JavaScript Reference


The encodeURIComponent() method encodes a Uniform Resource Identifier (URI) component by replacing each instance of certain characters by one, two, three, or four escape sequences representing the UTF-8 encoding of the character (will only be four escape sequences for characters composed of two "surrogate" characters).




String. A component of a URI.


encodeURIComponent escapes all characters except the following: alphabetic, decimal digits, - _ . ! ~ * ' ( )

Note that an URIError will be thrown if one attempts to encode a surrogate which is not part of a high-low pair, e.g.,

// high-low pair ok

// lone high surrogate throws "URIError: malformed URI sequence"

// lone low surrogate throws "URIError: malformed URI sequence"

To avoid unexpected requests to the server, you should call encodeURIComponent on any user-entered parameters that will be passed as part of a URI. For example, a user could type "Thyme &time=again" for a variable comment. Not using encodeURIComponent on this variable will give comment=Thyme%20&time=again. Note that the ampersand and the equal sign mark a new key and value pair. So instead of having a POST comment key equal to "Thyme &time=again", you have two POST keys, one equal to "Thyme " and another (time) equal to again.

For application/x-www-form-urlencoded, spaces are to be replaced by '+', so one may wish to follow a encodeURIComponent replacement with an additional replacement of "%20" with "+".

To be more stringent in adhering to RFC 3986 (which reserves !, ', (, ), and *), even though these characters have no formalized URI delimiting uses, the following can be safely used:

function fixedEncodeURIComponent (str) {
  return encodeURIComponent(str).replace(/[!'()*]/g, function(c) {
    return '%' + c.charCodeAt(0).toString(16);


The following example provides the special encoding required within UTF-8 Content-Disposition and Link server response header parameters (e.g., UTF-8 filenames):

var fileName = 'my file(2).txt';
var header = "Content-Disposition: attachment; filename*=UTF-8''" 
             + encodeRFC5987ValueChars(fileName);

// logs "Content-Disposition: attachment; filename*=UTF-8''my%20file%282%29.txt"

function encodeRFC5987ValueChars (str) {
    return encodeURIComponent(str).
        // Note that although RFC3986 reserves "!", RFC5987 does not,
        // so we do not need to escape it
        replace(/['()]/g, escape). // i.e., %27 %28 %29
        replace(/\*/g, '%2A').
            // The following are not required for percent-encoding per RFC5987, 
            // so we can allow for a little better readability over the wire: |`^
            replace(/%(?:7C|60|5E)/g, unescape);

 Created by Mozilla Contributors and licensed under CC-BY-SA 2.5