The use and non-use of the word function in TypeScript

Why when declare method in a class the compiler beeps, stating that the use of the word function is wrong.

In previous lessons from my learning to create a function, I first signed it as function .

abstract class Empregado {    
    abstract getFuncao():string;
}

class Professor extends Empregado {
    getFuncao():string {
        return "Professor";
    }
}

class Diretor extends Empregado {
    getFuncao():string {
        return "Diretor";
    }
}