isBase64.js

function isBase64(value){
  if (value == "")return false;
  for (let i = 0; i<value.length; ){
    const x = value.slice(i,i+=4);
    if (i==value.length-1 && !x.match(/^[a-zA-Z0-9]+={0,4}$/))return false;
    if (i<value.length-1 && !x.match(/^[a-zA-Z0-9]+$/))return false;
    if (x.length != 4)return false;
  }
  return true
}