TypeScript override ToString() [closed]
Solution 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(`${c}`); // outputs: Bar (Some name)
What can sometimes be an issue though is that it is not possible to access the toString
of a parent class:
console.log("" + (new Bar() as Foo));
Will run the toString on Bar, not on Foo.
Solution 2:
As pointed out by @Kruga, the example actually seemed to work in runtime JavaScript. The only problem with this is that TypeScript shows a type error.
TS2345: Argument of type 'Person' is not assignable to parameter of type 'string'.
To resolve this message, you must either:
- Call
.toString()
explicitly - Or concatenate the object with a string (e.g.
`${obj}`
orobj + ''
) - Or use
obj as any
(not recommended as you will lose type safety)