Takes a code_verifier string and makes a legal code_challenge out of it

pkce_code_challenge.js

const createCodeChallenge = ( codeVerifier ) => {
  if ( typeof window !== 'undefined' && !!( window.crypto ) && !!( window.crypto.subtle ) ) {
    return new Promise( ( resolve, reject ) => {
      let codeVerifierCharCodes = textEncodeLite( codeVerifier );
      crypto.subtle
        .digest( 'SHA-256', codeVerifierCharCodes )
        .then(
          hashedCharCodes => resolve( urlSafe( new Uint8Array(hashedCharCodes) ) ),
          error => reject( error )
        );
    });
  }
}

const textEncodeLite = ( str ) => {
  const charCodeBuffer = new Uint8Array( str.length );
  for ( let i = 0; i < str.length; i++ ) {
   charCodeBuffer[i] = str.charCodeAt( i );
  }
  return charCodeBuffer;
}

const urlSafe = ( buffer ) => {
  const encoded = base64.fromByteArray( new Uint8Array( buffer ) );
  return encoded.replace( /\+/g, '-' ).replace( /\//g, '_' ).replace( /=/g, '' );
}