Optional chaining allows you to safely access nested properties or methods without worrying about null or undefined values. It short-circuits the evaluation if any intermediate property is null or undefined.
const user = {
name: 'John',
address: {
city: 'New York',
postalCode: '12345'
}
};
const postalCode = user.address?.postalCode;
console.log(postalCode); // Output: 12345
const invalidCode = user.address?.postalCode?.toLowerCase();
console.log(invalidCode); // Output: undefinedThe nullish coalescing operator provides a default value when a variable is null or undefined.
const name = null;
const defaultName = name ?? 'Unknown';
console.log(defaultName); // Output: Unknown
const age = 0;
const defaultAge = age ?? 18;
console.log(defaultAge); // Output: 0Type assertion allows you to explicitly define the type of a variable when TypeScript is unable to infer it.
let userInput: unknown;
userInput = "Hello World";
const strLength = (userInput as string).length;
console.log(strLength); // Output: 11Generics enable you to create reusable components that can work with a variety of types.
function reverse<T>(items: T[]): T[] {
return items.reverse();
}
const numbers = [1, 2, 3, 4, 5];
const reversedNumbers = reverse(numbers);
console.log(reversedNumbers); // Output: [5, 4, 3, 2, 1]
const strings = ['a', 'b', 'c'];
const reversedStrings = reverse(strings);
console.log(reversedStrings); // Output: ['c', 'b', 'a']The keyof operator returns a union of all known property names of a given type.
interface User {
id: number;
name: string;
email: string;
}
function getUserProperty(user: User, property: keyof User) {
return user[property];
}
const user: User = {
id: 1,
name: 'John Doe',
email: 'john@example.com'
};
const name = getUserProperty(user, 'name');
console.log(name); // Output: John Doe
const invalidProperty = getUserProperty(user, 'age'); // Error: Argument of type '"age"' is not assignable to parameter of type '"id" | "name" | "email"'Type guards allow you to narrow down the type of a variable within a conditional block, based on a certain condition.
function logMessage(message: string | number) {
if (typeof message === 'string') {
console.log('Message: ' + message.toUpperCase());
} else {
console.log('Value: ' + message.toFixed(2));
}
}
logMessage('hello'); // Output: Message: HELLO
logMessage(3.14159); // Output: Value: 3.14Intersection types allow you to combine multiple types into a single type, creating a new type that has all the properties and methods of the intersected types.
interface Loggable {
log: () => void;
}
interface Serializable {
serialize: () => string;
}
type Logger = Loggable & Serializable;
class ConsoleLogger implements Loggable {
log() {
console.log('Logging to console...');
}
}
class FileLogger implements Loggable, Serializable {
log() {
console.log('Logging to file...');
}
serialize() {
return 'Serialized log data';
}
}
const logger1: Logger = new ConsoleLogger();
logger1.log(); // Output: Logging to console...
const logger2: Logger = new FileLogger();
logger2.log(); // Output: Logging to file...
console.log(logger2.serialize()); // Output: Serialized log data
8.Mapped Types:
Mapped types allow you to create new types by transforming the properties of an existing type.
interface User {
id: number;
name: string;
email: string;
}
type PartialUser = { [K in keyof User]?: User[K] };
const partialUser: PartialUser = {
name: 'John Doe',
email: 'john@example.com'
};
console.log(partialUser); // Output: { name: 'John Doe', email: 'john@example.com' }TypeScript supports string literal types and union types, which can be used to define specific sets of values for a variable.
type HttpMethod = 'GET' | 'POST' | 'PUT' | 'DELETE';
function sendRequest(url: string, method: HttpMethod) {
// send request logic here...
}
sendRequest('/users', 'GET');
sendRequest('/users', 'POST');
sendRequest('/users/1', 'PUT');
sendRequest('/users/1', 'DELETE');Decorators allow you to modify or extend the behavior of classes, methods, properties, and other declarations.
function uppercase(target: any, propertyKey: string) {
let value = target[propertyKey];
const getter = () => value;
const setter = (newValue: string) => {
value = newValue.toUpperCase();
};
Object.defineProperty(target, propertyKey, {
get: getter,
set: setter,
enumerable: true,
configurable: true
});
}
class Person {
@uppercase
name: string;
}
const person = new Person();
person.name = 'John Doe';
console.log(person.name); // Output: JOHN DOEIndex signatures allow you to define dynamic property names and their corresponding types in an interface or type.
interface Dictionary {
[key: string]: number;
}
const scores: Dictionary = {
math: 90,
science: 85,
history: 95
};
console.log(scores['math']); // Output: 90
console.log(scores['english']); // Output: undefinedTypeScript can infer the types based on conditional statements, allowing for more concise code.
function calculateTax(amount: number, isTaxable: boolean) {
if (isTaxable) {
return amount * 1.1; // Type: number
} else {
return amount; // Type: number
}
}
const taxableAmount = calculateTax(100, true);
console.log(taxableAmount.toFixed(2)); // Output: 110.00
const nonTaxableAmount = calculateTax(100, false);
console.log(nonTaxableAmount.toFixed(2)); // Output: 100.00TypeScript provides the readonly modifier to define properties that can't be modified after initialization.
class Circle {
readonly radius: number;
constructor(radius: number) {
this.radius = radius;
}
getArea() {
return Math.PI * this.radius ** 2;
}
}
const circle = new Circle(5);
console.log(circle.radius); // Output: 5
// circle.radius = 10; // Error: Cannot assign to 'radius' because it is a read-only property
console.log(circle.getArea()); // Output: 78.53981633974483Type aliases allow you to create custom names for existing types, providing more semantic meaning and improving code readability.
type Point = {
x: number;
y: number;
};
type Shape = 'circle' | 'square' | 'triangle';
function draw(shape: Shape, position: Point) {
console.log(`Drawing a ${shape} at (${position.x}, ${position.y})`);
}
const startPoint: Point = { x: 10, y: 20 };
draw('circle', startPoint); // Output: Drawing a circle at (10, 20)Type guards can also be used with classes to narrow down the type of an object instance.
class Animal {
name: string;
constructor(name: string) {
this.name = name;
}
}
class Dog extends Animal {
bark() {
console.log('Woof!');
}
}
function makeSound(animal: Animal) {
if (animal instanceof Dog) {
animal.bark(); // Type: Dog
} else {
console.log('Unknown animal');
}
}
const dog = new Dog('Buddy');
const animal = new Animal('Unknown');
makeSound(dog); // Output: Woof!
makeSound(animal); // Output: Unknown animal// Type '"hello"'
let x = "hello" as const;
// Type 'readonly [10, 20]'
let y = [10, 20] as const;
// Type '{ readonly text: "hello" }'
let z = { text: "hello" } as const;