diff --git a/archive/j/javascript/base64-encode-decode.js b/archive/j/javascript/base64-encode-decode.js
new file mode 100644
index 000000000..0487ff08d
--- /dev/null
+++ b/archive/j/javascript/base64-encode-decode.js
@@ -0,0 +1,60 @@
+#!/usr/bin/env node
+
+function usageAndExit() {
+  console.error("Usage: please provide a mode and a string to encode/decode");
+  process.exit(1);
+}
+
+function isValidBase64(str) {
+  // Check length multiple of 4
+  if (str.length % 4 !== 0) return false;
+  // Only A–Z, a–z, 0–9, +, /, = allowed
+  if (!/^[A-Za-z0-9+/]*={0,2}$/.test(str)) return false;
+  // = only allowed at end, at most two
+  const idx = str.indexOf("=");
+  if (idx !== -1 && idx < str.length - 2) {
+    // e.g. “ab==cd” invalid
+    return false;
+  }
+  return true;
+}
+
+function encodeBase64(input) {
+  // Use Buffer in Node.js
+  return Buffer.from(input, "ascii").toString("base64");
+}
+
+function decodeBase64(input) {
+  // Validate first
+  if (!isValidBase64(input)) {
+    usageAndExit();
+  }
+  // Buffer will ignore invalid trailing bits
+  return Buffer.from(input, "base64").toString("ascii");
+}
+
+function main() {
+  const args = process.argv.slice(2);
+  if (args.length < 2) {
+    usageAndExit();
+  }
+  const mode = args[0];
+  const text = args[1];
+
+  if (!text) {
+    usageAndExit();
+  }
+
+  if (mode === "encode") {
+    console.log(encodeBase64(text));
+  } else if (mode === "decode") {
+    console.log(decodeBase64(text));
+  } else {
+    usageAndExit();
+  }
+}
+
+if (require.main === module) {
+  main();
+}
+