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

Source: stackoverflow.com

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