learn-typescript

TS Simple Types

TypeScript supports some simple types (primitives) you may know.

There are three main primitives in JavaScript and TypeScript.

boolean - true or false values
number - whole numbers and floating point values
string - text values like “TypeScript Rocks”

There are also 2 less common primitives used in later versions of Javascript and TypeScript.

bigint - whole numbers and floating point values, but allows larger negative and positive numbers than the number type.
symbol are used to create a globally unique identifier.

Type Assignment

When creating a variable, there are two main ways TypeScript assigns a type:

Explicit
Implicit

In both examples below firstName is of type string

1. Explicit Type

Explicit - writing out the type:

let firstName: string = "Dylan";

Explicit type assignment are easier to read and more intentional.

2. Implicit Type

Implicit - TypeScript will “guess” the type, based on the assigned value:

let firstName = "Dylan";

Note: Having TypeScript “guess” the type of a value is called infer.

Implicit assignment forces TypeScript to infer the value.

Implicit type assignment are shorter, faster to type, and often used when developing and testing.

Error In Type Assignment

TypeScript will throw an error if data types do not match.

Example:

let firstName: string = "Developer Noyon"; // type string
firstName = 33; // attempts to re-assign the value to a different type

Implicit type assignment would have made firstName less noticeable as a string, but both will throw an error:

Example:

let firstName = "Developer Noyon"; // inferred to type string
firstName = 33; // attempts to re-assign the value to a different type

JavaScript will not throw an error for mismatched types.

Unable to Infer

TypeScript may not always properly infer what the type of a variable may be. In such cases, it will set the type to any which disables type checking.

Example:

// Implicit any as JSON.parse doesn't know what type of data it returns so it can be "any" thing...
const json = JSON.parse("55");
// Most expect json to be an object, but it can be a string or a number like this example
console.log(typeof json); // return number

This behavior can be disabled by enabling noImplicitAny as an option in a TypeScript’s project tsconfig.json. That is a JSON config file for customizing how some of TypeScript behaves.

Note: you may see primitive types capitalized like Boolean

boolean !== Boolean

For this tutorial just know to use the lower-cased values, the upper-case ones are for very specific circumstances.