Is there a more elegant solution for creating an object of type:
type ArrayWithA = [number, number, number] & { a: string };
I accomplished this by:
const obj : any = [1, 2, 3];
obj.a = "foo";
const arrayWithA : ArrayWithA = obj as ArrayWithA;
Inquiry: How can we achieve the same without resorting to using any?
Bonus Inquiry: What is a recommended approach for initializing an object of type:
type FuncWithA = ((string)=>void) & { a: string }A suggestion I would make is to utilize the Object.assign() method, which is represented in TypeScript's standard library as returning an intersection type similar to what you require. However, there is a bit of complexity in getting the compiler to infer that an array literal will be exactly like the type [number, number, number]. If you are fine with using
readonly [number, number, number]const assertion:
type ArrayWithA = readonly [number, number, number] & { a: string };
const arrayWithA: ArrayWithA = Object.assign([1, 2, 3] as const, { a: "foo" });
If not, there are several techniques you can try:
type ArrayWithA = [number, number, number] & { a: string };
const arr: [number, number, number] = [1, 2, 3]; // annotate extra variable
let arrayWithA: ArrayWithA = Object.assign(arr, { a: "foo" });
// type assertion
arrayWithA = Object.assign([1, 2, 3] as [number, number, number], { a: "foo" });
// helper function
const asTuple = <T extends any[]>(arr: [...T]) => arr;
arrayWithA = Object.assign(asTuple([1, 2, 3]), { a: "foo" });
When it comes to functions, you can apply the same principle with Object.assign():
type FuncWithA = ((x: string) => void) & { a: string }
let funcWithA: FuncWithA = Object.assign(
(x: string) => console.log(x.toUpperCase()),
{ a: "foo" }
);
Alternatively, you could just use a function statement and add the property later, especially since TypeScript 3.1 introduced expando functions:
function func(x: string) {
console.log(x);
}
func.a = "foo"; // no error
funcWithA = func; // works perfectly fine
Utilize Object.assign!
The result type of Object.assign is essentially a blend of the types from its arguments. This allows you to easily create hybrid objects by specifying their components and merging them in one go, instead of adding extra properties afterwards (which may require type conversions).
For example, you can achieve this for some scenarios like so:
type ArrayWithA = [number, number, number] & { a: string };
// Example usage:
// Ensure correct type inference by explicitly casting it as [number, number, number].
const obj2 = Object.assign([1, 2, 3] as [number, number, number], {a: "foo"}); // obj2 is now of type: [number, number, number] & { a: string } and can be assigned to ArrayWithA.
// Same principle applies to functions!
type FuncWithA = ((arg: string) => void) & { a: string };
const f1 = Object.assign((s: string) => {}, {a: "foo"}) // f1 is now of type: ((s: string) => void) & { a: string } and can be assigned to FuncWithA.
If I were to choose, my code snippet would look something like this:
type AArrayOfNumbers = [number, number, number] & { a: string };
namespace AArrayOfNumbers {
export function create(a: string, ...values: [number, number, number]): AArrayOfNumbers {
const obj = values as AArrayOfNumbers;
obj.a = a;
return obj;
}
}
const arrayObjectWithA = AArrayOfNumbers.create('a', 1, 2, 3);
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