decodeURIComponent using TextDecoder

decodeURIComponent.js

// Implementation of the standard decodeURIComponent using TextDecoder.
// The original JavaScript specification does not separate UTF-8 decoding this way.
// The purpose is to show the possible implementation in JavaScript language.

function hexDigit(code) {
  if (code >= '0'.charCodeAt(0) && code <= '9'.charCodeAt(0)) {
    return code - '0'.charCodeAt(0);
  } else if (code >= 'A'.charCodeAt(0) && code <= 'F'.charCodeAt(0)) {
    return code + (10 - 'A'.charCodeAt(0));
  } else if (code >= 'a'.charCodeAt(0) && code <= 'f'.charCodeAt(0)) {
    return code + (10 - 'a'.charCodeAt(0));
  } else {
    throw new URIError();
  }
}

function decodeURIComponent(string) {
  const input = String(string);
  let result = '';
  let k = 0;
  while (k < input.length) {
    const code = input.charCodeAt(k);
    if (code !== '%'.charCodeAt(0)) {
      result += String.fromCharCode(code);
      k += 1;
    } else {
      let octets = [];
      while (input.charCodeAt(k) === '%'.charCodeAt(0)) {
        if (k + 3 > input.length) {
          throw new URIError();
        }
        let octet = hexDigit(input.charCodeAt(k + 1)) * 16 + hexDigit(input.charCodeAt(k + 2));
        octets.push(octet);
        k += 3;
      }
      const textDecoder = new TextDecoder(undefined, {fatal: true, ignoreBOM: true});
      try {
        result += textDecoder.decode(Uint8Array.from(octets));
      } catch (error) {
        throw new URIError(error);
      }
    }
  }
  return result;
}