What is the use of Infer Keyword in TypeScript ?

The infer keyword in TypeScript is primarily associated with conditional types and is used to capture and assign a type to a type parameter within the scope of a conditional type. It allows TypeScript to infer the type from a given expression and use it within the definition of a more complex type.

Use Case: Conditional Types

Example without infer: This example shows without using infer, we would need to explicitly specify the type of R in the conditional type, losing the ability to dynamically capture the return type of exampleFunction.

Javascript

type ExtractReturnTypeWithoutInfer<T> = T extends
    (...args: any[]) => infer R ? R : never;

function exampleFunction(): string {
    return "Hello, TypeScript!";
}

type ResultWithoutInfer =
    ExtractReturnTypeWithoutInfer<typeof exampleFunction>;
// ResultWithoutInfer is 'string' in this case

Expected Behavior: The ResultWithoutInfer type is inferred to be 'string' because exampleFunction returns a string.

Example with infer: In this example, the infer keyword allows TypeScript to automatically deduce and assign the return type of exampleFunction to the type parameter R. This makes the ExtractReturnTypeWithInfer type more flexible and capable of adapting to different functions.

Javascript

type ExtractReturnTypeWithInfer<T> = T extends
    (...args: any[]) => infer R ? R : never;

function exampleFunction(): string {
    return "Hello, TypeScript!";
}

type ResultWithInfer =
    ExtractReturnTypeWithInfer<typeof exampleFunction>;
// ResultWithInfer is 'string' in this case

Expected Behavior: Similar to the previous example, the ResultWithInfer type is also inferred to be 'string' because the infer keyword allows TypeScript to automatically deduce and assign the return type of exampleFunction to the type parameter R.

Advantages:

1. Dynamic Inference:
   • The infer keyword enables TypeScript to dynamically infer and assign types based on the structure of the code.
2. Conditional Typing:
   • infer is commonly used in conditional types to capture and use types within the scope of the condition.
3. Flexible Type Inference:
   • It provides a level of flexibility by allowing TypeScript to infer types in a way that adapts to the actual structure and context of the code.