title: Why does using '=' in my if statement return undefined?
body: <p>When I use '=' in my if statement I get undefined, but when I use '===' I get my expected result. Can someone explain what's happening there?</p>
  <pre><code>function once(callback) {
    let counter = 0;
    let result;
    function sum(x) {
      if (counter === 0) {
        result = callback(x);
        counter++;
      }
      return result;
    }
    return sum;
  }
  const addByTwoOnce = once(function(num) {
    return num + 2;
  });
  
  // UNCOMMENT THESE TO TEST YOUR WORK!
  console.log(addByTwoOnce(5));  //should log 7
  console.log(addByTwoOnce(10));  //should log 7
  console.log(addByTwoOnce(9001));  //should log 7
  </code></pre>