PKCE Code Verifier and Code Challenge Generator without externals libs (vanilla js). Demo : https://codepen.io/bilelz/full/YzNozBe

pkce-generator_vanilla.js

function generateCodeVerifier() {
  return Array(128)
      .fill('0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-._~')
      .map(x => x[Math.floor(crypto.getRandomValues(new Uint32Array(1))[0] / (0xffffffff + 1) * x.length)])
      .join('');
}

async function generateCodeChallenge(code_verifier) {
  const encoder = new TextEncoder();
  const data = encoder.encode(code_verifier);
  const buffer = await window.crypto.subtle.digest('SHA-256', data);
  //ArrayBuffer to base64 https://gist.github.com/jonleighton/958841#gistcomment-2915919
  return btoa(new Uint8Array(buffer).reduce((data, byte) => {
    return data + String.fromCharCode(byte);
  }, '')).replace(/=/g, '').replace(/\+/g, '-').replace(/\//g, '_');
}