What is the difference between <type>variable and variable as type in Typescript

Issue

In Typescript there is two way to cast objects.

<type>variable

and

variable as type

Both of these seems to be permanent (compile-time) casting so the casted object will retain it's type. E.g:

let number: number = 2;

let string1 = <string>number;
let string2 = number as string;

string1; // -> the type is string
string1; // -> the type is string

My question: What is the difference between the two?

Solution

What is the difference between the two?

They are identical. x as Y was later created as an alternative syntax for <Y>x to enable non-ambiguous type-assertions in TSX files (as <Y>x would be otherwise recognized as a JSX expression). The x as Y syntax is preferred.

---

However, you have an apparent misconception here. This is not type-casting, but a compile-time type-assertion. The difference between the two is best demonstrated by what happens at runtime:

let number: number = 2;
let string = number as string;

console.log(typeof string); // "number"

TypeScript does not have a runtime framework to do automatic type-casting and type-checking, this is a design goal. (seriously?)

Answered By - John Weisz