TypeScript Types

Summary: in this tutorial, you’ll learn about the TypeScript types and their purposes.

What is a type in TypeScript

In TypeScript, a type is a convenient way to refer to different properties and functions that a value has.

A value is anything you can assign to a variable e.g., a number, a string, an array, an object, and a function.

For example, see the following value:

'Hello'

When you look at this value, you can say it is a string. This value has properties and methods that a string has.

For example, the 'Hello' value has a property called length that returns the number of characters:

console.log('Hello'.length); // 5

It also has many methods like match(), indexOf(), and toLocaleUpperCase(). For example:

console.log('Hello'.toLocaleUpperCase()); // HELLO 

When you look at the value 'Hello' and describe it by listing the properties and methods, it would be inconvenient.

A shorter way to refer to a value is to assign it a type. In this example, you say 'Hello' is a string. Then, you know that you can use the properties and methods of a string for the value 'Hello'.

In conclusion, in TypeScript:

• a type is a label that describes the different properties and methods that a value has

• every value has a type.

Types in TypeScript

TypeScript inherits the built-in types from JavaScript. TypeScript types are categorized into:

• Primitive types.

• Object types.

Primitive types

The following illustrates the primitive types in TypeScript:

Name	Description
string	Represents text data.
number	Represents numeric values.
boolean	Has true and false values.
null	Has one value: null.
undefined	Has one value: undefined. It is the default value of an uninitialized variable.
symbol	Represents a unique constant value.

Object types

Object types are functions, arrays, classes, etc. Later, you’ll learn how to create custom object types.

Purposes of types in TypeScript

There are two main purposes of types in TypeScript:

• First, types are used by the TypeScript compiler to analyze your code for errors.

• Second, types allow you to understand what values are associated with variables.

Examples of TypeScript types

The following example uses the querySelector() method to select the

element:

const heading = document.querySelector('h1');

The TypeScript compiler knows that the type of heading is HTMLHeadingElement:

And it shows a list of methods for the HTMLHeadingElement type that heading can access:

If you try to access a property or method that doesn’t exist, the TypeScript compiler will show an error. For example:

Summary

• In TypeScript, every value is associated with a type.

• A type is a label that describes the properties and methods that a value has.

• TypeScript compiler uses types to analyze your code for hunting bugs and errors.