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

encodeURI() is designed to encode a complete URL; it preserves characters that have special structural meaning in a URL (such as ':', '/', '?', '#', and '&'). In contrast, encodeURIComponent() encodes everything except standard unreserved characters (letters, numbers, '-', '_', '.', '!', '~', '*', ''', '(', ')'), making it the correct method for encoding individual query string keys and values.

When should spaces be encoded as %20 versus plus (+)?

Under standard RFC 3986 URI specifications, spaces must always be encoded as '%20'. However, HTML form submissions using the 'application/x-www-form-urlencoded' media type historically encode spaces as '+'. Modern web frameworks typically accept both in query parameters, but '%20' is required in URL paths and strict REST APIs.

Which characters are considered 'reserved' in RFC 3986?

RFC 3986 divides characters into 'unreserved' (letters A-Z, a-z, digits 0-9, and '-', '_', '.', '~') which never need encoding, and 'reserved' characters which serve as delimiters: gen-delims (':', '/', '?', '#', '[', ']', '@') and sub-delims ('!', '$', '&', ''', '(', ')', '*', '+', ',', ';', '='). Reserved characters must be percent-encoded when used as ordinary data.

What causes a 'URIError: malformed URI sequence' in JavaScript?

A URIError occurs when decodeURIComponent() encounters an invalid percent sequence. Common causes include a lone percent sign ('%') not followed by two hexadecimal digits, or an incomplete multi-byte UTF-8 character sequence (e.g. '%E2%82' without the final byte). Our tool detects and pinpoints these invalid sequences without crashing.

How does URL encoding handle international Unicode characters?

Per RFC 3986 guidelines, characters outside the US-ASCII range are first encoded into their UTF-8 byte representation (ranging from 1 to 4 bytes per character). Each UTF-8 byte is then converted into a corresponding '%XX' percent-encoded sequence. For example, the Euro symbol '€' has UTF-8 bytes 0xE2 0x82 0xAC, which encodes to '%E2%82%AC'.

Utility #23Client-Side JavaScript

URL Encoder & Decoder

Encode and decode URLs, URI components, and query parameters instantly in your browser. Supports standard RFC 3986 percent-encoding, form-urlencoded formatting, and query parameter parsing.

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URL Percent-Encoding Guide

Understanding RFC 3986 URI Percent-Encoding

Uniform Resource Identifiers (URIs) rely on a limited character set to maintain consistency across heterogeneous network topologies. Learn when to encode full URLs versus query components, and how to avoid encoding bugs.

1. Component vs Full URL

Use Component mode (encodeURIComponent) when inserting values into query parameters. Use Full URL mode (encodeURI) only when encoding a complete address to preserve structural delimiters like https:// and ?.

2. Spaces: %20 vs Plus (+)

Standard RFC 3986 encodes spaces strictly as %20. The plus sign + is an older HTML form encoding artifact (application/x-www-form-urlencoded). Using %20 prevents misinterpretation in URL path segments.

3. Multi-Byte UTF-8

International characters and emojis are converted to their UTF-8 byte sequences prior to percent-encoding. For example, the accented character é occupies two bytes (0xC3 0xA9) and encodes to %C3%A9.

Common Reserved Characters Reference Table

Character	Percent-Encoded	URI Role / Meaning	Encoding Rule
&	%26	Query parameter separator	Must encode when used inside a parameter value.
=	%3D	Key-value assignment delimiter	Must encode in values (e.g. base64 padding).
?	%3F	Query string prefix	Must encode if part of a search term.
/	%2F	Path hierarchy separator	Must encode if passing URLs inside query parameters.
#	%23	Fragment anchor identifier	Unencoded # terminates the query string immediately.

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Frequently Asked Questions

URL percent-encoding (defined by RFC 3986) is a mechanism for converting characters that are either reserved (like '/', '?', '&', '=') or non-ASCII (such as spaces or Unicode characters) into a US-ASCII safe representation consisting of a '%' followed by two hexadecimal digits representing the byte value. Without encoding, characters like '&' and '=' would disrupt URL query string parsing on web servers.