JSFiddle - React, Tailwind, and code Playground

JavaScript

var length = 2;
var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789".split('');
console.log(combine(length));

function combine(length, level, previous)
{
  var level = level || 1;
  var previous = previous || possible;
  var ret = [];
   
  previous.map(function(el, ind) {
     possible.map(function(letter) {
        ret.push(el + letter);
     });
  });

  if (level < length - 1) ret = combine(length, level + 1, ret);
     
  return ret;
}