typescript exclamation mark
//Normally when you declare a member (which doesn't include undefined in it's type)
//it has to be initialized directly or in the constructor.
//Otherwise Typescript will raise an error.
//If you add ! after the name, TypeScript will ignore this and not show an error
//if you don't immediately initialize it:
//Example
foo: string; // error: Property 'foo' has no initializer and is not definitely assigned in the constructor.
bar!: string; // no error