Javascript for loop index variables become part of global scope?

Question

Perhaps I'm not aware of how for loop index variables get scoped, but I was very surprised when one of my loops didn't complete, seemingly because a function called from within a loop contained an i for its for loop index as well.

Here's a little script I put together to demonstrate this behavior:

var loopOne = function(test) { for(i = 0; i < test.length; i++) console.log(getMask(test)); }; var getMask = function(pass) { var s = ""; for (i = 0; i < pass.length; i++) { s = s + "*"; } return s; }; loopOne('hello');

If I run this in Chrome and look at the console log, I should see ***** five times. However, I only see it once. Upon further inspection, if I type i in the Chrome javascript console, it will output 6 ( = 'hello'.length + 1). This makes me think that i has become a part of the global scope and is not limited to the scope of the for loop for which it was needed.

Is this correct? If so, what's a better practice for defining the index variable of a for loop in javascript?

deceze · Accepted Answer · 2011-05-18 02:03:18Z

12

In Javascript, variables are scoped with the var keyword. When declaring variables with var, the variable is scoped to the current function. When assigning to a variable without using the var keyword, it is assumed you're talking about an already defined variable in the same or a higher scope. If none is found, the variable is created in the highest scope.

Bottom line: declare all your variables using var.

edited May 18, 2011 at 2:03

answered May 18, 2011 at 1:36

deceze♦

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5 Comments

Ben McCormack Over a year ago

"If none is found, the variable is created in the highest scope." That's just evil. Thanks for the explanation about scoping using var. Now that you mention it, I should have remembered that.

deceze Over a year ago

@Ben Indeed, it's one of the many pitfalls of Javascript. AFAIK this is being/was rectified in the latest ECMAScript spec.

alex Over a year ago

I think only if you add "use strict".

RobG Over a year ago

Rember that javascript was intended as a simple, easy to use language. It also needs to support closures so assigning to undeclared idnetifiers is reasonabley common. So undeclared variables are added to the global object when they are first assigned a value, not before.

deceze Over a year ago

@Rob Added that as clarification.

Jordão · Accepted Answer · 2022-03-17 15:53:17Z

You should declare your loop index variable with let:

for (let i = 0; i < test.length; i++) ...

That will declare and set the proper scope for the variable.

alex · Accepted Answer · 2011-05-18 01:37:07Z

When you declare your variables with var, you scope them to the current execution context.

When you don't, they become properties of the global object (window in a browser).

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