Typescript: class implements interface issue?

Question

In Typescript 2.5.2 following does not work:

interface I { m?():void; } class C implements I { } let c = new C(); c.m(); <== Error : TS2339:Property 'm' does not exist on type 'C'.

Why is the m method not found?

If I add:

interface C extends I {}

It's ok! Or if I add:

let c = new C() as C & I;

It's ok too.

As I understand implements should combine C and I types as C & I. Is it an issue?

Interfaces are implemented abstract classes are extended. Interfaces are just a contract saying that any methods are going to be supported in the actual class. You need to implement m in class C — rjustin
– rjustin, Commented Sep 28, 2017 at 18:47

Fenton · Accepted Answer · 2017-09-28 18:48:26Z

The interface you have is a weak type that doesn't enforce the property m on implementers.

TypeScript is smart enough to see that m is definitely not on your class, but you can prevent the error by asking TypeScript to treat the variable as the interface:

let c: I = new C(); c.m();

Despite this trick, you'll still have a runtime problem, which TypeScript tried to warn you about.

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