this is what I want to achieve:
interface IFoo { ... }
function extendWithFoo<T extends {}>(x: T = {}): T & IFoo {
...
}
I'm getting error TS2322: Type '{}' is not assignable to type 'T'.
Is there a way to do this?
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2 Answers
You can do this:
function extendWithFoo<T extends {}>(x: T = {} as T): T & IFoo {
...
}
But using empty objects is problematic, as it accepts everything:
extendWithFoo(3); // ok
extendWithFoo("string"); // ok
extendWithFoo(true); // ok
extendWithFoo({}); // ok
extendWithFoo({ key: "value" }); // ok
extendWithFoo(new Array()); // ok
So my recommendation is to use something more specific.
In any case, you don't really need it, you can just:
function extendWithFoo<T>(x: T = {} as T): T & IFoo {
...
}
Which gives you the same thing.
Comments
In addition to Nitzan Tomer's suggestion, you can also introduce a type to restrict the input to object literals only
type ObjectLiteral = { [key: string]: any };
interface IFoo {
resultHasFoo(): void;
}
function extendWithFoo<T extends ObjectLiteral>(x: T = {} as T): T & IFoo {
return x as T & IFoo;
}
extendWithFoo(false); // error!
extendWithFoo(123); // error!
var p = extendWithFoo({
test: "one"
});
p.resultHasFoo(); // works!
Take a look at my related post... JavaScript to TypeScript: Intellisense and dynamic members
