Why does this variable have a value before I do "function MyVar() {}"?

Question

I'm trying to understand why this code behaves the way it does:

test.js

var User;

console.dir(User);

function User(name) { // User = function(name) { ... seems not to be the same?
    this.name = name;
}

node test.js

[Function: User]

Why does User have a value before the function() statement?

possible duplicate of [var functionName = function() {} vs function functionName() {}](http://stackoverflow.com/questions/336859/var-functionname-function-vs-function-functionname) — Barmar, Sep 21 '13 at 00:34
Function declaration statements are treated as if they appear at the very top of the function body, regardless of where they appear in the code. — Pointy, Sep 21 '13 at 00:34
Named function definitions are treated differently from function expressions; similarly to `var` statements, they don't take effect at the point they appear in the source code. I'm not sure of the details, and I don't have a standard quote handy, so this is a comment. — user2357112, Sep 21 '13 at 00:35

score 5 · Accepted Answer · answered Sep 21 '13 at 00:34

var and function statements are automatically hoisted to the top of the script!

This allows functions to be used before they are defined, like this:

dothis();
function dothis() { alert('wow'); }

edit function statements like this are not affected:

myfunc(); // won't work
var myfunc = function() { alert('nope'); }

score 2 · Answer 2 · edited May 23 '17 at 12:05

2

That function is defined at parse time.

edited May 23 '17 at 12:05

Community

answered Sep 21 '13 at 00:33

Daniel A. White

2 Answers2