Comparators & mutation

Sort with an explicit rule

Lesson 16 of 20 · 12 min · Module 5: Reliable data pipelines

Sorting changes the order in which a person encounters information. The order should follow a clear rule and preserve data owned by the caller.

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By the end, you’ll be able to…

Write numeric and tie-breaking comparisons
Protect the caller’s array order

The idea

JavaScript’s sort() changes its array in place. Make an array copy first. Return a negative, zero, or positive number from the comparator to describe the relative order.

const values = [10, 2, 30];
const ordered = [...values].sort((a, b) => a - b);
console.log(ordered);

01Understand the default

Without a comparator, sort compares string forms. That is why numeric values like 10 and 2 need an explicit numeric comparison.

02Return an ordering relationship

a.minutes - b.minutes is negative when a is shorter. A zero difference lets you move to the title rule.

03Separate array and object copying

This function changes only ordering, so copying the array is enough. It does not edit the session objects inside the array.

Your turn

Write shortestFirst(sessions). Return a new array sorted by minutes ascending, then title ascending for equal durations. Titles use ordinary ASCII letters. Do not reorder the original array.

What the checks look for

Sorts numerically rather than as text
Uses titles to break a duration tie
Leaves the original order unchanged
Accepts an empty list

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Take this with you

An explicit comparator makes ordering deliberate and testable.

Go a little further

Add descending order as an optional argument while preserving the same tie rule.