How to choose types conditionally from another type in TypeScript?

Published January 28, 2022

To choose types conditionally from another type, you can use the extends keyword to conditionally check if a particular condition is being satisfied by the target type followed by the ? symbol (question mark), then the type to choose when the condition becomes true, then the : symbol (colon), and finally the type to choose when the condition becomes false.

TL;DR

// a simple interface
interface Person {
  name: string;
  age: number;
}

// conditionally choose the type `string`
// when the condition becomes `true`
// or choose the type `number`
// when the condition becomes `fasle`
type ConditionalType = Person extends { name: string } ? string : number;

/*
  CONCLUSION: 👆
  If you hover over the `ConditonalType` type alias
  we can see that the type is `string` since the
  `Person` interface has a property 
  called `name` with the type of `string`
*/

To relate it in the JavaScript world, we have an operator called conditional ternary operator which has the same mechanism we discussed for the type conditional check.

For example, let's say we have an interface called Person with some properties like name having the type of string and another property called age having the type of number like this,

// a simple interface
interface Person {
  name: string;
  age: number;
}

Now we aim to choose either the string type if the condition is true or the number type if the condition is false by checking if the target type has a property of name with the type of string. In our case, the target type is the Person interface.

So to do that first we can make a new type alias called ConditionalType followed by writing the target type name Person followed by the extends keyword then the condition to check in our case is to check the property of name having the type of string ({ name: string }).

After the condition, we can write the type string to choose when the condition becomes true followed by the : symbol and then the type number to choose when the condition becomes false.

It can be done liek this,

// a simple interface
interface Person {
  name: string;
  age: number;
}

// conditionally choose the type `string`
// when the condition becomes `true`
// or choose the type `number`
// when the condition becomes `fasle`
type ConditionalType = Person extends { name: string } ? string : number;

/*
  CONCLUSION: 👆
  If you hover over the `ConditonalType` type alias
  we can see that the type is `string` since the
  `Person` interface has a property 
  called `name` has the type of `string`
*/

As you can see that the ConditionalType type alias has the type of string conditionally chosen since the Person interface has a property called name having the type of string.

We have successfully chosen types conditionally from another type in TypeScript. Yay 🥳!

See the above code live in codesandbox.

That's all 😃!

Feel free to share if you found this useful 😃.


Share on: Facebook Twitter