In the context of TypeScript, attempting to directly cast a Parent instance to a Child can be problematic. The compiler lacks enough information to confirm that the Parent instance is indeed an instance of Child. To address this issue, it is recommended to utilize type guards instead of explicit casting:
class Parent {
prop: string|null = null;
isChild(): this is Child {
return (this as any).otherProp !== undefined;
}
}
class Child extends Parent {
otherProp: string|null = null;
}
function test(p: Parent) {
if (p.isChild()) {
console.log(p.otherProp);
}
}
By utilizing a type guard like in the example above, within the guarded conditional statement, TypeScript recognizes p as a Child, while outside of it, p remains treated as a Parent.
An alternative approach involves overriding the type guard in the subclass, eliminating the need for explicit typecasting entirely:
class Parent {
prop: string|null = null;
isChild(): this is Child {
return false;
}
}
class Child extends Parent {
otherProp: string|null = null;
isChild(): this is Child {
return true;
}
}
function test(p: Parent) {
if (p.isChild()) {
console.log(p.otherProp);
}
}
The reasoning behind this behavior by TypeScript developers regarding whether it's intentional or accidental remains unconfirmed. It may serve as a safety measure to detect potential errors, but concrete documentation on this specific behavior is unavailable.
If you prefer a cleaner syntax, explicitly define the type of this in the castToChild method to streamline the process of converting to a Child:
class Parent {
prop: string|null = null;
castToChild(this: Parent): Child {
return this as Child;
}
}
class Child extends Parent {
otherProp: string|null = null;
}
function foo() {
let p: Parent = new Child();
let c = p.castToChild();
console.log(c.otherProp);
}
This modification essentially performs a double cast operation, albeit with a cleaner appearance. Nevertheless, it highlights the quirks in behavior wherein without specifying the this parameter, this defaults to Parent and becomes non-castable, whereas with the specified parameter, the casting operation succeeds.