This module contains the core of TypeStript, like variables of many types, typing functions, and using generic code for reusability.

TypeScript for React Developers

TypeScript is an in-demand language that enhances the functionalities of JavaScript. This means we can do everything in JavaScript with TypeScript and enjoy countless additional perks, including support for JS libraries, NPM, static typing, and much more.

In this course, we’ll start from the beginning and work our way up to more advanced concepts like type checking, iterators, and manipulating objects and arrays. We'll also be able to get plenty of hands-on practice along the way.

TypeScript for Beginners

# What is `reduce`?

`Array.reduce` is an extremely versatile function that lets you calculate results from all elements of an array. When calling `reduce` you specify a function that describes how to calculate the result for the next array element, given you already know the result for the previous array elements. It's like a functional version of a `foreach` loop with an additional variable that you change in the loop and return at the end.

TypeScript 3.7.5 Strict

Our `reduce` will be a function that accepts three arguments:
* `array` - the array to be reduced
* `reducer` - a function that accepts an element of the array and the partial result (calculated based on elements processed so far)
* `initialResult` - a value that will be passed to the `reducer` before any array elements have been processed

# Typing `reduce`

As you might have guessed, `reduce` is a generic, higher-order function. There will be two type arguments:
* `TElement` - the type of elements of the array
* `TResult` - the type of the calculated result

Let's look at the types of particular function arguments:
* `array` - since `TElement` is the type of array element, the type of the whole array is `TElement[]`
* `initialResult` - the type should be the same as the type of the final result; so it's `TResult`
* `reducer` - it's a function that takes a result (`TResult`) and an array element (`TElement`) and returns an updated result (`TResult`); therefore, it's type is `(result: TResult, el: TElement) => TResult`

Finally, the return type of the whole function is `TResult` (the type of the result we'd like to calculate).

Below you can find a simple implementation of `reduce` along with the usage examples with type arguments provided explicitly. 

This lesson walks through the implementation of an example generic function. Typing array utility functions is a great exercise that helps you fully understand the complexities of generics.

Generic Functions Example - Typing `reduce`

What is reduce?

What is `reduce`?