'use strict';
// ajax函數(shù)將返回Promise對(duì)象:
function ajax(method, url, data) {
    var request = new XMLHttpRequest();
    return new Promise(function (resolve, reject) {
        request.onreadystatechange = function () {
            if (request.readyState === 4) {
                if (request.status === 200) {
                    resolve(request.responseText);
                } else {
                    reject(request.status);
                }
            }
        };
        request.open(method, url);
        request.send(data);
    });
}

var log = document.getElementById('test-promise-ajax-result');
var p = ajax('GET', '/api/categories');
p.then(function (text) { // 如果AJAX成功，獲得響應(yīng)內(nèi)容
    log.innerText = text;
}).catch(function (status) { // 如果AJAX失敗，獲得響應(yīng)代碼
    log.innerText = 'ERROR: ' + status;
});

實(shí)現(xiàn)原理

//例1
function getUserId() {
    return new Promise(function(resolve) {
        //異步請(qǐng)求
        http.get(url, function(results) {
            resolve(results.id)
        })
    })
}

getUserId().then(function(id) {
    //一些處理
})

function Promise(fn) {
    var value = null,
        callbacks = [];  //callbacks為數(shù)組，因?yàn)榭赡芡瑫r(shí)有很多個(gè)回調(diào)

    this.then = function (onFulfilled) {
        callbacks.push(onFulfilled);
    };

    function resolve(value) {
        callbacks.forEach(function (callback) {
            callback(value);
        });
    }

    fn(resolve);
}

Promise.all()

var p1 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 500, 'P1');
});
var p2 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 600, 'P2');
});
// 同時(shí)執(zhí)行p1和p2，并在它們都完成后執(zhí)行then:
Promise.all([p1, p2]).then(function (results) {
    console.log(results); // 獲得一個(gè)Array: ['P1', 'P2']
});

Promise.race()

var p1 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 500, 'P1');
});
var p2 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 600, 'P2');
});
Promise.race([p1, p2]).then(function (result) {
    console.log(result); // 'P1'
});
//由于p1執(zhí)行較快，Promise的then()將獲得結(jié)果'P1'。p2仍在繼續(xù)執(zhí)行，但執(zhí)行結(jié)果將被丟棄