Exclamation Mark in type definition

Question

Currently I stumbled upon the ContentChildren decorator of Angular. In the first code example the following syntax is used:

Answer 1

My question is now whether the exclamation mark before a type definition has exactly the same meaning like in a normal context.

No that's actually not the same thing, in this context it does something different. Normally when you declare a member (which doesnt' include undefined in it's type) it has to be initialized directly or in the constructor. If you add ! after the name, TypeScript will ignore this and not show an error if you don't immediately initialize it:

class Foo {
  foo: string; // error: Property 'foo' has no initializer and is not definitely assigned in the constructor.
  bar!: string; // no error
}

The same thing actually applies to local variables as well:

let foo: string;
let bar!: string;

console.log(foo); // error: Variable 'foo' is used before being assigned.
console.log(bar); // no error

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