Introduction to Typescript and its Basic Types


17 min read

Introduction to Typescript and its Basic Types

Cover Image Credits: Photo by Luca Bravo

Typescript has become quite the popular topic of discussion among developers and companies in the tech space. So, what exactly is this Typescript everyone's talking about? This article will attempt to explain Typescript in the simplest form possible and also introduce you to the basic types available.

What is Typescript?

Typescript is commonly known as a Superset of Javascript. What does this mean? Typescript is a programming language built on top of Javascript. It introduces new features and advancements to the Javascript programming language. It offers the ability to use newer, more modern features when writing your Javascript code.

Typescript can not be run in a Javascript environment like the browser.

How then does Typescript work?

In addition to being a programming language, Typescript is also a compiler. It converts all Typescript codes to Javascript code so it can be run by environments that support Javascript, like your browser.

Why use Typescript?

Typescript in its simplest form is just typed javascript.

Take the below code for example:

function addTwoNumbers(num1, num2) {
    return num1 + num2;

addTwoNumbers(5, 6);
addTwoNumbers('5', '6');

In Javascript, the function above will run and output without errors. In the first function call, our output will be 11, while in the second function call, the output will be the string 56. As far as Javascript is concerned, we don't have any errors, but as you can see, our function is meant to add two numbers and not two strings. So in this case, our code fails the test silently by not throwing an error when either of num1 and num2 parameters are strings. These and other hidden issues expose the weaknesses we have in the Javascript programming language.

Typescript aims to solve this, in addition to adding other superpowers to Javascript.

Typescript Basics

Core Types and how to define them


8, -4, 2.8

Typescript regards all digits as a number type. Including single digits, negative integers, and floating-point numbers.

To define a number type in typescript we can do the following:

  • Option 1
let someNumber: number;

someNumber = 10;

In option 1 above, we are explicitly stating the type of the variable, in this case: someNumber. This option works when we don't want to assign a value on the spot. In this case, whatever value we assign to the variable in the future, typescript will treat it as a number type because we told typescript to do so. If we do this: someNumber = 'my name';, typescript would throw an error as we are attempting to assign a string value to a variable that has a number type definition.

  • Option 2
let someNumber: number = 5;

someNumber = 10;

Option 2 works when we are assigning a value on the spot and we still want to explicitly state the type. We can still change the value of the variable someNumber in the future.

  • Option 3
let someNumber = 5;

Option 3 is a bit different because we aren't explicitly stating the type. We just define the variable someNumber and assign a value to it which is 5. In this situation, typescript does something called Type Inference, where it uses the value assigned to infer the type that should be assigned to the variable. This is the same in all type definitions.


'my word', "my word", `my word`

All text values are considered strings, whether it's single quotes, double quotes, or the newer string literals.

For string types, we can define them as follows:

  • Option 1
let someString: string;

someString = "a string value";
  • Option 2
let someString: string = 'first string';

someString = 'new string value';
  • Option 3
let name: string = 'peter';

let someString = `this is some string ${name}`;

We can see the exact same method of definition here like we did with the number type definition. We can either explicitly define the string variable type or let typescript infer it. We can use single quotes, double quotes or string literals as well.


true, false

A boolean has either a true (1) or a false (0) value.

Boolean types can be defined as follows:

  • Option 1
let isTruthy: boolean;

isTruthy = false;
  • Option 2
let isTruthy: boolean = true;
  • Option 3
let isTruthy = true;


  name: 'Andy',
  age: 23,
  isEmployed: true

Objects look exactly like their vanilla javascript counterparts.

We can define objects in the following ways:

  • Option 1
let firstObject: {
  color: string;
  age: number;

firstObject = {
  color: 'blue',
  age: 30

By paying close attention to option 1 above, you can notice this method of definition is a bit different. We define the object with its properties and explicitly state the types for each property. If we try to assign a different value type to the properties, we will definitely get an error.

  • Option 2
let secondObject: {
  color: string;
  age: number;
} = {
  color: 'red',
  age: 25

The definition in option 2 above becomes useful when we want to assign a value on the spot. It still makes use of the object type definition with other types as properties.

  • Option 3
const thirdObject = {
  color: 'red',
  age: 25

For this definition, we are again just defining our object with properties and letting typescript infer the types based on the value we assign to the properties. I'm sure you're getting how this type definition thing works now.

Other valid but not recommended ways of defining object type

While there are situations where the methods below can be used, I wouldn't recommend using them, but if you have to, they should be used sparingly. The methods described above are my most recommended methods for object type definitions.

// Object Type
let thirdObject: object;
thirdObject = {
  color: 'red',
  age: 25

// Object Type
let fourthObject: object = {
  color: 'red',
  age: 25

If you take a look at each object definition shown in the snippet above, you will notice that they are defined in a way where we explicitly state the type: object. While this is okay, it doesn't properly define what type each individual property in the object should hold. So it isn't good practice to use this method as it isn't readable.


