for/in loop decrement javascript

Question

I am trying to decrement a for/in loop in js.

I have found lots of instruction on decrementing a for-loop, but had no luck finding a statement that decrements a for/in loop.

var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"];
for( var index in classRegister ) { 
 console.log(classRegister[index]);
}

This is my incrementing for/in loop statement that I want to modify to decrement.

Yea, it IS a bad idea to do this. Alas I do see people who DO do this. — Mark Schultheiss, Apr 13 '19 at 07:10

score 4 · Accepted Answer · answered Apr 13 '19 at 06:58

4

Use basic for loop & avoid for..in to iterate an array

var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"];
for (let i = classRegister.length - 1; i >= 0; i--) {
  console.log(classRegister[i]);
}

answered Apr 13 '19 at 06:58

brk

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note, due to the falsy you can just use `i` instead of `i >= 0` – Mark Schultheiss Apr 13 '19 at 07:02
It's not what I was hoping for. But it'll work. Thank you! – Bobbi Lusic Apr 13 '19 at 07:04
Yet that wasn't the question. – Ralph Ritoch Apr 13 '19 at 07:05

score 2 · Answer 2 · answered Apr 13 '19 at 07:04

2

If you want to use for/in you can, you simply need to subtract your index from the last index value.

var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"];
for( var index in classRegister ) { 
 console.log(classRegister[classRegister.length - 1 - index]);
}

answered Apr 13 '19 at 07:04

Ralph Ritoch

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Check this https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in#Array_iteration_and_for...in – brk Apr 13 '19 at 07:07
The question was simply HOW to do it in a for/in loop, not if there was a better way. – Ralph Ritoch Apr 13 '19 at 07:08
1

This does answer the question, but no-one is saying it is the right thing to do. :)k – Mark Schultheiss Apr 13 '19 at 07:08
Thank you Ralph. It looks like a solution that works around the limitation of for/in loops in js. Your answer as well as the other ones really help my understanding. – Bobbi Lusic Apr 13 '19 at 08:37

Code Maniac · Answer 3 · 2019-04-13T07:12:32.733

1

You should not use for..in for arrays instead use simple loop

var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"];
for(let index = classRegister.length-1; index >= 0; index-- ) { 
 console.log(classRegister[index]);
}

Or a while loop

var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"];
let index = classRegister.length-1;

while(index >= 0) { 
 console.log(classRegister[index]);
 index--;
}

There are hacky ways to do but you should never use them IMO

var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"];

let copy = [...classRegister].reverse()  // this reverse array in place

for( var index in copy ) { 
 console.log(copy[index]);
}

edited Apr 13 '19 at 07:12

answered Apr 13 '19 at 07:03

Code Maniac

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If you are going to do it this way you need to reverse a copy of the array, otherwise it destructs the array order. – Ralph Ritoch Apr 13 '19 at 07:11
@RalphRitoch yeah you're right, changed to incorporate this too – Code Maniac Apr 13 '19 at 07:13
`while(index)` is functionally the same as `while(index >= 0)` – Mark Schultheiss Apr 13 '19 at 07:17
@MarkSchultheiss no it is not, `index>=0` will not take you out of while loop at `index = 0` on the other hand `while(index)` will take you out of while loop at `index = 0` – Code Maniac Apr 13 '19 at 07:19
@MarkSchultheiss `while(index)` and `while(index > 0 )` will be same in functionality not `while(index >= 0)` – Code Maniac Apr 13 '19 at 07:22
1

Yea, you are right, my brain is asleep on that BUT consider `while(index--)` and avoid that `index--;` in the while... I think my brain is 'falsey" tonight also :) – Mark Schultheiss Apr 13 '19 at 07:23
@MarkSchultheiss yes that can be done, but i prefer this for sake of readality so you can easily see `oh this while loop run untill the value is greater than 0`, this line is `I think my brain is 'falsey" tonight also :)` awesome pun intended – Code Maniac Apr 13 '19 at 07:25
In older browsers (way older) the negative while is faster than a for loop, it is able to short cycle, in newer ones that has gone away for the most part.. But I digress here – Mark Schultheiss Apr 13 '19 at 07:27
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/191775/discussion-between-code-maniac-and-mark-schultheiss). – Code Maniac Apr 13 '19 at 07:32
1

Thank you all. I have very interesting options to play with now. – Bobbi Lusic Apr 13 '19 at 08:42
1

@BobbiLusic yes you can try things to learn, but as well as have a habit to follow good patterns so your code becomes better,readable and maintainable – Code Maniac Apr 13 '19 at 08:49

for/in loop decrement javascript

3 Answers3