Javascript for loop index variables becoming part of global scope?

Question

Perhaps I don’t know how the environment index variables for fall into the scope, but I was very surprised when one of my loops did not end, apparently because the function called from the loop contained i for its for loop index.

Here is a little script 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 see it only once. Upon further inspection, if I type i in the javascript Chrome console, it will output 6 ( = 'hello'.length + 1 ). This makes me think that i become part of the global scope and is not limited to the scope of the for loop for which it was necessary.

It is right? If so, what is better for defining the for loop index variable in javascript?

Answer 1

In Javascript, variables are covered by the var keyword. When declaring variables with var variable is bound to the current function. Assigning a variable without using the var keyword assumes that you are talking about an already defined variable in the same or higher area. If none are found, the variable is created in the highest region.

Bottom line: declare all your variables with var .

Answer 2

You should always declare your variables with var :

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

Answer 3

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

If you do not, they will become properties of the global object ( window in the browser).