03/04: add problem and a bit of solution

Author: kettanaito

exercises/03.assertions/04.problem.retryable-assertions/README.mdx

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+# Retryable assertions
+
+When testing asynchronous code, your system often arrives at the expected state _eventually_. That is why you often express your intentions in tests as "wait for X to happen" instead of claiming "X must happen immediately".
+
+In Promise-based APIs, that expectation is neatly abstracted behind the `async`/`await` keywords to keep your tests clean:
+
+```ts nonumber
+const response = await fetch('/api/songs')
+await expect(response.json()).toEqual(favoriteSongs)
+```
+
+> While fetching the list of songs takes time, that eventuality is represented as a Promise that you can `await`. This guaratees that your test will not continue until the data is fetched. Quite the same applies to reading the response body stream.
+
+But not all systems are designed like that. And even the systems that _are_ designed like that may not expose you the right Promises to await.
+
+In those situations, you likely reach to utilities like `waitFor()` to help you express the expected eventual state of your tested code.
+
+```ts nonumber nocopy highlight=3-5
+api.sideEffect()
+
+await vi.waitFor(() => {
+	expect(api.state).toEqual(expectedState)
+})
+```
+
+And this works great. But today, I'd like to show you a different approach to expressing eventual expectations.
+
+## Your task
+
+👨‍💼 In <InlineFile file="src/client.test.ts" />, a new challenge is waiting for you. Follow the instructions to replace `vi.waitFor()` with a new API called `expect.poll()`. Make sure your tests are passing once you're done, and see you soon!

exercises/03.assertions/04.problem.retryable-assertions/package.json

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+{
+  "type": "module",
+  "name": "exercises_03.assertions_04.solution.retryable-assertions",
+  "scripts": {
+    "dev": "vite",
+    "test": "vitest",
+    "build": "vite build"
+  },
+  "devDependencies": {
+    "vite": "^6.2.6",
+    "vitest": "^3.1.1"
+  }
+}

exercises/03.assertions/04.problem.retryable-assertions/src/client.test.ts

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+import { Client } from './client'
+
+test('receives a basket of fruits', async () => {
+	const client = new Client()
+	const responseListener = vi.fn()
+	client.request('fruits', responseListener)
+
+	// 💣 Delete this `vi.waitFor()` block.
+	// You will replace it with something else in a moment.
+	await vi.waitFor(() => {
+		expect(responseListener).toHaveBeenCalledWith(['apple', 'banana', 'cherry'])
+	})
+
+	// 🐨 Create a polling assertion by using `expect.poll()`.
+	// 💰 await expect.poll(() => received).toHaveBeenCalledWith(expected)
+
+	// 🐨 Use the same list of fruits as the expected value for the assertion.
+	// 💰 ['apple', 'banana', 'cherry']
+})

exercises/03.assertions/04.problem.retryable-assertions/src/client.ts

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+import { EventEmitter } from 'node:events'
+import { Server } from './server'
+
+export class Client extends EventEmitter {
+	private server: Server
+
+	constructor() {
+		super()
+		this.server = new Server()
+	}
+
+	public request(
+		requestType: string,
+		responseListener: (data: unknown) => void,
+	) {
+		this.server.emit('request', requestType, responseListener)
+	}
+}

exercises/03.assertions/04.problem.retryable-assertions/src/server.ts

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+import { EventEmitter } from 'node:events'
+
+export class Server extends EventEmitter {
+	constructor() {
+		super()
+		this.on('request', (requestType, respondWith) => {
+			queueMicrotask(() => {
+				switch (requestType) {
+					case 'fruits': {
+						return respondWith(['apple', 'banana', 'cherry'])
+					}
+
+					default: {
+						return respondWith('unknown request')
+					}
+				}
+			})
+		})
+	}
+}

exercises/03.assertions/04.problem.retryable-assertions/tsconfig.app.json

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+{
+	"extends": "./tsconfig.base.json",
+	"include": ["src/**/*"],
+	"exclude": ["src/**/*.test.ts*"]
+}

exercises/03.assertions/04.problem.retryable-assertions/tsconfig.base.json

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+{
+	"compilerOptions": {
+		"target": "ESNext",
+		"module": "ESNext",
+		"useDefineForClassFields": true,
+		"skipLibCheck": true,
+
+		/* Bundler mode */
+		"moduleResolution": "bundler",
+		"allowImportingTsExtensions": true,
+		"isolatedModules": true,
+		"moduleDetection": "force",
+		"noEmit": true,
+		"verbatimModuleSyntax": true,
+
+		/* Linting */
+		"strict": true,
+		"noUnusedLocals": false,
+		"noUnusedParameters": false,
+		"noFallthroughCasesInSwitch": true,
+		"noUncheckedSideEffectImports": true
+	}
+}

exercises/03.assertions/04.problem.retryable-assertions/tsconfig.json

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+{
+	"files": [],
+	"references": [
+		{ "path": "./tsconfig.app.json" },
+		{ "path": "./tsconfig.node.json" },
+		{ "path": "./tsconfig.test.json" }
+	]
+}

exercises/03.assertions/04.problem.retryable-assertions/tsconfig.node.json

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+{
+	"extends": "./tsconfig.base.json",
+	"compilerOptions": {
+		"lib": ["ES2023"]
+	},
+	"include": ["vitest.config.ts"]
+}

exercises/03.assertions/04.problem.retryable-assertions/tsconfig.test.json

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+{
+	"extends": "./tsconfig.app.json",
+	"include": ["src/**/*", "src/**/*.test.ts*"],
+	"exclude": [],
+	"compilerOptions": {
+		"types": ["vitest/globals"]
+	}
+}