Question 1

What is URL Encoding (Percent-Encoding) and why is it necessary?

Accepted Answer

Under RFC 3986 standards, URLs are restricted to a limited set of ASCII characters (unreserved characters include A-Z, a-z, 0-9, -, _, ., ~).

① Handling Non-ASCII & Unicode Characters:
Characters from non-Latin scripts (Chinese, Japanese, Arabic, etc.) and emojis must be converted into UTF-8 byte sequences, where each byte is represented by a percent sign followed by two hexadecimal digits (e.g., '中' becomes '%E4%B8%AD').

② Preventing Syntax Ambiguities:
In a URL, characters like '?' denote the start of query parameters, '=' separates key-value pairs, and '&' separates parameters. If a parameter value itself contains '&', '=', '?', or spaces, it must be percent-encoded to prevent server-side parsing errors.

Question 2

What is the difference between encodeURIComponent() and encodeURI()?

Accepted Answer

The primary difference lies in how reserved URL structural characters are treated:

① encodeURIComponent() (Recommended for Query Parameter Values):
Encodes all reserved characters including : / ? # [ ] @ ! $ & ' ( ) * + , ; =. This is essential for individual query parameter keys and values so they do not break the overall URL structure.

② encodeURI() (Used for Complete URL Strings):
Preserves URL structural syntax (e.g. ://, /, ?, &, # remain unencoded) and only encodes non-ASCII characters and spaces. It should not be used on parameter values containing '&' or '=' because it will not escape them.

③ Rule of Thumb: Use encodeURIComponent() for individual parameter keys/values, and encodeURI() when encoding a whole, valid URL string.

Question 3

Should space be encoded as '%20' or '+' in URLs?

Accepted Answer

This depends on the specification and context:

① RFC 3986 Standard (%20):
Under standard URI and HTTP specifications, spaces must be encoded as %20. In the path component of a URL (e.g., /my%20documents/), %20 is the only valid representation.

② application/x-www-form-urlencoded (+):
Historically, HTML form submissions using GET/POST with application/x-www-form-urlencoded encode spaces as '+'. Most backend frameworks (PHP, Spring, ASP.NET, Express) automatically decode both '+' and '%20' as spaces in query strings.

This tool provides a toggle switch to encode spaces as '+' if required by your specific backend.

Question 4

What is Double Encoding and how do I prevent it?

Accepted Answer

Double encoding occurs when an already percent-encoded string is accidentally encoded a second time:

① Mechanism:
A character like '中' is encoded to '%E4%B8%AD'. If encoded again, the '%' character is converted to '%25', resulting in '%25E4%25B8%25AD'.

② Negative Impact:
When the receiving server decodes the URL once, it obtains the string '%E4%B8%AD' rather than the original character '中', resulting in broken search queries, 404 file not found errors, or corrupted database records.

③ Prevention: Always ensure data is in its raw, unencoded state before applying encodeURIComponent().

Question 5

Why does decodeURIComponent() throw a 'URI malformed' error?

Accepted Answer

The 'URI malformed' JavaScript runtime error typically happens in the following cases:

① Truncated Percent Sequences:
A trailing '%' sign or an incomplete hex pair (e.g., '%E' instead of '%E4').

② Incomplete UTF-8 Byte Sequences:
Most international characters require 2 to 4 consecutive %XX byte sequences. If a string was trimmed or truncated midway (e.g., '%E4%B8' missing its third byte), decodeURIComponent() cannot reconstruct a valid Unicode character and throws an exception.

③ Legacy Non-UTF8 Encodings:
URLs encoded using legacy character sets (such as Big5 or ISO-8859-1) fail when processed by modern UTF-8 decoders.

Question 6

What is the difference between Base64 and URL encoding? Can they replace each other?

Accepted Answer

Base64 and URL encoding serve fundamentally different purposes and are not interchangeable:

① URL Encoding (Percent-encoding):
Selectively replaces invalid characters with %XX while keeping standard alphanumeric ASCII characters intact with minimal length overhead.

② Base64 Encoding:
Encodes arbitrary binary data or text into an ASCII string using a 64-character alphabet (A-Z, a-z, 0-9, +, /), increasing data size by ~33%. Standard Base64 contains '+', '/', and '=' which are reserved characters in URLs and must be URL-encoded or converted to 'Base64URL' format before being placed in a query string.

Question 7

Data Privacy & Zero Server Storage Guarantee

Accepted Answer

This tool operates 100% on the client side using pure browser JavaScript:

① Zero Server Transmission:
All URLs, API keys, bearer tokens, and sensitive query parameters you paste or inspect remain entirely within your local browser memory. Nothing is ever sent to, logged by, or stored on any remote server.

② Safe for Confidential Data:
You can safely inspect, parse, and debug sensitive OAuth redirect URLs, JWT tokens, and private API query parameters with total peace of mind.

URL Encoder & Decoder

Frequently Asked Questions (FAQ)7 Q&As