gistfile1.txt

 function generateCodePackagePromise() {
    var base64Url = function(buffer) {
    /*\
    |*|
    |*|  Base64 / binary data / UTF-8 strings utilities (#1)
    |*|
    |*|  https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding
    |*|
    |*|  Author: madmurphy
    |*|
    \*/

    var uint6ToB64 = function(nUint6) {

    return nUint6 < 26 ?
    nUint6 + 65 :
    nUint6 < 52 ?
    nUint6 + 71 :
    nUint6 < 62 ?
    nUint6 - 4 :
    nUint6 === 62 ?
    43 :
    nUint6 === 63 ?
    47 :
    65;

    }
    var base64EncArr = function(aBytes) {

    var eqLen = (3 - (aBytes.length % 3)) % 3,
    sB64Enc = "";

    for (var nMod3, nLen = aBytes.length, nUint24 = 0, nIdx = 0; nIdx < nLen; nIdx++) {
    nMod3 = nIdx % 3;
    /* Uncomment the following line in order to split the output in lines 76-character long: */
    /*
    if (nIdx > 0 && (nIdx * 4 / 3) % 76 === 0) { sB64Enc += "\r\n"; }
    */
    nUint24 |= aBytes[nIdx] << (16 >>> nMod3 & 24);
    if (nMod3 === 2 || aBytes.length - nIdx === 1) {
    sB64Enc += String.fromCharCode(uint6ToB64(nUint24 >>> 18 & 63), uint6ToB64(nUint24 >>> 12 & 63), uint6ToB64(nUint24 >>> 6 & 63), uint6ToB64(nUint24 & 63));
    nUint24 = 0;
    }
    }

    return eqLen === 0 ?
    sB64Enc :
    sB64Enc.substring(0, sB64Enc.length - eqLen) + (eqLen === 1 ? "=" : "==");

    };


     const base64 = base64EncArr(new Uint8Array(buffer));
     const base64url_no_padding = base64.replace(/\+/g, '-')
        .replace(/\//g, '_')
        .replace(/=/g, '');
     
    return base64url_no_padding;
    }
    var gen128x8bitNonce = function() {
    var array = new Uint8Array(128); //( generate 1024bits 8*128
    window.crypto.getRandomValues(array);
    return array;
    };
    var code_verifier = gen128x8bitNonce();
    
    var base64_verifier = base64Url(code_verifier.buffer);
     const encoder = new TextEncoder();
     const base64_arraybuffer = encoder.encode(base64_verifier); 
    return crypto.subtle.digest("SHA-256", base64_arraybuffer).then(function(code_challenge){

    return {
      code_verifer: base64_verifier,
      code_challenge: base64Url(code_challenge),
      code_challenge_method: "S256"
      }
    });
    }

    generateCodePackagePromise().then(function(package) {
      package.code_challenge;
      package.code_challenge_method;
      package.code_verifer;
     })