[1, 'second', false]

Arrays in typescript can be strict or flexible depending on how we want them.

Array type can be defined in the following ways:

  • Option 1
let firstArray = []   <--- // any[]

let secondArray = ['one', 'two']   <--- // string[]

let thirdArray = [1, 2]   <--- // number[]

let fourthArray = [true, false]   <--- // boolean[]

let fifthArray = ['one', 1, true]   <--- // (string | number | boolean)[]

let sixthArray: string[]; <--- // A string Array type

sixthArray = ['one', 'two']

let seventhArray: string[] = ['find', 'peace'];

firstArray is a flexible array as we did not explicitly state what kind of element the array would contain. On the other hand, secondArray, thirdArray, and fourthArray are strict arrays because the elements in each array are of one single type. fifthArray contains different element types and so typescript identifies that the array can only contain any of the three types. In this scenario, we can modify our fifthArray with any element that matches each of the three types: string or number or boolean.

  • Option 2
let eightArray: Array<number>;

eightArray = [2, 6, 4]

The method defined above uses what is called a generic array type definition. This works the same way let sampleArray: number[]; works. It is also a strict type array definition. What if we want a strict type where we only want two elements in the array of a defined type. This is where we can use a new type called Tuple.


[1, 'a string']

A tuple is a special type provided by typescript that aims to give us more control over defining and handling elements. Tuple exists in other programming languages like Python but is not available in Javascript.

A tuple can be defined as follows:

  • Option 1
let person: [number, string];

person = [40, 'peterson']    <--- // correct

person = ['peterson', 40]    <--- // ERROR

person = [40, 'peterson', 'james']    // ERROR --> Type '[number, string, string]' is not assignable to type '[number, string]'. Source has 3 element(s) but target allows only 2.

Looking at the definition above, we can get an idea of how powerful a Tuple is. We can define a fixed style of data structure and use it strictly. In the person tuple, we can only assign elements that match the types defined, and in addition, the elements must be in exactly the same shape as the definition and the same length. Notice that if we try to add three elements to the tuple, we get an error as it's only meant to have two elements.

  • Option 2
let identity: [number, string] = [2, 'james'];

identity = [20, 'Eren Yeager']; // Valid

For option 2, we initialize a value on the spot. A new value can be used to modify the existing tuple, as long as it matches the shape and length.

Take a look at the code below:

let animal = [number, string];

animal = [5, 'goats'];


While a tuple is a fixed-length definition, one caveat is that calling a method like push() on a tuple would actually work, as typescript does not throw an error when you push. This is something you should keep in mind while using tuples. The below code is the result of calling push on the tuple above:

[5, 'goats', 'birds']

Calling a method like push() on a tuple would actually work, as typescript does not throw an error when you push.

This is something you should keep in mind while using tuples.


enum { VALUE1, VALUE2, ... }

An enum is a user-defined data type. It is made up of named constant values that are normally referred to as enumerators, hence the name: Enum. Behind the scenes, the values are held as integers. Enums are natively available in other programming languages, like Java, Python, and others, but are not supported by Javascript natively. If we want to create an enum in javascript, we would define an object of the values with a const and then call freeze on the defined object.

However, in typescript, we have enums out of the box.

Enums are defined as follows:

enum Color = {

// To assess it
let paint: Color = Color.RED;

One use case for Enums would be to have pre-defined constant names for role-based authorization. This prevents us from having to type the roles for every single usage. You will mostly find enums written in all caps like BLUE but this isn't a restriction, as the case it is written in doesn't matter. It can also be written as Blue and still function the same.

Color.RED above isn't explicitly assigned a value, so the hidden value defaults to 0, and Color.YELLOW becomes 1, and so on. You catch the drift ๐Ÿ˜.

You can explicitly define the value, just like we did below:

enum Color = {
  RED = 100,

In this example, Color.RED takes on the value of 100, and since the values of YELLOW and BLUE aren't explicitly defined, they would become 101 and 102 respectively.

You can assign whatever value you want to an Enum. The values aren't restricted to just numbers, they could also be strings.


Typescript, which was first introduced in 2012 by Microsoft has become an incredible tool for Javascript developers, permitting type checking during development to prevent unforeseen errors. As more developers and organizations continue to adopt it and in addition to it being open-source, its popularity is bound to keep increasing. Whether you choose to use it in your project or not is entirely up to you.

If you however choose to adopt it, you would find a wealth of information on the Official Typescript Documentation.

If there are other typescript topics you'd like me to write on, you can drop a comment below ๐Ÿ‘‡

Till next time! Ciao! ๐Ÿ‘‹