How to make a union type from another object type in TypeScript?

Published January 18, 2022

To make a union type from another object's type, we can use the keyof operator followed by the object's type name in Typescript.

TL;DR

// Person type alias
type Person = {
  name: string;
  age: string;
  salary: string;
  userType: string;
};

// make union type from
// `Person` type alias
// using the `keyof` operator
type PersonKeys = keyof Person; // "name" | "age" | "salary" | "userType"

// make a variable and use the `PersonKeys` as its type
let john: PersonKeys = "age"; // ✅ Valid.
john = "hello"; // ❌ Invalid.

For example, let's say we have a type alias called Person with 4 properties called name, age, salary, and userType all having the type of string like this,

// Person type alias
type Person = {
  name: string;
  age: string;
  salary: string;
  userType: string;
};

Now to make a union type using the object's type Person, first let's make a type alias called PersonKeys and then use the keyof operator followed by the type alias name, in our case, it is the Person.

It can be done like this,

// Person type alias
type Person = {
  name: string;
  age: string;
  salary: string;
  userType: string;
};

// make union type from
// `Person` type alias
// using the `keyof` operator
type PersonKeys = keyof Person; // "name" | "age" | "salary" | "userType"

Now to prove that our PersonKeys union type is working let's use the PersonKeys as a type for a variable called john like this,

// Person type alias
type Person = {
  name: string;
  age: string;
  salary: string;
  userType: string;
};

// make union type from
// `Person` type alias
// using the `keyof` operator
type PersonKeys = keyof Person; // "name" | "age" | "salary" | "userType"

// make a variable and use the `PersonKeys` as its type
let john: PersonKeys = "age"; // ✅ Valid.
john = "hello"; // ❌ Invalid.

As you can see from the above code that assigning a value of age is valid because it is one of the union types from the PersonKeys union type and assigning the value of hello is invalid since it doesn't satisfy the PersonKeys union type.

Yay, we have successfully made a union type from another object's type. 🥳

See the above code live in codesandbox.

That's all 😃!

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