The as operator

The as operator

Recall how to write a type assertion:

var foo = <foo>bar;

Here we are asserting the variable bar to have the type foo. Since TypeScript also uses angle brackets for type assertions, JSX’s syntax introduces certain parsing difficulties. As a result, TypeScript disallows angle bracket type assertions in .tsx files.

To make up for this loss of functionality in .tsx files, a new type assertion operator has been added: as. The above example can easily be rewritten with the as operator.

var foo = bar as foo;

The as operator is available in both .ts and .tsx files, and is identical in behavior to the other type assertion style.