In Typescript I have two classes, Foo and Bar
export class Foo{
data: Bar<Types>
}
export class Bar<T>{
attributes: T
}
Next to that I also have a type
export type Types = Circle | Square
Next up I declare my variable
x: Bar<Circle>
But when I want to assign a value to this variable by doing
x = someRandonJsonData
I get the following type error:
Type 'Bar<Types>' is not assignable to type 'Bar<Circle>'.
I'm wondering where my mistake is in all of this ?
Short version:
let x: Bar<Circle>;
let z: Bar<Types>;
x = z;
The problem is that z is a wider type, because it could be the equivalent of Bar<Circle> or Bar<Square> - and the latter is not compatible; so you need to narrow it with a type guard:
function isCircle(item: Bar<Types>): item is Bar<Circle> {
return (item.attributes.hasOwnProperty('radius'));
}
if (isCircle(z)) {
x = z;
}
Here's a full example:
interface Circle {
radius: number;
}
interface Square {
width: number;
}
type Types = Circle | Square
class Foo{
data: Bar<Types>
}
class Bar<T>{
attributes: T
}
let x: Bar<Circle> = {
attributes: {
radius: 4
}
};
let y: Bar<Square> = {
attributes: {
width: 4
}
};
let z: Bar<Types> = {
attributes: {
radius: 4
}
};
// Error, as it may not be compatible
x = z;
function isCircle(item: Bar<Types>): item is Bar<Circle> {
return (item.attributes.hasOwnProperty('radius'));
}
if (isCircle(z)) {
// Works
x = z;
}
someRandonJsonData. If it isBar<Types>this error is expected.. and it's there for the same reason you can't assign a value of typeTypesto a variable of typeCircleBar<Types>. So how would I be able to make myBarproperty attributes to either beCircleorSquare?