TypeScript override ToString() [closed]

Question

Let's say I have a class Person which looks like this:

class Person {     constructor(         public firstName: string,         public lastName: string,         public age: number     ) {} } 

I have overridden the toString method as follows.

public toString(): string {     return this.firstName + ' ' + this.lastName; } 

Now I expect to be able to do the following, which works in runtime:

function alertMessage(message: string) {     alert(message); }  alertMessage(new Person('John', 'Smith', 20)); 

But this still gives me this error:

TS2345: Argument of type 'Person' is not assignable to parameter of type 'string'.

How can I assign a Person to this string argument?

Answer 1

Overriding toString works kind of as expected:

class Foo {     private id: number = 23423;     public toString = () : string => {         return `Foo (id: ${this.id})`;     } }  class Bar extends Foo {    private name:string = "Some name";     public toString = () : string => {         return `Bar (${this.name})`;     } }  let a: Foo = new Foo(); // Calling log like this will not automatically invoke toString console.log(a); // outputs: Foo { id: 23423, toString: [Function] }  // To string will be called when concatenating strings console.log("" + a); // outputs: Foo (id: 23423) console.log(`${a}`); // outputs: Foo (id: 23423)  // and for overridden toString in subclass.. let b: Bar = new Bar(); console.log(b); // outputs: Bar { id: 23423, toString: [Function], name: 'Some name' } console.log("" + b); // outputs: Bar (Some name) console.log(`${b}`); // outputs: Bar (Some name)  // This also works as expected; toString is run on Bar instance.  let c: Foo = new Bar(); console.log(c); // outputs: Bar { id: 23423, toString: [Function], name: 'Some name' } console.log("" + c); // outputs: Bar (Some name) console.log(`${c}`); // outputs: Bar (Some name) 

What can sometimes be an issue though is that it is not possible to access the toString of a parent class:

console.log("" + (new Bar() as Foo)); 

Will run the toString on Bar, not on Foo.