Learn TypeScript 3 by Building Web Applications

Late binding and TypeScript definite assignment assertions

If you take a close look at the code for the previous UserSettings example component, you may notice a weird exclamation mark:

@Inject("username") 
username!: string; // definite assignment assertion 

Now, what is it and why did we need it?

As the official TypeScript documentation (https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html#definite-assignment-assertions) states, definite assignment assertions can be used to tell the compiler that variables will indeed be assigned, even if TypeScript can't determine this on its own.

The definite assignment assertion is used through the ! character, and is defined after a property or variable declaration.

If we consider ...