Don't mutate, make a copy

When you change an array in place, what it used to hold is lost, and with it the answer to the simplest question there is: what changed? Undo, comparisons and change detection in every UI library are built on that question.

The alternative is for every change to produce a new value: you write [...list, item] instead of push, list.filter(x => x !== y) instead of splice, and {...obj, key: v} instead of changing the property directly.

The array here is frozen with Object.freeze, so in strict mode any attempt to change it throws an error instead of failing silently.