Using TypeScript with React

How Strong Typing Helps You Spot Errors You Never Knew Were There


So what is Typing anyway?

To understand what TypeScript is, first you must be familiar with the concept of typing. JavaScript developers will probably be aware of the difference between Equality (==) and Identity (===) Operators. These operators differentiate between entities like the string "123" and the integer 123. This is because numbers and characters are fundamentally stored differently in memory. In JavaScript, all variables are declared using the keyword var (or const/let). However, this is not the case in strongly typed languages, such as Java and C, which CANNOT declare any variable without an explicitly defined data type.

JavaScript is ‘interpreted’, not ‘compiled’

A data type is a formalization of the way in which information is represented in a computer system, such as the case with Integers and Strings. Languages such as Java and C are referred to as strongly typed languages, because they depend on these types being defined at compile time, before the high-level language is converted into machine code or byte-code. JavaScript is an interpreted language (it is not compiled) It uses dynamic typing because, as the name implies, JavaScript was created with script-building in mind. Scripts generally automate the execution of code that is exterior to the script itself (such as compiled programs). As JavaScript development has become increasingly complex, there has arisen greater need for more explicit typing. This need was partially met by tools such as PropTypes. However, TypeScript, which was created by Microsoft, is especially helpful for validating the consistency of user-designed data objects.

Type Checking

Type checking is the idea that the shape of a data object remains consistent as it is passed between different parts of a program. TypeScript extends the JavaScript language, making it possible to explicitly define the shape of objects when they are declared, or passed into / returned from a function.

When I first started to use TypeScript linting, I immediately found all kinds of subtle errors in my code that I had never realized were there. We may set out to write a program with a particular idea in mind, but as that idea starts to take shape, we have to change our code to suit disparate purposes, and add new features. It is not always obvious how changing the shape of an object in one area will affect the code we have used in a myriad of other places. Even with relatively simple programs, it’s not hard to lose track of what is really going on, particularly when working with platforms like Redux.js, where it is necessary to keep track of references to data objects that are splashed across a constellation of files, all of which need to be modified in concert. This is where TypeScript really shines. It can tell us that the data we are passing in is not what is being expected. This makes it a lot easier to track down minute flaws in our logic that can be difficult to otherwise identify.

Awesome! So how do I use it?

The official documentation is a great place to start.

First install the package

npm install -g typescript

Define a variable with an explicit type

var a: number = myFunctionReturnsInt()
var b: string = myFunctionReturnsString()

Define a function that takes explicit type and returns explicit type (ES6)

dayOfWeek = (day: number): string => {
    switch(day) {
        case 1: return "Sunday";
        case 2: return "Monday";

Define your own data type

interface Person {
    firstName: string;
    lastName: string;

function greeter(person: Person) {
    return "Hello, " + person.firstName + " " + person.lastName;

let user = { firstName: "Jane", lastName: "User" };


Finding errors you never knew were there

It’s quite easy to do something like this:

<th className='tableHead' colSpan='2'>

You may not ever realize that this is wrong, but fortunately TypeScript is there to call attention to your error, because TypeScript understands that a number is expected, not a string


Now you can correct it

<th className='tableHead' colSpan={2}>