Typescript - Beginner Guide to type keyword with example

typescript type keyword tutorials example In this short tutorial, learn about the type keyword in typescript with examples.

Type is a keyword in typescript, used as an alias for existing or custom types.

type keyword examples

Following are example

How to declare a type nullable

Let us declare Employee Interface in ts file

interface Employee {
  name: string;
  id: number;
  salary: number;

Create an empty typed array of an interface or class

Employee object holds different types of data.

Using generics type declaration

First, array variable can be declared with a generic array of Employeeby assigning an empty array

let employees: Array<Employee> = [];

Thus, create a variable store’s empty typed array

type assertion

`type assertion is like assigning an object of one type to a variable. It will have no performance impact at runtime. however, It used to avoid errors at compile time.

two syntaxes are used. The first is as syntax and the second is angle-bracket syntax.

The following is an example for creating an empty typed array with as syntax.

let empArray1 = [] as Employee[];

The above syntax can also be written with angle-bracket syntax as follows

let empArray = <Employee[]>[]; 

Both are equal in terms of usage and performance.

Array constructor

Finally, array constructors are generic usage and used by every developer. It uses a new operator to create an empty array.

let empArray = new Array<Employee>(); 
let empArray: Array<Employee> = new Array<Employee>();

It looks good in readability, however, performance impacts creating a reference in memory.

How to create empty typed string array with examples

There are many ways typed string array’s can be created with following syntaxes

let emptyStringArray: string[] = [];
let emptyStringArray1 = new Array<string>(); 
let emptyStringArray2: Array<string> = new Array<string>();

Create and initialize typed empty objects in an array

In this example, create a non-empty array with object data that is empty or default values.

let us declare an interface for User

interface User {
  name: string;
  id: number;
  password: string;

Following is an example with array initialized with three empty user objects. Partial in typescript allows you to construct object with optional fields Partial<User> return User with optional id,name and password

users: Partial<User>[] = [{}, {}, {}];

And other option is to manual set values for each object with default values of an object

users1: User[] = [{ id: null, name: null, password:null }{ id: null, name: null, password:null }]


In Conclusion, you learned different ways to create a typed empty array, and also how to create an array with empty objects.

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