What is a 'type assertion' between the function name and parameters, on calling a function?

Question

In this Typescript React starter guide it gives:

import { createStore } from 'redux';

interface StoreState {
    languageName: string;
    enthusiasmLevel: number;
}

function enthusiasm(state: StoreState, action: EnthusiasmAction): StoreState {
    // returns a StoreState
}
const store = createStore<StoreState>(enthusiasm, {
     enthusiasmLevel: 1,
     languageName: 'TypeScript',   
});

What is this assertion doing there?

I couldn't find where this syntax is defined, and can't "deduce" what it means.

Answer 1

This isn't a type assertion, it is a type argument, which is used for generic types.

Type Assertions

Firstly, here's a type assertion...

const x = <HTMLAnchorElement>document.getElementById('myLink');

The type assertion appears before the expression and says "actually this is an anchor, not just a general element".

Generics

Now let's look at generics...

This function takes a string and returns it.

function example(input: string): string {
    return input;
}

Now we can add another function that takes a number and returns it, but actually our function doesn't care about the type of the parameter, or the return type - as long as they are the same...

So instead of repeating the function for each type, we can say, "the type will be T, where T will be defined later on".

function example<T>(input: T): T {
    return input;
}

Explicit Type Argument

When you use something generic (it can be a class, or a function, or a method), you can supply the type argument explicitly, like this:

function example<T>(input: T): T {
    return input;
}

const str = example<string>('str');

const num = example<number>(3);

They look a bit like type assertions, but they appear in a different location.

Implicit Type Argument

In many cases, you don't need to explicitly pass the type argument, as the compiler can work it out for you.

function example<T>(input: T): T {
    return input;
}

const str = example('str');

const num = example(3);