TypeScript Reference : Cheat Sheets

If you are a developer working with TypeScript, having a cheat sheet can be a valuable resource. A cheat sheet is a concise reference tool that provides a quick overview of the language’s syntax, features, and best practices. Whether you are a beginner or an experienced programmer, having a cheat sheet can help you save time and improve your coding efficiency.

When it comes to TypeScript, there are several cheat sheets available online that can help you quickly find the information you need. These cheat sheets cover a wide range of topics, including the basics of the language, data types, functions, classes, and modules. They also provide examples and explanations to help you understand the concepts better.

One of the advantages of using a cheat sheet is that it allows you to find the syntax or feature you’re looking for without having to browse through lengthy documentation or search the web. With a cheat sheet, you can quickly find the information you need and get back to coding.

Whether you’re looking for a quick reference guide or a comprehensive resource, a TypeScript cheat sheet can be an invaluable tool for any developer. It can help you save time, improve your productivity, and become a more effective TypeScript programmer. So, next time you find yourself needing to look up something in TypeScript, don’t forget to consult your cheat sheet!

Basic Syntax and Types

Variables

In TypeScript, you declare variables using the let or const keyword. The let keyword is used for variables whose values can be reassigned, while the const keyword is used for variables whose values cannot be changed once assigned.

Example:

let x = 10;

const y = 20;

Data Types

TypeScript supports various data types, including:

• Boolean: Represents a logical value – true or false.
• Number: Represents both integer and floating-point numbers.
• String: Represents a sequence of characters.
• Array: Represents a collection of elements of the same type.
• Tuple: Represents an array with a fixed number of elements, where each element may have a different type.
• Object: Represents a non-primitive type.
• Any: Represents any type.
• Void: Represents the absence of a value.
• Null and Undefined: Represents the absence of a value.
• Enum: Represents a set of named constants.
• Union: Represents a value that can be one of several types.
• Intersection: Represents a type that combines multiple types.
• Type Alias: Represents a custom type definition.

Function

A function in TypeScript is defined using the function keyword, followed by the name of the function and a pair of parentheses containing the function parameters. The function body is enclosed in curly braces ({}) and the return type is specified after the parentheses.

Example:

function add(x: number, y: number): number {

return x + y;
}

Control Flow

Control flow statements in TypeScript are similar to JavaScript. They include:

• If…else: Executes a block of code if a specified condition is true, and another block of code if it is false.
• Switch: Evaluates an expression and executes code block(s) that correspond to matching case label(s).
• For: Executes a block of code a specified number of times.
• While: Executes a block of code as long as a specified condition is true.
• Do…while: Executes a block of code once, and then repeats the execution as long as a specified condition is true.
• Break: Terminates the current loop, switch, or label statement.
• Continue: Jumps to the next iteration of a loop.

Type Assertion

Type assertion is a way to explicitly inform the TypeScript compiler about the type of a value. It is used when you know more about a value’s type than TypeScript does.

Example:

let someValue: any = "hello world";

let strLength: number = (someValue as string).length;

Modules

Modules in TypeScript are used to organize code and provide the ability to reuse and share code between different files.

Example:

// module.ts

export function greet(name: string) {
return "Hello, " + name + "!";
}
// main.ts
import {greet} from "./module";
console.log(greet("John")); // Output: Hello, John!

Classes

Classes in TypeScript are used to create objects with properties and methods. They provide a way to define blueprint for creating objects.

Example:

class Person {

name: string;
age: number;
constructor(name: string, age: number) {
this.name = name;
this.age = age;
}
greet() {
console.log("Hello, my name is " + this.name + " and I am " + this.age + " years old.");
}
}
let person = new Person("John", 25);
person.greet(); // Output: Hello, my name is John and I am 25 years old.

Interfaces

Interfaces in TypeScript are used to define the shape of an object. They specify properties and methods that must be implemented by an object.

Example:

interface Vehicle {

brand: string;
speed: number;
accelerate(): void;
brake(): void;
}
class Car implements Vehicle {
brand: string;
speed: number;
constructor(brand: string) {
this.brand = brand;
this.speed = 0;
}
accelerate() {
this.speed += 10;
}
brake() {
this.speed -= 10;
}
}
let car = new Car("Ford");
car.accelerate();
console.log(car.speed); // Output: 10

Generics

Generics in TypeScript allow you to create functions and classes that can work with different types.

