What is the type assertion operator for in TypeScript?

Question

The spec doesn't say much about where the type assertion operator might be helpful in TypeScript. I didn't need it in my code. So I am curious what sort of problems it is supposed to solve. Any ideas?

Answer 1

It's somewhat like type casting however as it does not come with runtime support (its a compile time assertion only) TypeScript choses to call it 'Type Assertion'. Consider this example :

var element1 = document.getElementById('canvas'); // Determined to be HTMLElement
element1.getContext('2d'); // ERROR as it is HTMLElement 



// Determined to be canvas due to your assertion
var element2 = <HTMLCanvasElement>document.getElementById('canvas'); 
element2.getContext('2d'); // Valid 

You will need it whenever typescript type inference would prevent you to assign things around due to incompatible inferred types.

Answer 2

There are two forms of Typescript type assertions. Examples from the Typescript handbook here.

1. Angle bracket syntax for type assertions (doesn't work within a tsx file):

let strLength: number = (<string>someValue).length;

2. as syntax for type assertions (works within ts or tsx alike):

let strLength: number = (someValue as string).length;

Type assertions override the (limited) type inference capabilities of the ever current version of Typescript, which can be a good thing if you are correct, but there is also a risk that your judgment is incorrect and TS will believe you. If there's a likely mismatch between what you infer and what TS infers, TS will still balk, then you can more strongly overrule the TS inference by as unknown as string.