Iterator in a loop is a 'string' instead of 'number'

[andrewgwallace]

Version: v12.16.1, v15.14.0 (tested)
Platform(s) (tested):

• Linux 7b473cfe85f6 5.8.0-1032-gcp fix LICENSE #34~20.04.1-Ubuntu SMP Wed May 19 18:19:35 UTC 2021 x86_64 GNU/Linux
• Darwin Kernel Version 20.5.0

What steps will reproduce the bug?

Create test array and console log out the typeof of the iterator to see the result

Sample snippet:

const movies = [
  { 
    title: 'Requiem for a Dream',
    duration: 128
  }
]

function iteratorType(array) {
  for (let i in array) {
    console.log(typeof i)
  }
}

iteratorType(movies)

How often does it reproduce? Is there a required condition?

Every time. Conditions: An array with at least one value.

What is the expected behavior?

The iterator should be typeof number.

What do you see instead?

A typeof of 'string' is returned

[andrewgwallace]

I guess I'm probably missing some logical reason as to why an iterator is a string but it just seems odd -- When I was writing up a quick sort algorithm, I had to do parseInt(i) in order to make the comparison operator work with the index of the item I was comparing it to. Seems like an unnecessary step.

[Trott]

This is the way JavaScript works and Node.js can't realistically change it. You will get the same result running your code in a browser.

for...in is not recommended for Arrays generally. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in for explanation, references, and alternatives.

[mscdex]

Option 1: If you don't want to create a new data structure from the existing data, you'll have to use a vanilla for loop (this is the simplest and fastest approach).

Option 2: You can instead iterate over both the keys and values using movies.entries() instead of just movies and using for-of. Doing so will get you a number type for the index:

const movies = [
  { 
    title: 'Requiem for a Dream',
    duration: 128
  }
]

function iteratorType(array) {
  for (let [i, val] of array.entries()) {
    console.log(typeof i)
  }
}

iteratorType(movies)

This option can be handy if you also need the value.

[Trott]

I'm going to close this because there isn't anything we're likely to do to change this behavior. But feel free to leave additional comments/questions if we've missed something.

[andrewgwallace]

Thank you all for the valuable feedback! ☺️