Undefined compare slow in Firefox

esIterTest.js

// Interestingly, the compare with undefined below is about ~10x slower than the typeof compare in Firefox. 
// See this by running myTiming() and myTimingUndefinedCompare()

class MyArrayIterator {

  constructor(source) {
    this._index = 0;
    this._source = source;
  }

  next() {
    "use strict";
    if (this._index >= this._source.length) {
      return undefined;
    }
    return this._source[this._index++];
  }
}


function mySum(it) {
    "use strict";

  let v;
  let r = 0;
  while (typeof (v = it.next()) !== 'undefined') {
    r += v;
  }
  return r;
}

function mySumUndefinedCompare(it) {
    "use strict";

  let v;
  let r = 0;
  while ((v = it.next()) !== undefined) {
    r += v;
  }
  return r;
}
  
function makeData() {
  let data = [];
  for (let i = 0; i < 1000000; ++i) {
    data.push(Math.random());
  }
  return data;
}

var data = makeData();

function myTiming() {
    "use strict";

  let it = new MyArrayIterator(data);
  let t1 = performance.now();
  let r = mySum(it);
  let t2 = performance.now();
  return [r, t2 - t1];
}

function myTimingUndefinedCompare() {
    "use strict";

  let it = new MyArrayIterator(data);
  let t1 = performance.now();
  let r = mySumUndefinedCompare(it);
  let t2 = performance.now();
  return [r, t2 - t1];
}