Type definition in object literal in TypeScript

Question

In TypeScript classes it\'s possible to declare types for properties, for example:

class className {
  property: string;
};

How do declare

Answer 1

If your properties have the same type, you could use predefined utility type Record :

type Keys = "property" | "property2"

const obj: Record<Keys, string> = {
  property: "my first prop",
  property2: "my second prop",
};

You can of course go further and define a custom type for your property values:

type Keys = "property" | "property2"
type Values = "my prop" | "my other allowed prop"

const obj: Record<Keys, Values> = {
  property: "my prop",
  property2: "my second prop", // TS Error: Type '"my second prop"' is not assignable to type 'Values'.
};

Answer 2

In TypeScript if we are declaring object then we'd use the following syntax:

[access modifier] variable name : { /* structure of object */ }

For example:

private Object:{ Key1: string, Key2: number }

Answer 3

You're pretty close, you just need to replace the = with a :. You can use an object type literal (see spec section 3.5.3) or an interface. Using an object type literal is close to what you have:

var obj: { property: string; } = { property: "foo" };

But you can also use an interface

interface MyObjLayout {
    property: string;
}

var obj: MyObjLayout = { property: "foo" };