euler002.js

function genSeqUntil(valFunc, limitPredicate, seq = []) {
    var nextVal = valFunc(seq.length);
    if (limitPredicate(seq.length, nextVal)) {
      seq[seq.length] = nextVal;
      return genSeqUntil(valFunc, limitPredicate, seq);
    }
    return seq;
}
const memoize = (fn) => (function () {
      var cache = {};
      return function () {
        var strArgs = JSON.stringify(arguments);
        if (!(strArgs in cache)) {
          cache[strArgs] = fn.apply(undefined, arguments);
        }
        return cache[strArgs];
      }
  }()),
  fibby = memoize((num) =>
    num <= 1 ? num : fibby(num - 1) + fibby(num - 2));

genSeqUntil(fibby, (i, v) => v < 4000000)
  .filter((num) => num % 2 === 0)
  .reduce((accum, v) => accum + v, 0);

(num) => num <= 1 ? num : fibby(...)
I'd throw a ternary on line 22