Promise实现

function Promise(fn) { 
    var state = 'pending',
        value = '',
        callbacks = [];
 
    this.then = function(onfulfilled) {
        if(state === 'pending') {     // 判断状态，当执行过resolve，之后链式的then直接执行
            callbacks.push(onfulfilled);
            return this;
        }
        onfulfilled(value);
        return this;
    }

    function resolve(newValue) {
        value = newValue;
        state = 'fulfilled';
        callbacks.forEach(function(callback) {
            callback(value)
        })
    }

    fn(resolve)
}

上面代码存在一个问题，在then方法注册回调之前，resolve就已经执行了，为了保证注册回调在resolve之前执行，可以通过延时将resolve执行回调的方法放到执行队列队尾。

function resolve(newValue) {
    value = newValue;
    state = 'fulfilled';
    setTimeout(function() {
        callbacks.forEach(function(callback) {
            callback(value)
        })
    })
}

var hcx = new Promise(function(resolve) {
    resolve(‘my name is hcx!’)
}).then(function(result) {
    // do something
})

加入失败处理：

function Promised(fn) {
    var callbacks = [],
        value = '',
        state = 'pending';
    this.then = function(onFulfilled, onReject) {
        return new Promised(function(resolve, reject) {
            handle({
                onFulfilled: onFulfilled,
                onReject: onReject,
                resolve: resolve,
                reject: reject
            })
        })
    }
    function handle(callback) {
        if(state === 'pending') {
            callbacks.push(callback);
            return;
        }
        var cb = state === 'fulfilled'? callback.onFulfilled: callback.onRejected;
        if(!cb) {
            cb = state === 'fulfilled'? callback.resolve: callback.reject;
            cb(value);
            return;
        }
        try {
            callback.resolve(cb(value));
        } catch(err) {
             callback.reject(err);
        }
    }
    function resolve(data) {
        value = data;
        state = 'fulfilled';
        execute();
    }
    function reject(data) {
        value = data;
        state = 'rejected';
        execute();
    }
    function execute() {
        setTimeout(function() {
            for(var i = 0; i < callbacks.length; i++) {
                handle(callbacks[i]);
            } 
        });
    }
    fn(resolve, reject);
}