What does a variable directly followed by empty curly braces mean in Javascript?

Question

Going through the Eloquent Javascript book where I encountered something I haven't seen before.

In the code snippet below the variable 'map' is followed by empty curly braces. Can someone please explain what this means? Does this do anything tot he function that follows.

Also, can someone explain what map[event] = phi; does exactly? I take it this map refers to the variable 'map' we declared in the first line...

var map = {};
function storePhi (event, phi) {
  map[event] = phi;
}

storePhi("pizza", 0.069);

Answer 1

The {} represents an empty object .

map[event] = phi will add (or overwrite) a property on the map object with the name event and assign it to a value of phi . This way, you can do map.EVENT_NAME to get the value of phi for that event.

After performing storePhi("pizza", 0.069); , map will look like this:

console.log(map);
map = {
  pizza: 0.069
}

console.log(map.pizza);
map.pizza = 0.069

Answer 2

It means the variable is a dictionary that stores key value pairs. The subscript or the value within the [] brackets is the key and the value on the right side is the value.