Add utility belt example

examples/utility-belt/package.json

@@ -0,0 +1,25 @@
+{
+  "name": "utility-belt",
+  "version": "1.0.0",
+  "main": "src/index.js",
+  "type": "module",
+  "scripts": {
+    "start": "vitest --ui",
+    "test": "vitest"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/stevekinney/testing-javascript.git"
+  },
+  "author": "Steve Kinney <hello@stevekinney.net>",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/stevekinney/testing-javascript/issues"
+  },
+  "homepage": "https://github.com/stevekinney/testing-javascript#readme",
+  "devDependencies": {
+    "@vitest/ui": "^2.1.1",
+    "vite": "^5.4.5",
+    "vitest": "^2.1.1"
+  }
+}

examples/utility-belt/src/string-to-number.js

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+/**
+ * Converts a string to a number or throws an error if the string is not a number.
+ * @param {string} value
+ * @returns {number} The number.
+ */
+export const stringToNumber = (value) => {
+  const number = Number(value);
+
+  if (isNaN(number)) {
+    throw new Error(`'${value}' cannot be parsed as a number.`);
+  }
+
+  return number;
+};

examples/utility-belt/src/string-to-number.test.js

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+import { describe, it, expect } from 'vitest';
+import { stringToNumber } from './string-to-number';
+
+describe('stringToNumber', () => {
+  it('converts a string to a number', () => {
+    expect(stringToNumber('42')).toBe(42);
+  });
+
+  it('throws an error if given a string that is not a number', () => {
+    const value = 'foo';
+    expect(() => stringToNumber(value)).toThrowError(
+      `cannot be parsed as a number`,
+    );
+  });
+});