JavaScript splice: Edit a Schedule and Inspect Removed Items

The task

Replace one scheduled activity with two activities, then inspect a nonmutating slice of the revised schedule without repeating the extraction fixture.

Runnable Node 22 example

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

const schedule = ['briefing', 'tour', 'review'];
const removed = schedule.splice(1, 1, 'setup', 'practice');
assert.deepEqual(removed, ['tour']);
assert.deepEqual(schedule, ['briefing', 'setup', 'practice', 'review']);
const view = schedule.slice(1, 3);
assert.deepEqual(view, ['setup', 'practice']);
assert.equal(schedule.length, 4);
const inserted = schedule.splice(schedule.length, 0, 'close');
assert.deepEqual(inserted, []);
console.log(schedule.join(' -> '));

Expected output and explanation

The first edit starts at index one, removes one activity and inserts two. The returned array contains tour, while schedule now holds four activities. The result of splice is therefore not the edited schedule.

The slice reads the two inserted activities without removing them. The final splice uses a delete count of zero to append close and returns an empty removed-elements array. An empty return can therefore accompany a successful insertion.

Pitfall to check

Passing only a start value to splice removes through the end, while passing a zero delete count inserts without deletion. Make that argument explicit when accidental truncation would be costly. Preserve or copy the source first when its original membership must remain available.