Typescript Book

At typescriptbook.dev, our mission is to provide a comprehensive and accessible resource for individuals who are interested in learning the TypeScript programming language. We strive to create a welcoming and inclusive community where learners of all levels can come together to share knowledge, ask questions, and collaborate on projects. Our goal is to empower individuals to become proficient in TypeScript, and to help them develop the skills and confidence they need to succeed in their careers as developers.

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TypeScript Cheatsheet

This cheatsheet is designed to provide a quick reference guide for those who are new to TypeScript. It covers the basic concepts, topics, and categories related to TypeScript programming language.

Table of Contents

• Introduction
• Basic Concepts
• Data Types
• Variables
• Functions
• Classes
• Interfaces
• Enums
• Generics
• Modules
• Type Inference
• Type Compatibility
• Type Guards
• Type Aliases
• Type Assertions
• Conclusion

Introduction

TypeScript is a superset of JavaScript that adds optional static typing and other features to the language. It is designed to make it easier to write and maintain large-scale JavaScript applications.

TypeScript is developed and maintained by Microsoft, and it is open source and free to use. It is widely used in web development, and it is supported by popular frameworks like Angular and React.

Basic Concepts

• TypeScript Compiler: The TypeScript compiler is a tool that converts TypeScript code into JavaScript code. It checks the code for errors and generates JavaScript code that can be run in a browser or on a server.

• Type Annotations: Type annotations are used to specify the type of a variable, function parameter, or function return value. They are optional in TypeScript, but they can help catch errors early and make the code easier to understand.

• Type Inference: TypeScript can infer the type of a variable based on its value. This means that you don't always have to specify the type explicitly.

• Type Compatibility: TypeScript has a type system that allows for type checking and type inference. It also has a type compatibility system that allows for more flexible type checking.

• Type Guards: Type guards are used to narrow down the type of a variable based on a condition. They are often used with union types.

• Type Aliases: Type aliases are used to create a new name for an existing type. They can be used to make the code more readable and maintainable.

• Type Assertions: Type assertions are used to tell TypeScript that a value has a certain type, even if TypeScript can't infer it. They are often used when working with third-party libraries that don't have type definitions.

Data Types

TypeScript has several built-in data types:

• Boolean: Represents a logical value of true or false.
• Number: Represents a numeric value.
• String: Represents a sequence of characters.
• Array: Represents a collection of values of the same type.
• Tuple: Represents an array with a fixed number of elements, each with a specific type.
• Enum: Represents a set of named constants.
• Any: Represents any type.
• Void: Represents the absence of a value.
• Null: Represents a null value.
• Undefined: Represents an undefined value.
• Never: Represents a value that will never occur.

Variables

Variables in TypeScript can be declared using the let, const, or var keywords. let and const are block-scoped, while var is function-scoped.

let x: number = 10;
const y: string = "hello";
var z: boolean = true;

Functions

Functions in TypeScript can be declared using the function keyword. They can have optional and default parameters, and they can return a value of a specific type.

function add(x: number, y: number): number {
  return x + y;
}

function greet(name: string, message: string = "Hello"): void {
  console.log(`${message}, ${name}!`);
}

Classes

Classes in TypeScript are similar to classes in other object-oriented languages. They can have properties, methods, and constructors. They can also implement interfaces and extend other classes.

class Person {
  name: string;
  age: number;

  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }

  greet(): void {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old.`);
  }
}

const person = new Person("John", 30);
person.greet();

Interfaces

Interfaces in TypeScript are used to define the shape of an object. They can be used to enforce a certain structure on an object, and they can be implemented by classes.

interface Person {
  name: string;
  age: number;
}

function greet(person: Person): void {
  console.log(`Hello, my name is ${person.name} and I am ${person.age} years old.`);
}

const john: Person = { name: "John", age: 30 };
greet(john);

Enums

Enums in TypeScript are used to define a set of named constants. They can be used to make the code more readable and maintainable.

enum Color {
  Red,
  Green,
  Blue,
}

const color: Color = Color.Red;
console.log(color); // 0

Generics

Generics in TypeScript are used to create reusable code that can work with different types. They can be used with functions, classes, and interfaces.

function identity<T>(value: T): T {
  return value;
}

const result: number = identity<number>(10);
console.log(result); // 10

Modules

Modules in TypeScript are used to organize code into separate files. They can be imported and exported using the import and export keywords.

// math.ts
export function add(x: number, y: number): number {
  return x + y;
}

// app.ts
import { add } from "./math";

const result: number = add(10, 20);
console.log(result); // 30

Type Inference

TypeScript can infer the type of a variable based on its value. This means that you don't always have to specify the type explicitly.

const x = 10; // x is inferred to be of type number
const y = "hello"; // y is inferred to be of type string

Type Compatibility

TypeScript has a type system that allows for type checking and type inference. It also has a type compatibility system that allows for more flexible type checking.

interface Person {
  name: string;
}

let person: Person = { name: "John", age: 30 };
let person2 = person; // person2 is inferred to be of type Person

Type Guards

Type guards are used to narrow down the type of a variable based on a condition. They are often used with union types.

interface Cat {
  name: string;
  meow(): void;
}

interface Dog {
  name: string;
  bark(): void;
}

function makeSound(animal: Cat | Dog): void {
  if ("meow" in animal) {
    animal.meow();
  } else {
    animal.bark();
  }
}

Type Aliases

Type aliases are used to create a new name for an existing type. They can be used to make the code more readable and maintainable.

type Person = {
  name: string;
  age: number;
};

const john: Person = { name: "John", age: 30 };

Type Assertions

Type assertions are used to tell TypeScript that a value has a certain type, even if TypeScript can't infer it. They are often used when working with third-party libraries that don't have type definitions.

const element = document.getElementById("my-element") as HTMLInputElement;
element.value = "hello";

Conclusion

This cheatsheet covered the basic concepts, topics, and categories related to TypeScript programming language. It is designed to provide a quick reference guide for those who are new to TypeScript. With this cheatsheet, you should be able to get started with TypeScript and start building your own applications.

Common Terms, Definitions and Jargon

1. TypeScript - A superset of JavaScript that adds optional static typing and other features to the language.
2. Static Typing - A type system where types are checked at compile-time rather than runtime.
3. Dynamic Typing - A type system where types are checked at runtime rather than compile-time.
4. Compiler - A program that translates source code into machine code or another form of executable code.
5. Type Inference - The ability of a compiler to automatically deduce the type of a variable based on its usage.
6. Type Annotation - The act of explicitly specifying the type of a variable or function parameter.
7. Interface - A contract that defines the structure and behavior of an object.
8. Class - A blueprint for creating objects that encapsulates data and behavior.
9. Object - An instance of a class that has its own state and behavior.
10. Function - A block of code that performs a specific task and can be called multiple times.
11. Arrow Function - A shorthand syntax for defining functions in JavaScript and TypeScript.
12. Type Alias - A way to create a new name for an existing type.
13. Union Type - A type that can be one of several different types.
14. Intersection Type - A type that represents the combination of two or more types.
15. Type Guard - A way to narrow down the type of a variable based on a condition.
16. Type Assertion - A way to tell the compiler the type of a variable when it cannot be inferred.
17. Type Compatibility - The ability of two types to be used interchangeably in a given context.
18. Type System - The set of rules that govern the use of types in a programming language.
19. Type Safety - The property of a type system that prevents certain types of errors at compile-time.
20. Type Erasure - The process of removing type information from a program during compilation.

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