Any difference between type assertions and the newer `as` operator in TypeScript?

160
votes

Is there any difference between what the TypeScript spec calls a type assertion:

var circle = <Circle> createShape("circle");

And the newer as operator:

var circle = createShape("circle") as Circle;

Both of which are typically used for compile-time casting?

2
castingtypescript

2 Answers

190
votes

The difference is that as Circle works in TSX files, but <Circle> conflicts with JSX syntax. as was introduced for this reason.

For example, the following code in a .tsx file:

var circle = <Circle> createShape("circle");

Will result in the following error:

error TS17002: Expected corresponding JSX closing tag for 'Circle'.

However, as Circle will work just fine.

Use as Circle from now on. It's the recommended syntax.

41
votes

From Wiki page: "What's new in TypeScript [1.6]":

New .tsx file extension and as operator

TypeScript 1.6 introduces a new .tsx file extension. This extension does two things: it enables JSX inside of TypeScript files, and it makes the new as operator the default way to cast (removing any ambiguity between JSX expressions and the TypeScript prefix cast operator). For example:

var x = <any> foo; 
// is equivalent to:
var x = foo as any;