Learning TypeScript

Chapter 10. Generics

Variables you

declare in the type system?

A whole new (typed) world!

All the type syntaxes you’ve learned about so far are meant to be used with types that are completely known when they’re being written. Sometimes, however, a piece of code may be intended to work with various different types depending on how it’s called.

Take this identity function in JavaScript meant to receive an input of any possible type and return that same input as output. How would you describe its parameter type and return type?

function identity(input) {
    return input;
}

identity("abc");
identity(123);
identity({ quote: "I think your self emerges more clearly over time." });

We could declare input as any, but then the return type of the function would also be any:

function identity(input: any) {
    return input;
}

let value = identity(42); // Type of value: any

Given that input is allowed to be any input, we need a way to say that there is a relationship between the input type and the type the function returns. TypeScript captures relationships between types using generics.

In TypeScript, constructs such as functions may declare any number of generic type parameters: types that are determined for each usage of the generic construct. These type parameters are used as types in the construct to represent some type that can be different in each instance of the construct. Type parameters may be provided with different types, referred to as type arguments, for each instance of the construct ...