feat: add octal to decimal conversion

Conversions/OctalToDecimal.js

@@ -0,0 +1,24 @@
+/**
+ * @function octalToDecimal
+ * @description Converts an octal number (base-8) string to its decimal
+ *   (base-10) equivalent without using JavaScript's built-in parseInt.
+ * @param {string} octal - A string representing an octal number
+ * @returns {number} - The decimal equivalent of the octal number
+ * @see https://en.wikipedia.org/wiki/Octal
+ * @example octalToDecimal("10") => 8
+ * @example octalToDecimal("377") => 255
+ * @example octalToDecimal("0") => 0
+ */
+const octalToDecimal = (octal) => {
+  if (typeof octal !== 'string' || !/^[0-7]+$/.test(octal)) {
+    throw new TypeError(
+      'Input must be an octal string containing only digits 0-7'
+    )
+  }
+  return octal
+    .split('')
+    .reverse()
+    .reduce((acc, digit, index) => acc + Number(digit) * Math.pow(8, index), 0)
+}
+
+export { octalToDecimal }

Conversions/test/OctalToDecimal.test.js

@@ -0,0 +1,43 @@
+import { octalToDecimal } from '../OctalToDecimal.js'
+
+describe('octalToDecimal', () => {
+  test("octalToDecimal('0') returns 0", () => {
+    expect(octalToDecimal('0')).toBe(0)
+  })
+
+  test("octalToDecimal('1') returns 1", () => {
+    expect(octalToDecimal('1')).toBe(1)
+  })
+
+  test("octalToDecimal('7') returns 7", () => {
+    expect(octalToDecimal('7')).toBe(7)
+  })
+
+  test("octalToDecimal('10') returns 8", () => {
+    expect(octalToDecimal('10')).toBe(8)
+  })
+
+  test("octalToDecimal('100') returns 64", () => {
+    expect(octalToDecimal('100')).toBe(64)
+  })
+
+  test("octalToDecimal('377') returns 255", () => {
+    expect(octalToDecimal('377')).toBe(255)
+  })
+
+  test("octalToDecimal('1000') returns 512", () => {
+    expect(octalToDecimal('1000')).toBe(512)
+  })
+
+  test('throws TypeError for invalid octal string', () => {
+    expect(() => octalToDecimal('89')).toThrow(TypeError)
+  })
+
+  test('throws TypeError for empty string', () => {
+    expect(() => octalToDecimal('')).toThrow(TypeError)
+  })
+
+  test('throws TypeError for non-string input', () => {
+    expect(() => octalToDecimal(10)).toThrow(TypeError)
+  })
+})