Javascript: Basic for loop is not working

Question

Is there any reason why the following would not work:

for (i=0;i < someArray.length;i++) {
 if (someArray[i].indexOf("something") !== -1) {
 //do something here
 }
}

The most basic "for" loop possible. But it doesn't work. On the first line (declaration of the loop, not inside the loop), I get "Uncaught reference error; i is not defined."

I have this page open in one Chrome tab, and another earlier version of the page open in another tab. In the other tab, this loop works just fine; in the first tab, this code throws an error.

EDIT - July 2 2015

The response about strict mode was helpful. After reading up a bit and going through the code I've got a handle on what's going on.

The confusing bit was that both versions of the code look like this, with some minor differences (requirejs module):

define(
 'viewModels/someViewModel',
 ['dependency1', 'dependency2', 'dependency3'],
 function(dep1, dep2, dep3) {
 "use strict";
 function SomeViewModel(arg1, arg2) {
 var self = this;
 self.initialize();
 self.removeRefinement = function(refinementString) {
 var refinementArray = refinementString.split("&");
 for (i=0;i < navigationArray.length;i++) { //<-- error
 }
 }
 }
 }
);

One version throws the reference error. One doesn't.

This is a large web application with many other pages and Javascript files. The only thing I could think of was that in one version of the code, maybe i had been inadvertently globally defined somewhere else in the app, where strict mode wasn't enabled. After running to the breakpoint and checking "window" I see that's exactly what's happening.

Thanks =D

• ARLCode: Wait, why can't you change it?

Answer 1

If you are in strict mode, you'll get the error Uncaught reference error; i is not defined. If you're not in strict mode, you won't get the error.

This will throw the error

'use strict'
var someArray = ['aaa', 'bbb', 'ccc'];
for (i=0;i < someArray.length;i++) {
 console.log(i)
 if (someArray[i].indexOf("something") !== -1) {
 //do something here
 }
}

This won't

var someArray = ['aaa', 'bbb', 'ccc'];
for (i=0;i < someArray.length;i++) {
 console.log(i)
 if (someArray[i].indexOf("something") !== -1) {
 //do something here
 }
}

Answer 2

when you declare a variable it must be declared like this var i = 0;

a for loop looks like this:

JavaScript

for(var i = 0; i == 10; i++)
{
}

• John Sterling: Not the correct answer. If you read the question carefully, the user explains that he cannot add the var declaration. Also, the answer is not complete and is only valid in strict mode.
• ARLCode: Whoops, yeah. I noticed it after I submitted the answer hence why I commented to the users post.
• pllee: Your first statement is false, global variables can be declared without var.