Self Driving Car Exercise - Step 29 - Notation

Hi, people

I’m taking the typescript course and in the self-driving car exercise I couldn’t understand the notation used in the step #29.

In brief, there’s an object with two keys and randonmly generated booleans assigned to them.
Ex:

example = {
key1: value1,
key2: value2
}

When the instructor assess those keys to find out if they are truthy, it codes:
!example[key1]

My question is: why can’t I use: !example.key1