JS mutation and variable re-assignment. This somehow still trips me up.

let arr = [9, 'a', true];

let newArr = arr;

newArr[0] = 'changed';

console.log(arr) // -> ['changed', 'a', true]

console.log(arr === newArr) // -> true

both arr and newArr are pointing to the exact same object in memory, so their equality is complete. It doesn't matter that the variables are different, the equality check is looking to see which exact object is being referenced, and in this case it's the same.

To duplicate without mutation, we can use the spread operator, slice(), Array.from(arr), and .concat()