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

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 = 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.