How to use Recursive Conditional Types In Javascript
In this approach, we leverage recursive conditional types to infer the first element of a tuple as a tuple type. By recursively checking the type of each element until reaching the first one, we can extract its type as a tuple.
Syntax:
type FirstElementAsTuple<T extends any[]> =
T extends [infer First, ...infer _Rest] ?
[First] :
never;
Example: The following code demonstrates the usage of recursive conditional types to infer the first element of a tuple as a tuple type.
type MyTuple = [string, number, boolean];
type FirstElementAsTuple<T extends any[]> =
T extends [infer First, ...infer _Rest] ?
[First] :
never;
const tuple: MyTuple = ['hello', 42, true];
const firstElement: FirstElementAsTuple<MyTuple> = ['hello'];
console.log("Tuple:", tuple);
console.log("First Element as Tuple:", firstElement);
Output
Tuple: [ 'hello', 42, true ]
First Element as Tuple: [ 'hello' ]
How to Infer First Element of a Tuple as a Tuple Type ?
Tuples can be represented as arrays or objects in TypeScript. This will be achieved by knowing Typescript’s type system and methods such as manual inference as well as the use of the advanced type system of TypeScript.
Defining a tuple type in TypeScript resembles the array syntax with the possibility to specify the type for each element position.
Table of Content
- Using TypeScript native tuple support
- Conditional type using infer
- Mapped Types
- Manual Inference to Extract First Element as Tuple Type
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