Prefer to replace array[array.length - 1] with array.at(-1)

Problem/Motivation

In the core, to get the last item in an array, use array[array.length - 1]. Modern JavaScript makes this code shorter.

Proposed resolution

Use array.at(-1).
For example, in /themes/claro/js/mobile.install.js

Before:

    if (steps[steps.length - 1].className === 'done') {

After:

    if (steps.at(-1).className === 'done') {

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet