Basic Types: String, Number, Boolean

TypeScript has various basic (primitive) types. Common ones include string, number, and boolean. These are the fundamentals of TypeScript's type system.

The string type

A string represents text data, it can be a single character, a word, sentance, or paragraph. In TypeScript, you can use single quotes, double quotes, or template literals (backticks):

let firstName: string = "Homer";
let lastName: string = 'Simpson';
let message: string = `Hello, ${firstName}!`;
console.log(message);

let firstName: string = "Homer";
let lastName: string = 'Simpson';
let message: string = `Hello, ${firstName}!`;
console.log(message);

String methods

All regular JavaScript string methods are available in TypeScript, and TypeScript knows the return types:

let text: string = "Hello, TypeScript!";

text.toUpperCase(); // Returns: string
text.length; // Returns: number

console.log(typeof(text.toUpperCase()));
console.log(typeof(text.length));

let text: string = "Hello, TypeScript!";

text.toUpperCase(); // Returns: string
text.length; // Returns: number

console.log(typeof(text.toUpperCase()));
console.log(typeof(text.length));

TypeScript will warn you if you try to use a method that doesn't exist:

let text: string = "Hello, TypeScript!";
text.toUppercase(); // typo- should be toUpperCase()

let text: string = "Hello, TypeScript!";
text.toUppercase(); // typo- should be toUpperCase()

The number type

TypeScript has only one number type (unlike some languages that have int, float, double, etc). Used for both integers and floating-point numbers:

let age: number = 40;
let count: number = 7;
let temperature: number = -5.5;
let cost: number = 29.99;

Number operations

TypeScript understands number operations and also their return types:

let a: number = 10;
let b: number = 5;

let add: number = a + b; // 15
let subtract: number = a - b; // 5
let multiply: number = a * b; // 50
let divide: number = a / b; // 2
let remainder: number = a % b; // 0

console.log(add)
console.log(typeof(add))

let a: number = 10;
let b: number = 5;

let add: number = a + b; // 15
let subtract: number = a - b; // 5
let multiply: number = a * b; // 50
let divide: number = a / b; // 2
let remainder: number = a % b; // 0

console.log(add)
console.log(typeof(add))

Number methods

Numbers have methods available through the Number object:

let num: number = 3.14159;

console.log(typeof(Number.parseInt("42"))); // number
console.log(typeof(Number.parseFloat("3.14"))); // number
console.log(typeof(num.toFixed(2))); // string

let num: number = 3.14159;

console.log(typeof(Number.parseInt("42"))); // number
console.log(typeof(Number.parseFloat("3.14"))); // number
console.log(typeof(num.toFixed(2))); // string

• toFixed() formats a number to a specific number of decimal places. Resulting in a string value (e.g. "3.14" instead of 3.14)

The boolean type

A boolean type represents a true or false value:

let isLoggedIn: boolean = true;
let admin: boolean = false;
let hasPermission: boolean = true;

Boolean operations

You can use logical operators with booleans:

let isLoggedIn: boolean = true;
let isAdmin: boolean = false;

let canEdit: boolean = isLoggedIn && isAdmin; // false
let canView: boolean = isLoggedIn || isAdmin; // true

console.log(canEdit);
console.log(canView);

let isLoggedIn: boolean = true;
let isAdmin: boolean = false;

let canEdit: boolean = isLoggedIn && isAdmin; // false
let canView: boolean = isLoggedIn || isAdmin; // true

console.log(canEdit);
console.log(canView);

Type safety examples

TypeScript prevents common mistakes related to types:

// Strings
let name: string = "Marge";
name = 42; // Error: Type 'number' is not assignable to type 'string'

// Numbers
let age: number = 30;
age = "thirty"; // Error: Type 'string' is not assignable to type 'number'

// Booleans
let isActive: boolean = true;
isActive = "false"; // Error: Type 'string' is not assignable to type 'boolean'

// Strings
let name: string = "Marge";
name = 42; // Error: Type 'number' is not assignable to type 'string'

// Numbers
let age: number = 30;
age = "thirty"; // Error: Type 'string' is not assignable to type 'number'

// Booleans
let isActive: boolean = true;
isActive = "false"; // Error: Type 'string' is not assignable to type 'boolean'

Combining types in functions

You can use these types in function parameters and return types:

function welcome(name: string): string {
  return `Hello, ${name}!`;
}

function canOrder(isAvailable: boolean, hasIngredients: boolean): boolean {
  return isAvailable && hasIngredients;
}

function calculateTotal(price: number, quantity: number): number {
  return price * quantity;
}

• (name: string) this specifies the name passed to the function must be a string
• : string specifies a string as the function return type

These three types (string, number, boolean) are the most fundamental types in TypeScript. In the next lesson, we will explore the types of null, undefined, void, and never.