From 4c625f0a6c3068b79131aa0bfc86a024b458a695 Mon Sep 17 00:00:00 2001
From: sarthakgupta1907-cell <sarthakgupta1907@gmail.com>
Date: Mon, 2 Nov 2026 18:35:31 +0530
Subject: [PATCH] feat: add hexadecimal to decimal conversion

---
 Conversions/HexadecimalToDecimal.js           | 29 ++++++++++++
 Conversions/test/HexadecimalToDecimal.test.js | 47 +++++++++++++++++++
 2 files changed, 76 insertions(+)
 create mode 100644 Conversions/HexadecimalToDecimal.js
 create mode 100644 Conversions/test/HexadecimalToDecimal.test.js

diff --git a/Conversions/HexadecimalToDecimal.js b/Conversions/HexadecimalToDecimal.js
new file mode 100644
index 0000000000..1680d627c0
--- /dev/null
+++ b/Conversions/HexadecimalToDecimal.js
@@ -0,0 +1,29 @@
+/**
+ * @function hexadecimalToDecimal
+ * @description Converts a hexadecimal number (base-16) string to its decimal
+ *   (base-10) equivalent without using JavaScript's built-in parseInt.
+ * @param {string} hex - A string representing a hexadecimal number
+ * @returns {number} - The decimal equivalent of the hexadecimal number
+ * @see https://en.wikipedia.org/wiki/Hexadecimal
+ * @example hexadecimalToDecimal("FF") => 255
+ * @example hexadecimalToDecimal("10") => 16
+ * @example hexadecimalToDecimal("0") => 0
+ */
+const hexadecimalToDecimal = (hex) => {
+  if (typeof hex !== 'string' || !/^[0-9a-fA-F]+$/.test(hex)) {
+    throw new TypeError(
+      'Input must be a valid hexadecimal string containing only 0-9 and A-F'
+    )
+  }
+  return hex
+    .toUpperCase()
+    .split('')
+    .reverse()
+    .reduce((acc, char, index) => {
+      const value =
+        char >= 'A' ? char.charCodeAt(0) - 55 : char.charCodeAt(0) - 48
+      return acc + value * Math.pow(16, index)
+    }, 0)
+}
+
+export { hexadecimalToDecimal }
diff --git a/Conversions/test/HexadecimalToDecimal.test.js b/Conversions/test/HexadecimalToDecimal.test.js
new file mode 100644
index 0000000000..c428f376ed
--- /dev/null
+++ b/Conversions/test/HexadecimalToDecimal.test.js
@@ -0,0 +1,47 @@
+import { hexadecimalToDecimal } from '../HexadecimalToDecimal.js'
+
+describe('hexadecimalToDecimal', () => {
+  test("hexadecimalToDecimal('0') returns 0", () => {
+    expect(hexadecimalToDecimal('0')).toBe(0)
+  })
+
+  test("hexadecimalToDecimal('A') returns 10", () => {
+    expect(hexadecimalToDecimal('A')).toBe(10)
+  })
+
+  test("hexadecimalToDecimal('F') returns 15", () => {
+    expect(hexadecimalToDecimal('F')).toBe(15)
+  })
+
+  test("hexadecimalToDecimal('10') returns 16", () => {
+    expect(hexadecimalToDecimal('10')).toBe(16)
+  })
+
+  test("hexadecimalToDecimal('FF') returns 255", () => {
+    expect(hexadecimalToDecimal('FF')).toBe(255)
+  })
+
+  test("hexadecimalToDecimal('100') returns 256", () => {
+    expect(hexadecimalToDecimal('100')).toBe(256)
+  })
+
+  test("hexadecimalToDecimal lowercase 'ff' returns 255", () => {
+    expect(hexadecimalToDecimal('ff')).toBe(255)
+  })
+
+  test("hexadecimalToDecimal('1000') returns 4096", () => {
+    expect(hexadecimalToDecimal('1000')).toBe(4096)
+  })
+
+  test('throws TypeError for invalid hex string', () => {
+    expect(() => hexadecimalToDecimal('GHI')).toThrow(TypeError)
+  })
+
+  test('throws TypeError for empty string', () => {
+    expect(() => hexadecimalToDecimal('')).toThrow(TypeError)
+  })
+
+  test('throws TypeError for non-string input', () => {
+    expect(() => hexadecimalToDecimal(255)).toThrow(TypeError)
+  })
+})