encodeURIComponent()

uriComponent

A string to be encoded as a URI component (a path, query string, fragment, etc.). Other values are converted to strings.

A new string representing the provided uriComponent encoded as a URI component.

URIError

Thrown if uriComponent contains a lone surrogate.

encodeURIComponent() is a function property of the global object.

encodeURIComponent() uses the same encoding algorithm as described in encodeURI(). It escapes all characters except:

A–Z a–z 0–9 - _ . ! ~ * ' ( )

Compared to encodeURI(), encodeURIComponent() escapes a larger set of characters. Use encodeURIComponent() on user-entered fields from forms POST'd to the server — this will encode & symbols that may inadvertently be generated during data entry for character references or other characters that require encoding/decoding. For example, if a user writes Jack & Jill, without encodeURIComponent(), the ampersand could be interpreted on the server as the start of a new field and jeopardize the integrity of the data.

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 +.

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

const fileName = "my file(2).txt";
const header = `Content-Disposition: attachment; filename*=UTF-8''${encodeRFC5987ValueChars(
  fileName,
)}`;

console.log(header);
// "Content-Disposition: attachment; filename*=UTF-8''my%20file%282%29.txt"

function encodeRFC5987ValueChars(str) {
  return (
    encodeURIComponent(str)
      // The following creates the sequences %27 %28 %29 %2A (Note that
      // the valid encoding of "*" is %2A, which necessitates calling
      // toUpperCase() to properly encode). Although RFC3986 reserves "!",
      // RFC5987 does not, so we do not need to escape it.
      .replace(
        /['()*]/g,
        (c) => `%${c.charCodeAt(0).toString(16).toUpperCase()}`,
      )
      // 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, (str, hex) =>
        String.fromCharCode(parseInt(hex, 16)),
      )
  );
}

The more recent RFC3986 reserves !, ', (, ), and *, even though these characters have no formalized URI delimiting uses. The following function encodes a string for RFC3986-compliant URL component format. It also encodes [ and ], which are part of the IPv6 URI syntax. An RFC3986-compliant encodeURI implementation should not escape them, which is demonstrated in the encodeURI() example.

function encodeRFC3986URIComponent(str) {
  return encodeURIComponent(str).replace(
    /[!'()*]/g,
    (c) => `%${c.charCodeAt(0).toString(16).toUpperCase()}`,
  );
}

A URIError will be thrown if one attempts to encode a surrogate which is not part of a high-low pair. For example:

// High-low pair OK
encodeURIComponent("\uD800\uDFFF"); // "%F0%90%8F%BF"

// Lone high-surrogate code unit throws "URIError: malformed URI sequence"
encodeURIComponent("\uD800");

// Lone high-surrogate code unit throws "URIError: malformed URI sequence"
encodeURIComponent("\uDFFF");

You can use String.prototype.toWellFormed(), which replaces lone surrogates with the Unicode replacement character (U+FFFD), to avoid this error. You can also use String.prototype.isWellFormed() to check if a string contains lone surrogates before passing it to encodeURIComponent().

• decodeURI()
• encodeURI()
• decodeURIComponent()