Reversed For Loop - First iteration undefined

Question

Can someone explain why the first iteration of a reverse for-loop logs undefined?

var arr = [1, 2, 3, 4, 5, 6, 7, 8]

for (var i = arr.length; i > -1; i--) {
  console.log(arr[i]);
}

just for the sake of adding something: `arr.reverse().map( e => console.log(e) );` using lambda and the reverse prototype, which is supported since ecmascript's first edition. https://jsfiddle.net/uoe33kk6/ — briosheje, Dec 21 '16 at 16:19

Denys Séguret · Accepted Answer · 2016-12-21T16:21:34.473

3

At first iteration, i is arr.length.

There's no element at index arr.length. The last element of a zero-based array is at index arr.length-1.

The usual way to iterate in reverse is

for (let i=arr.length; i-- > 0; ) {

(note that the condition decrements the counter and is executed before the content of the loop)

This can can also be simplified in JavaScript into

for (let i=arr.length; i--;) {

edited Dec 21 '16 at 16:21

answered Dec 21 '16 at 16:05

Denys Séguret

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http://stackoverflow.com/questions/1642028/what-is-the-name-of-the-operator-in-c @DenysSeguret – roberrrt-s Dec 21 '16 at 16:08

Nina Scholz · Answer 2 · 2016-12-21T16:13:29.540

2

Why not use a while loop with a postfix decrement -- instead of a justified for loop?

var arr = [1, 2, 3, 4, 5, 6, 7, 8],
    i = arr.length;
  
while (i--) {
    console.log(arr[i]);
}

edited Dec 21 '16 at 16:13

answered Dec 21 '16 at 16:07

Nina Scholz

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`while(i--)` What? – roberrrt-s Dec 21 '16 at 16:08
1

it just iterates to zero. – Nina Scholz Dec 21 '16 at 16:09
There's no real reason to downvote here. But note that this kind of loop doesn't make it easy to limit the scope of the counter to the loop (as is now possible with `let`). – Denys Séguret Dec 21 '16 at 16:10
Ah, shorthand, how silly of mine, carry on :). (I wasn't the guy that downvoted). – roberrrt-s Dec 21 '16 at 16:10
While this code is perfectly valid, it's got a very high wtf factor. It relies on the fact that 0 evaluates to false, confounding semantics with implementation peculiarities. – Joel Cornett Dec 21 '16 at 19:59

score 0 · Answer 3 · answered Dec 21 '16 at 16:09

this happens because the length of the array from arr.length returns 8, and since the arrays in JavaScript are Zero-indexed (counting starts from 0), the last element in an array is at the index arr.length - 1

so to avoid the undefined, do this:

$(function(){

  var arr = [1,2,3,4,5,6,7,8]

  for (var i = arr.length - 1; i > -1; i--) {
    console.log(arr[i]);
  }

})

Reversed For Loop - First iteration undefined

3 Answers3

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