Example:

function identity(arg: T): T {

return arg;
}
let result = identity(10);
console.log(result); // Output: 10

Variable Declarations and Scopes

Variables in TypeScript

In TypeScript, variables can be declared using the let, const, or var keywords. The let keyword is used for block-scoped variables, the const keyword is used for block-scoped variables that cannot be reassigned, and the var keyword is used for function-scoped variables.

Variable Types

In TypeScript, variable types can be explicitly specified using a colon followed by the type name. For example, let count: number; declares a variable named count of type number. TypeScript also supports type inference, which means that the type of a variable can be automatically inferred based on its initial value. For example, let name = 'John'; declares a variable named name with type string.

Variable Scopes

TypeScript supports block-scoped variables using the let and const keywords. Block-scoped variables are only accessible within the block (i.e., the pair of curly braces) in which they are defined. For example:

if (true) {

let message = 'Hello!';
console.log(message); // Output: Hello!
}
console.log(message); // Error: Cannot find name 'message'

On the other hand, variables declared using the var keyword are function-scoped. This means that they are accessible throughout the entire function in which they are declared. For example:

function printNumber() {

for (var i = 0; i < 5;="" i++)="">
console.log(i);
}
console.log(i); // Output: 5
}
printNumber();

Global Variables

Variables declared outside of any function or block are considered global variables and are accessible everywhere in the program. However, it is generally recommended to minimize the use of global variables to avoid potential naming conflicts and unintended side effects.

Shadowing Variables

In TypeScript, it is possible to declare a variable with the same name as an existing variable in an outer scope. This is called variable shadowing. When a variable is shadowed, the inner variable takes precedence over the outer variable within its scope. For example:

let count = 5;

function printCount() {
let count = 10;
console.log(count); // Output: 10
}
printCount();
console.log(count); // Output: 5

Hoisting

In JavaScript, variables declared using the var keyword are hoisted to the top of their containing function or script. This means that you can use a variable before it is declared, but its value will be undefined. TypeScript inherits this behavior from JavaScript. However, variables declared using the let or const keywords are not hoisted and will result in a reference error if used before they are declared.

Conclusion

Understanding variable declarations and scopes is essential for writing robust and maintainable TypeScript code. By using the appropriate keywords and understanding the scoping rules, you can ensure that your variables are properly defined and accessed within your program.

Functions and Arrow Functions

Function Types

In TypeScript, we can define function types using the following syntax:

let myFunction: (param1: type1, param2: type2, ...) => returnType;

Here:

• myFunction is the name of the function,
• param1, param2, … are the parameters of the function,
• type1, type2, … are the types of the parameters,
• returnType is the type of the value that the function returns.

Defining Functions

In TypeScript, functions can be defined using the function keyword or using arrow function notation.

// Function declaration

function add(x: number, y: number): number {
return x + y;
}
// Arrow function
let multiply = (x: number, y: number): number => {
return x * y;
};

In the above example:

• The add function takes two parameters x and y of type number, and returns a value of type number.
• The multiply arrow function also takes two parameters x and y of type number, and returns a value of type number. The arrow function is assigned to the variable multiply.

Optional Parameters

In TypeScript, function parameters can be made optional by appending a question mark ? to their names.

function greet(name: string, age?: number): void {

if(age) {
console.log("Hello " + name + ", you are " + age + " years old.");
} else {
console.log("Hello " + name);
}
}
greet("John");
greet("Alice", 25);

In the above example, the age parameter is marked as optional using the ? symbol. When invoking the greet function, we can choose to omit or provide a value for the age parameter.

Default Parameters

In TypeScript, function parameters can have default values by using the assignment operator (=) to assign a default value.

function greet(name: string, age: number = 18): void {

console.log("Hello " + name + ", you are " + age + " years old.");
}
greet("John");
greet("Alice", 25);

In the above example, the age parameter is assigned a default value of 18. If the age parameter is not provided when invoking the greet function, it will automatically default to 18.

Rest Parameters

In TypeScript, functions can have rest parameters to accept variable-length arguments. Rest parameters are denoted by prefixing the parameter name with three dots (...).

function sum(...numbers: number[]): number {

let total = 0;
numbers.forEach(num => total += num);
return total;
}
let result = sum(1, 2, 3, 4, 5);
console.log(result); // Output: 15

In the above example, the sum function accepts any number of arguments and calculates their sum. The rest parameter numbers is an array of numbers.

