回调内部变量的范围

Question

var net = require('net');
net.createServer(function(socket){
    console.log('CONNECTED: ' + socket.remoteAddress +':'+ socket.remotePort);
    var id = '123';
    socket.on('data',function(data){
        console.log(id);
        setTimeout(function(){
            console.log(id);
        },5000);
    });
}).listen(10000,'');

In this code it should print 123 and undefined right? but it prints 123 and 123. As per my understanding setTimeout executes after sometime, at that point of time it cannot access id variable. I am really confused. why is this happening and where am i wrong.

Answer 1

The callbacks for the socket.on and for the setTimeout are both inside the scope that defines the id var. Since neither callback defines its own local id they both print 123.

Answer 2

In this code it should print 123 and undefined right? but it prints 123 and 123.

That is because setTimeout function doesn't have local id variable.

Following will print the output you are looking for

 setTimeout(function(){  
        var id;
        console.log(id);
    },5000);

This is because even though setTimeout's function also has its own scope, it will use the scope of outer function if the variable is not defined in the inner function.

Answer 3

Note: this polyfill only run browser. If you use this code in nodejs, you change somewhere..

/*  setTimeout argument supports    */
if (document.all && !window.setTimeout.isPolyfill) {
    var __nativeST__ = window.setTimeout;
    window.setTimeout = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) {
        var aArgs = Array.prototype.slice.call(arguments, 2);
        return __nativeST__(vCallback instanceof Function ? function () {
            vCallback.apply(null, aArgs);
        } : vCallback, nDelay);
    };
    window.setTimeout.isPolyfill = true;
}

if (document.all && !window.setInterval.isPolyfill) {
    var __nativeSI__ = window.setInterval;
    window.setInterval = function (vCallback, nDelay /*, argumentToPass1, argumentToPass2, etc. */) {
        var aArgs = Array.prototype.slice.call(arguments, 2);
        return __nativeSI__(vCallback instanceof Function ? function () {
            vCallback.apply(null, aArgs);
        } : vCallback, nDelay);
    };
    window.setInterval.isPolyfill = true;
}

Example:

setTimeout(function(id){
            console.log(id);
        },5000,"hello");

//5 second then //hello;

Detail info this

Changed Code:

var net = require('net');
net.createServer(function(socket){
    console.log('CONNECTED: ' + socket.remoteAddress +':'+ socket.remotePort);
    var id = '123';
    socket.on('data',function(data){
        console.log(id);
        setTimeout(function(id){
            console.log(id);
        },5000, id);
    });
}).listen(10000,'');