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Why is using “for…in” with array iteration a bad idea in javascript?
Using for..in loops in JavaScript with array iteration is a bad idea because of the following behavior −
Using normal iteration loops −
Example
let arr = [] arr[4] = 5 for (let i = 0; i < arr.length; i ++) { console.log(arr[i]) }
Output
undefined undefined undefined undefined 5
If we had iterated over this array using the for in construct, we'd have gotten −
Example
let arr = [] arr[4] = 5 for (let i in arr) { console.log(arr[i]) }
Output
5
Note that the length of the array is 5, but this still iterates over only one value in the array.
This happens because the purpose of the for-in statement is to enumerate over object properties. This statement will go up in the prototype chain, also enumerating over inherited properties, a thing that sometimes is not desired.
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