I’m encountering some unexpected behavior in my JavaScript code which I’d like to understand. I want a one-line function to detect if a number exists inside an array of integers, the number being checked is the iterator of a loop in another array. I’m using the standard includes() function, should note the same happens with an indexOf() >= 0 check. For this test we have the following code:

const array1 = [0, 1, 2, 3];
const array2 = [1, 2];
for(let i in array1)
    console.log(array2.includes(i));

To my surprise each output returns false instead of the expected false true true false sequence. How can this be? I know I’m checking i not arr[i], but i still corresponds to the numbers included in those arrays: 0 is still 0, 1 is still 1, etc. Using arr.includes(1) does return true, so why not arr.includes(i) when i is also 1?

>Solution :

for...in loops through all properties, not values. Arrays are objects, and their properties are 0 ... array.length, so i inside the for loop is 1, 2, 3 and 4.

You should be using a for...of loop instead:

const array1 = [0, 1, 2, 3];
const array2 = [1, 2];
for(let i of array1)
    console.log(array2.includes(i));