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JavaScript JavaScript Arrays Loop Through Arrays Using a 'for' Loop to Iterate Over an Array

Alex Hort-Francis
Alex Hort-Francis
17,074 Points
Posted by Alex Hort-Francis
Alex Hort-Francis
17,074 Points

Is 'let' compulsory when declaring 'i' in the for loop?

I've noticed that the following code will work in the browser (Chrome):

for (i=0; i<10; i++) {
  console.log(i);
}

The output is:

0
1
2
3
4
5
6
7
8
9

With that in mind, I am interested whether it is technically compulsory to use let to declare i in the for loop syntax.

Does this have something to do with 'strict mode' and avoiding potentially buggy code?

Thanks! :)

4 Answers

Jason Larson
Jason Larson
8,359 Points

You are correct. In JavaScript, by default you are not required to declare variables before using them, so the following statements are basically the same (other than scope):

i = 0;
let i = 0;
var i = 0;

The problems come in when strict mode is enabled or using other frameworks like React that require strict mode. Additionally, you can run into some bugs that are harder to troubleshoot when you don't declare the variable type because they will become global variables by default.

Alex Hort-Francis
Alex Hort-Francis
17,074 Points
Alex Hort-Francis
Alex Hort-Francis
17,074 Points

I'm back here again, I Googled my own question haha

I think the takeaway is just use let in the for loop, for best practice -- at least that's what I'll be doing from now on..

Alex Hort-Francis
Alex Hort-Francis
17,074 Points
Alex Hort-Francis
Alex Hort-Francis
17,074 Points

That is very interesting.

I suppose i in this case would be automatically initialised as a let? Or would it be a var?

It sounds like it would be best to always declare i properly in a for loop then (using let).

Otherwise, if i is being automatically created as a global variable, would that mean that any other javascript run on that page would be able to access it? So if another i is used in a separate script it could cause collision problems?

Thanks!

Jason Larson
Jason Larson
8,359 Points

i would be automatically declared as a var, since var is global. The let declaration is block-scoped, so if you declare it in a for loop, it will go out of scope once you exit the loop. Honestly, I'm not sure about collisions from another script, as it may depend on how the script is accessed, but certainly any other parts of the same script could read or write to the variable.

Alex Hort-Francis
Alex Hort-Francis
12,924 Points
Alex Hort-Francis
Alex Hort-Francis
12,924 Points

Update for 2023

According to our good friend GPT-4:

In JavaScript, it's not compulsory to use let when declaring a loop variable like i in a for loop, but it's a recommended best practice for various reasons. Here's a deeper dive into your question:

  1. Without let in Non-strict Mode: If you don't use let (or var) and you're not in strict mode, then the variable i will be implicitly declared in the global scope. This is problematic because it can lead to unexpected bugs or overwrite other variables with the same name in the global scope.
   for (i=0; i<10; i++) {
     console.log(i);
   }
   console.log(window.i);  // 10 (in browsers)
  1. Without let in Strict Mode: If you enable strict mode with 'use strict';, the above code will throw an error because strict mode doesn't allow implicit global variable declarations.
   'use strict';
   for (i=0; i<10; i++) {
     console.log(i);
   }
   // Uncaught ReferenceError: i is not defined
  1. Using let: When you use let to declare the loop variable, it ensures the variable only exists in the loop's block scope. This is the recommended approach because it avoids polluting the global scope or the surrounding function scope.
   for (let i=0; i<10; i++) {
     console.log(i);
   }
   console.log(typeof i);  // "undefined"

  1. Historical Context with var: Before let was introduced in ES6 (ES2015), the only option for variable declarations was var. Unlike let, var has function scope instead of block scope. This could lead to subtle bugs and was one of the reasons let (with its block scoping) was introduced.

  2. Strict Mode: Enabling strict mode ('use strict';) helps catch common coding mistakes. In the context of your question, strict mode will prevent implicit global declarations, thereby forcing you to declare your loop variable explicitly.

In summary, while you might be able to get away without using let in some contexts, it's a good habit to always declare loop variables with let to avoid potential issues and make your code more maintainable.