Base64 / URL / HTML Encoder & Decoder

Encode or decode text using Base64, URL percent-encoding, or HTML entities. Processes text client-side with no external calls.

Input

• The text string to encode or decode
• Encoding type: base64 (default), url, or html
• Direction: encode (default) or decode

Output

• The transformed string
• A brief note on the encoding type and direction applied

Instructions

Base64 (type: base64)

Encode:

1. Take the input string.
2. Convert each character to its UTF-8 byte sequence (handle non-ASCII/Unicode correctly).
3. Apply Base64 encoding using the standard alphabet (A–Z, a–z, 0–9, +, /).
4. Pad with = characters to make the length a multiple of 4.
5. The algorithm equivalent is: btoa(unescape(encodeURIComponent(input))).

Decode:

1. Take the Base64-encoded input.
2. Validate it contains only valid Base64 characters (A–Z, a–z, 0–9, +, /, =).
3. Decode using: decodeURIComponent(escape(atob(input))).
4. Return the original UTF-8 string.

URL Percent-Encoding (type: url)

Encode:

1. Apply encodeURIComponent semantics: encode every character except A–Z a–z 0–9 - _ . ! ~ * ' ( ).
2. Spaces become %20 (not +).
3. Non-ASCII characters are UTF-8 encoded then percent-escaped.

Decode:

1. Replace each %XX sequence with the corresponding byte.
2. Interpret the resulting bytes as UTF-8.
3. Equivalent to decodeURIComponent(input).

HTML Entities (type: html)

Encode: Replace these characters with their named HTML entities:

• \x3C → <
• > → >
• & → &
• " → "
• ' → '

Decode: Reverse the mapping — replace each HTML entity with its literal character.

Options

• type: base64 | url | html — default: base64
• direction: encode | decode — default: encode

Examples

Base64 encode: Input: Hello, World! Output: SGVsbG8sIFdvcmxkIQ==

Base64 encode (Unicode): Input: Héllo Output: SMOpbGxv

Base64 decode: Input: SGVsbG8sIFdvcmxkIQ== Output: Hello, World!

URL encode: Input: name=John Doe&city=New York Output: name%3DJohn%20Doe%26city%3DNew%20York

URL decode: Input: hello%20world%21 Output: hello world!

HTML encode: Input: \x3Cscript>alert("XSS")\x3C/script> Output: <script>alert("XSS")</script>

HTML decode: Input: <h1>Hello & welcome</h1> Output: \x3Ch1>Hello & welcome\x3C/h1>

Error Handling

• Invalid Base64 input for decode: If the string contains characters outside the Base64 alphabet or has incorrect padding, report: Error: Invalid Base64 string. Ask the user to verify the input.
• Invalid URL encoding for decode: If a %XX sequence uses non-hex digits or the sequence is incomplete, report: Error: Invalid URL encoded string.
• Empty input: Return an empty string with a note that no input was provided.
• Binary/non-text data: Warn the user that Base64 encoding of binary data requires the raw bytes, which cannot be provided as plain text — suggest they use a tool that accepts file uploads.