Where this matters
When adding an item to a list, keep the mutated array separate from the return value of push. Assigning that return value elsewhere gives a length.
JavaScript · read-only example
const values = [1, 2];
console.log(values.push(3));
console.log(JSON.stringify(values));Read the full explanation
Push mutates the array and returns the new length, which is three. The second log shows the mutated array.
Trace the code
- The original array has two entries.
- push appends 3 to the existing array and returns its new length.
- The next log serializes that same array after the mutation.
Why the other predictions differ
3 [1,2]— Push appends to the original array even when it is bound with const.[1,2,3] [1,2,3]— Push returns a number, not the array itself.
3 [1,2,3]
Reading this explanation does not record mastery. A correct submitted attempt can do so, including after review.
Change one thing
Append a different number to show that the return value is the length, not the appended item.
Predict the result, then run this self-contained snippet in your browser’s developer console. It uses only sample values and makes no network requests.
const values = [1, 2]; console.log(values.push(99)); console.log(JSON.stringify(values));Check the variation’s output
3 [1,2,99]
Explain which binding, value, or operation changed before moving to the next challenge.