---
title: "Base64 Decode action for ActionClip"
canonical_url: "https://actionclip.app/actions/base64-decode"
document_type: "standalone_action"
official: true
action_count: 1
catalog_schema_version: 1
catalog_revision: "2026-08-21.1"
catalog_published_at: "2026-08-20T18:50:00Z"
---

# Base64 Decode action for ActionClip

Decode Base64 into validated UTF-8 text.

## What it does

Accepts padded or unpadded standard and URL-safe Base64, ignores whitespace, and rejects malformed Base64 or invalid UTF-8 bytes.

## Action details

| Field | Value |
| --- | --- |
| Category | Developer Tools |
| Action type | JavaScript |
| Result | Replace or copy text |
| Internet | Not required |

## Action and outcome

| Action | Outcome |
| --- | --- |
| **Base64 Decode** — Selected text: QWN0aW9uQ2xpcA== | ActionClip |

## How to decode Base64 into validated UTF-8 text with ActionClip

1. Select the text you want to use.
2. Choose **Base64 Decode** from ActionClip.
3. Review the result, then replace the selection or copy the new text.

## Requirements

No setup required.

## Privacy

Runs locally in ActionClip’s JavaScript sandbox. Selected text does not leave your Mac.

## Action configuration

````javascript
function run(selected_text) {
  const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  let input = selected_text.replace(/\s+/g, "").replace(/-/g, "+").replace(/_/g, "/");
  if (!/^[A-Za-z0-9+/]*={0,2}$/.test(input) || input.length % 4 === 1) {
    throw new Error("Invalid Base64 text.");
  }
  input = input.replace(/=+$/, "");
  while (input.length % 4 !== 0) input += "=";

  const bytes = [];
  for (let index = 0; index < input.length; index += 4) {
    const a = alphabet.indexOf(input[index]);
    const b = alphabet.indexOf(input[index + 1]);
    const c = input[index + 2] === "=" ? 0 : alphabet.indexOf(input[index + 2]);
    const d = input[index + 3] === "=" ? 0 : alphabet.indexOf(input[index + 3]);
    if (a < 0 || b < 0 || c < 0 || d < 0) throw new Error("Invalid Base64 text.");
    const value = (a << 18) | (b << 12) | (c << 6) | d;
    bytes.push((value >> 16) & 255);
    if (input[index + 2] !== "=") bytes.push((value >> 8) & 255);
    if (input[index + 3] !== "=") bytes.push(value & 255);
  }

  let output = "";
  for (let index = 0; index < bytes.length;) {
    const first = bytes[index++];
    let codePoint;
    if (first <= 0x7F) {
      codePoint = first;
    } else if (first >= 0xC2 && first <= 0xDF) {
      if (index >= bytes.length || (bytes[index] & 0xC0) !== 0x80) throw new Error("Decoded bytes are not valid UTF-8.");
      codePoint = ((first & 0x1F) << 6) | (bytes[index++] & 0x3F);
    } else if (first >= 0xE0 && first <= 0xEF) {
      if (index + 1 >= bytes.length || (bytes[index] & 0xC0) !== 0x80 || (bytes[index + 1] & 0xC0) !== 0x80) {
        throw new Error("Decoded bytes are not valid UTF-8.");
      }
      const second = bytes[index++];
      const third = bytes[index++];
      if ((first === 0xE0 && second < 0xA0) || (first === 0xED && second >= 0xA0)) {
        throw new Error("Decoded bytes are not valid UTF-8.");
      }
      codePoint = ((first & 0x0F) << 12) | ((second & 0x3F) << 6) | (third & 0x3F);
    } else if (first >= 0xF0 && first <= 0xF4) {
      if (index + 2 >= bytes.length || (bytes[index] & 0xC0) !== 0x80
        || (bytes[index + 1] & 0xC0) !== 0x80 || (bytes[index + 2] & 0xC0) !== 0x80) {
        throw new Error("Decoded bytes are not valid UTF-8.");
      }
      const second = bytes[index++];
      const third = bytes[index++];
      const fourth = bytes[index++];
      if ((first === 0xF0 && second < 0x90) || (first === 0xF4 && second >= 0x90)) {
        throw new Error("Decoded bytes are not valid UTF-8.");
      }
      codePoint = ((first & 7) << 18) | ((second & 0x3F) << 12)
        | ((third & 0x3F) << 6) | (fourth & 0x3F);
    } else {
      throw new Error("Decoded bytes are not valid UTF-8.");
    }
    output += String.fromCodePoint(codePoint);
  }
  return output;
}
````

## Links

- [HTML listing](https://actionclip.app/actions/base64-decode)
- [Download action definition](https://actionclip.app/actions/base64-decode.actionclip)
- [Browse Developer Tools actions](https://actionclip.app/marketplace#marketplace-developerTools)
- [All Marketplace listings](https://actionclip.app/marketplace)