ES6 - Mutate an Array Declared with const

JavaScript
1

Tell us what’s happening:
I successfully completed this challenge. I included some quality checks using console log and there is a quirk. I’m wondering why the console prints:
“Unshifted 3”, instead of “Unshifted 2”?

Your code so far

const s = [5, 7, 2];
function editInPlace() {
  // Only change code below this line
  let Popped = s.pop();
  console.log(s);
  console.log("Popped " + Popped);
  // Using s = [2, 5, 7] would be invalid
  let Unshifted = s.unshift(Popped);
  console.log(s);
  console.log("Unshifted " + Unshifted);
  // Only change code above this line
}
editInPlace();

Your browser information:

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Challenge: ES6 - Mutate an Array Declared with const

Link to the challenge:

2

Read about the Return value of unshift here. It is the same Return value returned from the push method.

1 Like
3

Oh. Interesting. It returns the array length. I played with the array values and lengths and confirm this. Thank you!
I made a small change in my last console.log

console.log("Repositioned the popped value: " + Popped);
4

Just adding other details that you may find interesting:

  • unshift is kind of opposite to push, both return length
  • shift is kind of opposite to pop, both return an element (first or last, respectively.)

All of these Array methods mutate the array.

1 Like