0

I've always seen express apps being initialised like:

var express = require('express')
var app = express()

But today I stumbled upon a example with new operator:

var Express = require('express')
var app = new Express()

Is there any difference?

Source: https://github.com/erikras/react-redux-universal-hot-example/blob/0d7c49318a8ed78bdef4022b9f0adc4bfb04bdba/src/server.js#L25

Pavel 'Strajk' Dolecek
  • 678
  • 7
  • 16
  • the second example is not correctly copied from the code you linked. it uses `import Express from 'express';` not `var Express = require('express')`. look up import vs require. – Claies Nov 18 '15 at 20:59
  • @Claies for others who happen on this, what difference is that? – TbWill4321 Nov 18 '15 at 21:16
  • I'm not an expert on script imports, but I know for sure they are entirely different. `import` is from ES6. I was merely trying to point out that your question is flawed because you posted the code incorrectly. – Claies Nov 18 '15 at 21:19
  • ES6 compilers (e.g. Babel) currently convert `import` statements into `require` calls. There's not a significant difference unless using the `as` statement or destructuring. – TbWill4321 Nov 18 '15 at 21:27

1 Answers1

0

I think there is a subtle difference, but one which doesn't make much (any?) difference in this case.

With Node.js's require statement what you end up with is the object that the module assigns to module.exports. This could be anything, even just a simple value, but it could also be a function.

Thus, suppose you have the following module in myModule.js:

function complexOperation() { 
   // blah blah 
   }
module.exports = complexOperation;

When you do:

var myModule = require("myModule.js");

You end up with myModule set to complexOperation, and you would call it just like any regular function. Straightforward. But note that it wouldn't necessarily make sense to do something like new complexOperation() here - all depends on what complexOperation is.

In the case of Express.js the object returned is itself a function, namely, createApplication (defined in this file: https://github.com/strongloop/express/blob/master/lib/express.js). So require('express') returns a function, which is assigned to express, and which is then run on the next line. So app ends up with the result of the execution of the function createApplication.

The second variation (ignoring the fact that, as one comment-er pointed out, you misquoted that source file) uses the new keyword. When you do:

var a = new Something();

Something should be a function - a constructor function. And what this statement does is:

  • create a new object
  • invokes Something() with this set to the new object
  • returns the result of Something()

(There is more going on, with the object's prototypes being setup. Full explanation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new).

So in the above case, the statement var app = new Express() creates a new object, invokes Express() with this set up to refer to the newly created object, and then returns the result.

In the case of Express.js, because the thing exported is itself a function, much the same result is achieved. A new object is created, createApplication is run with this setup accordingly, and then app gets the result of createApplication.