12

Here is my code for loop

var username = ['Sam', 'Adarsh', 'Rohit', 'Rajat'];
for(var i in username){
  console.log(username[i]);
}

it's outputing the same as needed, But I am not sure why Let declaration needed. I understand the concept of VAR and LET but not sure in which cases var create problem in for loops ?

Any body please help me to understant the concept. I am new noob and trying to figure out :)

Thanks for your help.

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5
  • The problem with that code isn't with var, it's with the fact that you're using a for..in loop to iterate over an array, which is a bad idea. Commented Nov 10, 2016 at 2:31
  • I just wanna know let and var role in any for loop ... Can you give me a example ? Commented Nov 10, 2016 at 2:33
  • 2
    Use neither, and go for Array.forEach Commented Nov 10, 2016 at 2:35
  • @RajatSharma let create the block scope in for(in) {}, so outside the for(in) {} loop, you can't get the value of i Commented Nov 10, 2016 at 2:35
  • But use for .. in loop to array is bad usage. Commented Nov 10, 2016 at 2:36

1 Answer 1

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19

When you use var:

var username = ['Sam', 'Adarsh', 'Rohit', 'Rajat'];
for(var i in username){
  console.log(username[i]);
}
i // 3

When you use let

var username = ['Sam', 'Adarsh', 'Rohit', 'Rajat'];
for(let i in username){
  console.log(username[i]);
}
i // Uncaught ReferenceError: i is not defined

let in ES6 will create block scope to function scope

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12 Comments

let create the block scope in for(in) {}, so outside the for(in) {} loop, you can't get the value of i.
Actually I am studying about "Problem with var in for loops" ... with closures function ...which is over come by let -- Can you please help me about this topic .. Thanks in advance
No one seems to care but it is bothering me and worth mentioning: for...in should not be used for collections, it is meant only for object property traversal and does not guarantee the traversal order.
@JesseRezaKhorasanee for in always need to be used with hasOwnProperty. Your question can see this: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…
@tomioion @DaveEveritt it's funny, the value of i on this Stack Overflow page is undefined for some reason. So if you are testing in the inspector on this page, you will not get the reference error. Swap i out for another variable name (a) and you will get the error.
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