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?

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Answer 1

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.

Javascript for loop index variables become part of global scope?

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