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castingtypescript

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

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?

Solution

  • 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.