JavaScript Array pop: Read the Last Item or Remove It

The task

Inspect the newest undo action, remove it, and verify the previous action remains. Check empty behavior separately instead of assuming every stack contains an item.

Runnable Node 22 example

const assert = require('node:assert/strict');

const undo = ['rename', 'move'];
assert.equal(undo.at(-1), 'move');
assert.equal(undo[undo.length - 1], 'move');
assert.equal(undo.length, 2);
const removed = undo.pop();
assert.equal(removed, 'move');
assert.deepEqual(undo, ['rename']);
assert.equal([].at(-1), undefined);
assert.equal([].pop(), undefined);
console.log(`removed=${removed}; next=${undo.at(-1)}; size=${undo.length}`);

Expected output and explanation

The two initial reads agree without changing the stack length. pop returns move and changes the container so rename becomes the last item. A function that merely wants a preview should not consume the action by calling pop.

The empty checks both produce undefined, but for different operations: one reads an absent position and one tries to remove from an empty array. If undefined is itself a permitted stack value, inspect length before removing when you need to distinguish those cases.

Pitfall to check

Array[-1] is an ordinary property lookup, not the negative-position behavior of at(-1). Do not substitute the bracket form with a literal −1 and assume it finds the last element.