TypeScript function return type based on input parameter

Question

I have a few different interfaces and objects that each have a type property. Let\'s say these are objects stored in a NoSQL db. How can I create a generic

Answer 1

Conditional Types to the rescue:

interface Circle {
    type: "circle";
    radius: number;
}

interface Square {
    type: "square";
    length: number;
}

type TypeName = "circle" | "square"; 

type ObjectType = 
    T extends "circle" ? Circle :
    T extends "square" ? Square :
    never;

const shapes: (Circle | Square)[] = [
    { type: "circle", radius: 1 },
    { type: "circle", radius: 2 },
    { type: "square", length: 10 }];

function getItems(type: T) : ObjectType[]  {
    return shapes.filter(s => s.type == type) as ObjectType[];
}

const circles = getItems("circle");
for (const circle of circles) {
    console.log(circle.radius);
}

Thanks Silvio for pointing me in the right direction.