Why is this lambda function not an error in typescript?

Question

I imagined this code

let x: (a: { b: number }) => void = (a: { b: number, c: string }) => { alert(a.c) };
x({ b: 123 });

should produce an error, since the lambda function requires an additional property on the a argument, so the signatures should not be compatible. But trying this in the latest typescript playground does not produce any errors! Why is that?

Answer 1

When you said

let x: (a: { b: number }) => void

You are saying "x is a function that takes an object with a b key."

You then assigned it a lambda that took an object with a b key and a c key. That passes the typechecker because any object that gets passed in must have a b key.

Then you pass it an object with a b key. There's no error.

If you want it to fail because of the c key, make the type of x the following:

let x: (a: { b: number, c: string }) => void