Arrow Functions

Arrow functions in TypeScript provide a more concise syntax for defining functions.

let square = (x: number): number => x * x;

console.log(square(5)); // Output: 25

In the above example, the square arrow function takes a parameter x of type number and returns the square of x by multiplying it with itself.

Function Overloading

In TypeScript, we can declare multiple function signatures for the same function to handle different parameter types or different numbers of parameters. This is known as function overloading.

function combine(a: string, b: string): string;

function combine(a: number, b: number): number;
function combine(a: any, b: any): any {
return a + b;
}
let result1 = combine("Hello", "TypeScript");
let result2 = combine(5, 10);
console.log(result1); // Output: "HelloTypeScript"
console.log(result2); // Output: 15

In the above example, we have defined multiple function signatures for the combine function. It can accept two strings and return a string, or it can accept two numbers and return a number. The actual implementation of the function comes in the last function signature, which accepts parameters of type any.

Object-oriented Programming

Introduction

Object-oriented programming (OOP) is a programming paradigm that organizes code into objects, which are instances of classes. It allows for modularity and reusability of code, making it easier to manage and maintain large codebases.

Classes and Objects

In OOP, a class is a blueprint for creating objects. Objects are instances of classes that have their own unique attributes and behaviors. Classes define the properties and methods that objects of the class will have.

To create a new object of a class, you use the new keyword followed by the class name and parentheses. For example:

class Car {

make: string;
model: string;
year: number;
constructor(make: string, model: string, year: number) {
this.make = make;
this.model = model;
this.year = year;
}
start() {
console.log(`Starting ${this.make} ${this.model}`);
}
}
const myCar = new Car("Honda", "Civic", 2021);
myCar.start();

This example defines a Car class with properties (make, model, and year) and a method (start). It then creates a new object of the Car class named myCar using the constructor function.

Inheritance

Inheritance is a key concept in OOP. It allows you to create a new class based on an existing class, inheriting its properties and methods. The new class is called a subclass or derived class, and the existing class is called the parent class or base class.

To create a subclass, you use the extends keyword followed by the name of the parent class. The subclass can override existing properties and methods or add new ones.

class ElectricCar extends Car {

batteryCapacity: number;
constructor(make: string, model: string, year: number, batteryCapacity: number) {
super(make, model, year);
this.batteryCapacity = batteryCapacity;
}
charge() {
console.log(`Charging ${this.make} ${this.model} with capacity ${this.batteryCapacity}`);
}
}
const myElectricCar = new ElectricCar("Tesla", "Model 3", 2022, 75);
myElectricCar.start();
myElectricCar.charge();

In this example, the ElectricCar class is a subclass of the Car class. It adds a new property (batteryCapacity) and a new method (charge). The subclass also calls the parent class’s constructor using the super keyword.

Encapsulation

Encapsulation is the practice of hiding internal details and exposing only what is necessary. It helps to prevent the direct modification of properties and methods from outside the class.

In TypeScript, you can use access modifiers to specify the visibility of properties and methods within a class. There are three access modifiers: public, private, and protected.

• public – Can be accessed from anywhere.
• private – Can only be accessed within the same class.
• protected – Can be accessed within the same class and its subclasses.

class Person {

private name: string;
constructor(name: string) {
this.name = name;
}
greet() {
console.log(`Hello, my name is ${this.name}`);
}
}
const person = new Person("John");
person.greet();
console.log(person.name); // Error: 'name' is private

In this example, the name property is marked as private. It can only be accessed within the class itself. Attempting to access it from outside the class will result in an error.

Polymorphism

Polymorphism allows objects of different types to be treated as objects of a common type. It allows for flexibility and extensibility in the code.

TypeScript supports polymorphism through method overriding and method overloading.

• Method overriding – A subclass can provide its own implementation of a method defined in the parent class.
• Method overloading – A class can have multiple methods with the same name but different parameters.

class Animal {

makeSound() {
console.log("Animal makes a sound");
}
}
class Dog extends Animal {
makeSound() {
console.log("Dog barks");
}
}
class Cat extends Animal {
makeSound() {
console.log("Cat meows");
}
}
const animals: Animal[] = [new Dog(), new Cat()];
animals.forEach(animal => animal.makeSound());

In this example, the Animal class has a makeSound method. The Dog and Cat classes override this method with their own implementations. The forEach loop demonstrates polymorphism by treating the objects as Animal instances, even though they are actually instances of the subclasses.

Conclusion

Object-oriented programming allows for organized and modular code through the use of classes, objects, inheritance, encapsulation, and polymorphism. It provides a powerful way to structure and maintain complex software systems.

