Using Objects for Lookups Dot Notation vs Bracket Notation

Tell us what’s happening:
My code does pass, but my question is why does it pass only when I use bracket notation. At first, I used the dot notation and it won’t let me pass. I googled and found that they both should have work, so I’m just curious why that is. Is it a bug or is there a reason to why a dot notation will not work in this situation. Thank you for your time and help.

Your code so far

// Setup
function phoneticLookup(val) {
  var result = "";

  // Only change code below this line
  var lookup = {
    "alpha": "Adams",
    "bravo": "Boston",
    "charlie": "Chicago",
    "delta": "Denver",
    "echo": "Easy",
    "foxtrot": "Frank"
  result = lookup[val];
  // Only change code above this line
  return result;

// Change this value to test

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Link to the challenge:

Thanks for clarifying this up for me.