how is using a function before it was defined valid?

Question

Below is a snippet from node url module source.

var punycode = require('punycode');
var util = require('util');

exports.parse = urlParse;
exports.resolve = urlResolve;
exports.resolveObject = urlResolveObject;
exports.format = urlFormat;

exports.Url = Url;

function Url() {
  this.protocol = null;
  this.slashes = null;
  this.auth = null;
  this.host = null;
  this.port = null;
  this.hostname = null;
  this.hash = null;
  this.search = null;
  this.query = null;
  this.pathname = null;
  this.path = null;
  this.href = null;
}

As you can see, the 'Url' is used before function 'Url' was defined. As far as I know this is not valid javascript, but it works ok.

Can someone tell me why this is ok? And why node writers love this convention?

EDIT : Thanks. I had no understanding of 'function hoisting' because the former title was the wrong question, modified.

function declerations are always hoisted to the top of the scope in javascript, so it's perfectly valid. — adeneo, Jul 07 '14 at 16:20
http://designpepper.com/blog/drips/variable-and-function-hoisting ... if it was not valid, no one would write code like that. Since it works, it must be valid. — Felix Kling, Jul 07 '14 at 16:23

score 1 · Accepted Answer · answered Jul 07 '14 at 16:29

1

function like 'function a(){}' will be defined first even it's placed behind. function like 'var a = function(){}' will be defined as the normal variable define order. check this code:

alert(a);
function a(){}

alert(b);
var b = function(){}

http://jsfiddle.net/j4v7E/

answered Jul 07 '14 at 16:29

TomWan

319
2
9

What if I have `var a = function b(){}`? Do both rules apply now? – Felix Kling Jul 07 '14 at 16:37
@FelixKling NO, the b will be ignored here, it's as the same as `var a = function(){}` – TomWan Jul 08 '14 at 02:07

how is using a function before it was defined valid?

1 Answers1