 成功时的回调
        onFulfilled(this.value)
    }
    if (this.status === "rejected") {
        // 失败时的回调
        onRejected(this.reason);
    }
}

下面比较完整的写法

function Promise(executer) {
    this.status = "pending";
    this.value = undefined;
    this.reason = undefined;
    this.resolveCallbacks = [];
    this.rejectCallbacks = [];
    let resolve = (value) => {
        if (this.status == "pending") {
            this.value = value;
            this.status = "resolved";
            this.resolveCallbacks.forEach(item => item(this.value))
        }
    };
    let reject = (reason) => {
        if (this.status == "pending") {
            this.reason = reason;
            this.status = "rejected";
            this.rejectCallbacks.forEach(item => item(this.reason))
        }
    };
    try {
        executer(resolve, reject)
    } catch (e) {
        reject(e)
    }
}
Promise.prototype.then = function (onFulfilled, onREjected) {
    if (this.status == "resolved") {
        return new Promise((resolve, reject) => {
            let val = onFulfilled(this.value);
            if (val instanceof Promise) {
                val.then(resolve, reject)
            } else {
                resolve(val);
            }
        })
    }
    if (this.status == "rejected") {
        return new Promise((resolve, reject) => {
            let val = onREjected(this.value);
            if (val instanceof Promise) {
                val.then(resolve, reject)
            } else {
                resolve(val);
            }
        })
    }
    if (this.status == "pending") {
        return new Promise((resolve, reject) => {
            this.resolveCallbacks.push(() => {
                let val = onFulfilled(this.value);
                if (val instanceof Promise) {
                    val.then(resolve, reject)
                } else {
                    resolve(val);
                }
            });
            this.rejectCallbacks.push(() => {
                let val = onREjected(this.value);
                if (val instanceof Promise) {
                    val.then(resolve, reject)
                } else {
                    resolve(val);
                }
            });
        })
    }
};
module.exports = Promise;