04/03: add solution

Author: kettanaito

exercises/04.assertions/03.solution.custom-equality-testers/README.mdx

@@ -1,5 +1,11 @@
 # Custom equality testers
 
 - [docs](https://vitest.dev/api/expect.html#expect-addequalitytesters)
+- Unlike matchers, equality testers are _applied recursively_! This is extremely useful if your custom data structure is nested in some other data structure.
+- Gotcha: Equality testers must be _synchronous_.
 
-Really neat way to define equality testers to compare otherwise incomparable objects, like `Response`.
+Really neat way to define equality testers to compare otherwise incomparable objects, like `Response`. - `Response` isn't the best example because equality testers _must be synchronous_.
+
+## TODO
+
+- [ ] Better name for the test. We aren't testing the `Measurement` class but something that uses it.

exercises/04.assertions/03.solution.custom-equality-testers/index.html

@@ -0,0 +1,17 @@
+import { Measurement } from './measurement'
+
+test('compares measurements', () => {
+	expect(new Measurement(10, 'in')).toEqual(new Measurement(25.4, 'cm'))
+})
+
+test('compares nested measurements', () => {
+	expect({
+		id: 1,
+		name: 'fabric',
+		length: new Measurement(10, 'in'),
+	}).toEqual({
+		id: 1,
+		name: 'fabric',
+		length: new Measurement(25.4, 'cm'),
+	})
+})

exercises/04.assertions/03.solution.custom-equality-testers/public/vite.svg

@@ -0,0 +1,19 @@
+type MeasurementUnit = 'in' | 'cm'
+
+export class Measurement {
+	constructor(
+		public readonly value: number,
+		public readonly unit: MeasurementUnit,
+	) {}
+
+	public equals(other: Measurement): boolean {
+		if (this.unit === other.unit) {
+			return this.value === other.value
+		}
+
+		const thisValue = this.unit === 'in' ? this.value * 2.54 : this.value
+		const otherValue = other.unit === 'in' ? other.value * 2.54 : other.value
+
+		return thisValue === otherValue
+	}
+}