JavaScript variable life time

Question

I am learning node.js and JavaScript. Following is the code. I found the var http is outside the function start2. I am wondering why it works? Since we only exports function start2, right?

Is this some concept about closure? (I've tried put the var http inside the start2. It works for sure.)

var http = require('http');

function start2(){
    function onRequest(request,response){
        console.log("Request recieved");
        response.writeHead(200,{"Content-Type":"text/html"});
        response.write("<h1>Hello world</h1>");
        response.end(); 
    }
    http.createServer(onRequest).listen(8888);
    console.log("Server has started.");
}
exports.start=start2;

Answer 1

You could put var http = require('http'); inside your function but most often that's not how people code it. People place it at the top like you have it. Why? This is a call to the module system to load module http. Most often, you want to do this once and make it available for your entire file. If you are doing to use this module multiple times in your file you don't want to call require again and again.

If a module it is rarely used and perhaps expensive to load, then it may make sense to have the require call inside a function, instead of paying the cost of loading it each and every time:

function calledUnderExceptionalCircumstances() {
    var expensive = require('expensive');
    expensive.foo();
}