Here's something interesting. You don't have to put a value into a let box right away. You can create an empty box now and fill it later:
let favoriteSong;
console.log(favoriteSong);
favoriteSong = "Happy Birthday";
console.log(favoriteSong);It prints
undefined Happy Birthday
Whoa, what's undefined?
When you create a variable without giving it a value, JavaScript puts a special value called undefined inside. It means "this box exists, but nothing has been put in it yet." It's JavaScript's way of saying "¯\\\_(ツ)\_/¯".
This gives us some useful vocabulary:
- Declaring a variable means creating the box:
let favoriteSong; - Assigning a value means putting something in the box:
favoriteSong = "Happy Birthday"; - Initializing means doing both at the same time:
let favoriteSong = "Happy Birthday";
Watch outUsing a box that doesn't exist
undefined (the box exists but is empty) is different from a variable that was never created at all:
console.log(favoriteMovie);It stops with an error
Uncaught ReferenceError: favoriteMovie is not defined
"Not defined" means "there's no box with that label anywhere." This usually happens because of a typo. Check that the spelling and capital letters match exactly where you created the variable!