How to Declare Specific Type of Keys in an Object in TypeScript ?

In TypeScript, object definitions can include specific key-value types using index signatures. You can declare specific types of keys in an object by using different methods as listed below:

Using Mapped Types

You can define your mapped type using the type alias in TypeScript that can be used to specify the keys of a particular type in an object.

Syntax:

type customType = {};

Example: The below code explains the use of the mapped types to declare specific type keys in an object.

Javascript

type MyObject = {
  [key: string]: number;
};

const obj: MyObject = {
  a: 1,
  b: 2,
  c: 3
};
console.log(obj); 

Output:

{ a: 1, b: 2, c: 3 }

Using Interface

You can also use a interface to define your custom type where the keys of the object are typed to a specific type and can later be used to type interface.

Syntax:

Interface interfaceName{}

Example: The below code example explains the use of the interface to declare specific type of keys in an object.

Javascript

interface MyObject {
    [key: string]: number;
}

const obj: MyObject = {
    a: 1,
    b: 2,
    c: 3
};

console.log(obj); 

Output:

{ a: 1, b: 2, c: 3 }

Using Inline Mapped Types with type

You can define a mapped type inline using the type keyword in TypeScript. This allows you to specify the keys of a particular type directly within the type declaration.

Syntax:

type TypeName = { [KeyType]: ValueType };

Example: This TypeScript code defines a type MyObject allowing string keys with number values. It creates an object obj with properties assigned numeric values and logs it.

JavaScript

type MyObject = { [key: string]: number };

const obj: MyObject = {
    a: 1,
    b: 2,
    c: 3
};

console.log(obj);

Output:

{
  "a": 1,
  "b": 2,
  "c": 3
}