Typescript: adding only an optional property removes type checking

Question

I'm using typescript 1.4.1 and have the following code:

var hello: {message: string};
hello = {world: 't'};

As intended, the typescript compiler outputs an error:

Type '{ world: string; }' is not assignable to type '{ message: string; }'. Property 'message' is missing in type '{ world: string; }'. (var) hello: { message: string; }

But if the type contains only an optional property such as in:

var hello: {message?: string};
hello = {world: 't'};

Then the compiler is perfectly happy, but I would like the error to be detected.

What am'I doing wrong? or is it a typescript bug?

Thanks for your help!

Answer 1

Then the compiler is perfectly happy, but I would the error to be detected. What am'I doing wrong? or is it a typescript bug?

No this is by design. extra information is okay as long as the required information (in this case nothing) is present.