Interfaces in Typescript fail to enforce explicit type checking on function arguments

Upon compiling the code below, I anticipated encountering TWO errors. However, to my surprise, Typescript compiled it flawlessly without any errors.

interface IFoo{
    Bar(callback: (arg:string) => void):void;
}

class Foo implements IFoo {
    public Bar(callback: () => void):void{
        callback();
    }
}

var foo: IFoo;
foo = new Foo();
foo.Bar(() => {
    console.log("Hi");
})

Expected Error 1: The method IFoo.Bar mandates a function argument that accepts a string parameter. Nonetheless, when implementing IFoo in the class Foo, the method Foo.Bar is defined with a function argument that lacks any parameters. This discrepancy should logically trigger a type error.

Expected Error 2: Variable foo is of type IFoo. When calling foo.Bar with a function argument that does not align with the specified parameters of Bar within IFoo, an error should ideally be thrown for violating the interface's definition.

Evidently, there seems to be a lapse in enforcing the function signature types both during implementation and invocation of methods defined by an interface in Typescript. Could someone shed light on why this code compiles without issue?

Answer №1

The answer to this question can be found in the FAQ section of TypeScript. Here is a summary of the explanation provided:

It is considered normal and expected behavior. To better understand why, take a look at the "substitutability" concept explained at the beginning of the FAQ -- essentially, an argument like `handler` can be used in place of `callback` because it can safely ignore any additional parameters.

Another scenario worth considering is demonstrated here:

let items = [1, 2, 3];
items.forEach(arg => console.log(arg));

This example mirrors the situation that was initially thought to be incorrect. During runtime, `forEach` calls the callback function with three arguments (value, index, array), even though often only one or two of these arguments are actually utilized. This practice is quite common in JavaScript coding patterns, as explicitly declaring every parameter would be cumbersome and unnecessary.

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