"this is undefined" in static function

Question

class Example{
    static foo():string{
        return this.bar();
    }

    static bar():string{
        return "bar";
    }
}

class Broken{
    static breaksThis(callback: () => string):string{
        return callback();
    }
}

console.log(Example.foo()); // works
console.log(Broken.breaksThis(Example.foo)); // Error at line 3: "this is undefined"

An interactive Example can be found here.

I would like to understand why the first log works as intended but the second fails. And how could I fix it?

Answer 1

You are abusing this in a static method of a class.
Try this in order to fix:

class Example {
    static foo(): string {
        return Example.bar(); /* Here is the fix */
    }

    static bar(): string {
        return "bar";
    }
}

class Broken {
    static breaksThis(callback: () => string): string {
        return callback();
    }
}

console.log(Example.foo()); // works
console.log(Broken.breaksThis(Example.foo));