feat: add formatNumber (#39)

src/js/format-number.js

@@ -0,0 +1,22 @@
+/**
+ * Format a number to a human-readable string
+ * @param num The number to format
+ * @param decimalPlaces The number of decimal places to include in the formatted string
+ * @returns {string}
+ */
+function formatNumber(num, decimalPlaces = 1) {
+    if (num >= 1000000) {
+        return (num / 1000000).toFixed(decimalPlaces) + 'M';
+    } else if (num >= 1000) {
+        return (num / 1000).toFixed(decimalPlaces) + 'k';
+    } else {
+        return num.toFixed(decimalPlaces);
+    }
+}
+
+// Expose to the global scope
+if (typeof window !== 'undefined') {
+    window.formatNumber = formatNumber;
+}
+
+module.exports = formatNumber;

tests/format-number.test.js

@@ -0,0 +1,26 @@
+import {
+    describe,
+    expect,
+    test,
+} from '@jest/globals';
+
+const formatNumber = require('../src/js/format-number');
+
+describe('formatNumber function', () => {
+    test.each([
+        [1234567, 1, '1.2M'],  // 1.2 million
+        [1234567, 2, '1.23M'],  // 1.23 million
+        [1234, 1, '1.2k'],  // 1.2 thousand
+        [1234, 2, '1.23k'],  // 1.23 thousand
+        [123, 1, '123.0'],  // 123 with 1 decimal place
+        [123, 2, '123.00'],  // 123 with 2 decimal places
+    ])('formats %i with %i decimal places as %s', (num, decimalPlaces, expected) => {
+        expect(formatNumber(num, decimalPlaces)).toBe(expected);
+    });
+
+    test('defaults to 1 decimal place if not provided', () => {
+        expect(formatNumber(1234567)).toBe('1.2M');
+        expect(formatNumber(1234)).toBe('1.2k');
+        expect(formatNumber(123)).toBe('123.0');
+    });
+});