Custom equality testers
03 03 Problem
Assertions in tests often come down to value comparison. You expect the result of
sum(2, 3) to equal 5; you expect cart to have a property items of type Array, and so on. And, sometimes, comparing things isn't as straightforward.For example, imagine testing software that deals with measurements. Each measurement has a
value (e.g. 1, 5, or 57) and a unit (inches, millimeters, centimeters, etc). It's quite easy to compare two measurements if both their values and units are the same:expect(new Measurement(1, 'in')).toEqual(new Measurement(1, 'in')) // ✅
It's a bit harder if those two are different.
expect(new Measurement(1, 'in')).toEqual(new Measurement(2.54, 'cm')) // ❌
Semantically, these two measurements are equal. 1 inch is, indeed, 2.54 centimeters. But syntactically these two measurements produce different class instances that cannot be compared literally:
// If you unwrap measurements, you can imagine them as plain objects.
// Vitest will attempt to compare them as such, which will fail.
expect({ value: 1, unit: 'in' }).toEqual({ value: 2.54, unit: 'cm' }) // ❌
Although you can, technically, solve this problem by introducing a custom matcher (something like
.toEqualMeasurement()), that would be like using a fire hose to pour oneself a glass of water. It will work, but it's rather an overkill.In situations like this, all you want is to teach Vitest how to compare custom values. That would still be equality comparison, but the one you fine-tune to compare measurements for what they stand for and not what objects they produce.
You need to extend
.toEqual() itself.Your task
👨💼 In this exercise, your task is to implement a custom equality checker for Vitest. Head to the
file and follow the instructions to do that.
As always, run the tests (
npm test) to verify your solution. The tests shall pass!