JS Array in a for loop

Question

This piece of code will print [1,2,3,4,5,6,7,8,9,10] on the console it means in every iteration arr.length change and this is reflected in the loop body also.

let arr = [1, 2, 3];
for (e of arr) {
  arr.push(arr[arr.length - 1] + 1);
  if (arr.length >= 10) break;
}
console.log(arr)

But here, The output will be [4,5,6] and that mean the shift() function is not considering the expansion of the array.

let arr = [1, 2, 3];
for (e of arr) {
  arr.push(arr[arr.length - 1] + 1);
  if (arr[arr.length - 1] >= 10) break;
  arr.shift();
}
console.log(arr)

My question is why? I expected [8,9,10] output from second code

I would suggest you to remove arr.shift() and use arr = arr.slice(-4,-1) instead — flyingfox
– flyingfox, Commented Nov 6, 2022 at 13:27
your array doesnt go beyond 3 elemetns in the second case. so in the first iteration its e=1 , second e=3, third e=5. can visualize with pythontutor.com/javascript.html if u want — cmgchess
– cmgchess, Commented Nov 6, 2022 at 13:31
If you console.log(arr) immediately after arr.shift(); you'll see what's happening. — Andy
– Andy, Commented Nov 6, 2022 at 13:34
The issue has been identified above, but as an aside, mutating an array as you iterate through it is generally a really bad idea. — Rory McCrossan
– Rory McCrossan, Commented Nov 6, 2022 at 13:34
Thank you guys I just understand I expected the for..loop work with value instead of index! — RainClick
– RainClick, Commented Nov 6, 2022 at 14:08

flyingfox · Accepted Answer · 2022-11-06 13:36:57Z

2

The reason is that you have add element at first,then you invoke shift() to remove the element,so the array size will not change and it will only iterate once

let arr = [1, 2, 3];
for (e of arr) {
  arr.push(arr[arr.length - 1] + 1); // add element
  if (arr[arr.length - 1] >= 10) break;
  arr.shift(); // remove element
}
console.log(arr)

In order to get your expected result,we need to make sure it can iterate more than once until got the expected result. So we can change for to while

let arr = [1, 2, 3];
while(arr.at(-1) < 10) {
  arr.push(arr.at(-1) + 1);
  arr.shift();
}
console.log(arr)

edited Nov 6, 2022 at 13:36

answered Nov 6, 2022 at 13:30

flyingfox

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1 Comment

flyingfox Over a year ago

@pilchard Glad to see you again and thanks for your advice

damonholden · Accepted Answer · 2022-11-06 13:42:33Z

1

The second for ...of loop is working correctly:

  let arr = [1, 2, 3];

  for (e of arr) {
    arr.push(arr[arr.length - 1] + 1); // 4, 5, 6
    if (arr[arr.length - 1] >= 10) break; // false, false, false
    arr.shift(); // 1, 2, 3
  }

  console.log(arr); // [ 4, 5, 6 ]

Because you are removing the first element of the array in each loop with shift(), your array never exceeds 3 elements at the beginning of the loop, so it will only loop 3 times.

hope this helps.

edited Nov 6, 2022 at 13:42

answered Nov 6, 2022 at 13:37

damonholden

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