node.js实现回调的方法示例

向回调函数传递额外的参数

在调用函数中，使用匿名函数中实现需传递的参数，再次匿名函数内调用回调函数。

var events = require("events"); function CarShow() { events.EventEmitter.call(this); this.seeCar = function (make) { this.emit('sawCar', make); } } CarShow.prototype.__proto__ = events.EventEmitter.prototype; var show = new CarShow(); function logCar(make) { console.log("Saw a "+make); } function logColorCar(make, color) { console.log("Saw a %s %s ", color, make); } show.on("sawCar", logCar); show.on("sawCar", function (make) { var colors = ["red", "blue", "black", "pink", "green"]; var color = colors[Math.floor(Math.random()*3)]; logColorCar(make, color); }); show.seeCar("Ferrari"); show.seeCar("Porsche"); show.seeCar("Bugatti");

在回调中实现闭包

如果某个回调函数需要访问父函数的作用域的变量，就需要使用闭包，在函数块内部封装一个异步调用，并传入所需要的变量。

function logCar(logMsg, callback) { process.nextTick(function () { callback(logMsg); }); } var cars = ["猎豹", "捷达", "朗逸"]; for(var idx in cars){ var msg = "Saw a "+cars[idx]; logCar(msg, function () { console.log("Normal Callback "+ msg); }); } for(var idx in cars){ var msg = "Saw a "+cars[idx]; (function (msg) { logCar(msg, function () { console.log("Closure Callback "+ msg); }) })(msg); } //Normal Callback Saw a 朗逸 //Normal Callback Saw a 朗逸 //Normal Callback Saw a 朗逸 //Closure Callback Saw a 猎豹 //Closure Callback Saw a 捷达 //Closure Callback Saw a 朗逸

链式回调

使用异步函数时，如果两个函数都在事件队列上，则无法保证它们的运行顺序。解决方法是让来自异步函数的回调再次调用该函数，直到没有更多的工作要做，以执行链式回调。

function logCar(car, callback) { console.log("Saw a %$", car); if(cars.length){ process.nextTick(function () { callback(); }); } } function logCars(cars) { var car = cars.pop(); logCar(car, function () { logCars(cars); }); } var cars = ["猎豹", "捷达", "朗逸"]; logCars(cars);