Arrays and Tuples

Arrays

An array is a collection of elements of the same type. In TypeScript, you can define an array using square brackets [] and the type of its elements.

Array Declaration:

• let arr: number[]; – declares an array of numbers.
• let arr: string[]; – declares an array of strings.
• let arr: boolean[]; – declares an array of booleans.
• let arr: any[]; – declares an array of any type.

Array Initialization:

An array can be initialized using the following syntax:

let arr: number[] = [1, 2, 3];

let arr: string[] = ['apple', 'banana', 'cherry'];

Array Access:

You can access individual elements of an array using square brackets [] and the index of the element:

let arr: number[] = [1, 2, 3];

let firstElement: number = arr[0]; // access the first element of the array

Tuples

A tuple is similar to an array, but it can contain elements of different types. In TypeScript, you can define a tuple using square brackets [] and the types of its elements separated by commas.

Tuple Declaration:

let tuple: [number, string, boolean];

Tuple Initialization:

A tuple can be initialized using the following syntax:

let tuple: [number, string, boolean] = [1, 'hello', true];

Tuple Access:

You can access individual elements of a tuple using square brackets [] and the index of the element:

let tuple: [number, string, boolean] = [1, 'hello', true];

let secondElement: string = tuple[1]; // access the second element of the tuple

Tuple Update:

You can update individual elements of a tuple by assigning new values:

let tuple: [number, string, boolean] = [1, 'hello', true];

tuple[0] = 10; // update the first element

Tuple Unpacking:

You can unpack a tuple into multiple variables:

let tuple: [number, string] = [1, 'hello'];

let [numberValue, stringValue] = tuple;

Tuple Rest Elements:

A tuple can have rest elements, which are prefixed with three dots …:

let tuple: [number, string, ...boolean[]] = [1, 'hello', true, false, true];

Tuple Read-only Elements:

You can make individual elements of a tuple read-only:

let tuple: readonly [number, string] = [1, 'hello'];

tuple[0] = 10; // Error: Index signature in type 'readonly [number, string]' only permits reading

Conclusion

Arrays and tuples are powerful data structures in TypeScript that provide flexibility and type safety. They allow you to store and manipulate collections of values with different types.

Interfaces and Type Aliases

Interfaces

An interface in TypeScript is a way to define the structure of an object. It is a contract that describes the properties and methods that an object must have. Here is an example of an interface:

interface Person {

name: string;
age: number;
greet: () => void;
}

In the above example, the interface defines that a person object must have a name property of type string, an age property of type number, and a greet method that takes no arguments and returns void.

Interfaces can also be extended to inherit properties and methods from another interface:

interface Employee extends Person {

employeeId: number;
}

In the above example, the Employee interface extends the Person interface, which means that an employee object must have all the properties and methods defined in both interfaces.

Type Aliases

A type alias in TypeScript allows you to create a new name for a type. It can be used to simplify complex types or to create reusable types. Here is an example of a type alias:

type Person = {

name: string;
age: number;
greet: () => void;
};

In the above example, the Person type alias is equivalent to the Person interface defined earlier. Both can be used to define objects with the same structure.

Type aliases can also be used with union types, intersection types, and more complex type expressions to create more flexible and expressive types.

Choosing Between Interfaces and Type Aliases

In general, you can use either interfaces or type aliases to define the structure of an object. However, there are some subtle differences between them:

• Interfaces can be implemented by classes, while type aliases cannot.
• Type aliases can represent computed properties using mapped types.
• Interfaces can be merged or extended, while type aliases cannot.
• Type aliases are more flexible when it comes to creating complex types using union types, intersection types, and conditional types.

It is recommended to use interfaces for most cases, especially when working with libraries or frameworks that expect objects to implement certain interfaces. However, type aliases can be a better choice when you need to define more complex types or when you want to create reusable types.

Generics and Decorators

Generics

Generics allow you to create reusable components by creating a placeholder for a type that can be used in multiple places in the code. Using generics increases code flexibility and reduces the need for code duplication.

To create a generic type or function, you use the syntax T inside angle brackets, <>

Here’s an example of a generic function that swaps the values of two variables:

function swap<T>(a: T, b: T): void {
let temp: T = a;
a = b;
b = temp;
}
let a: number = 5;
let b: number = 10;
swap(a, b);
console.log(a, b); // Output: 10, 5

Decorators

Decorators are a design pattern that allows you to add behavior or metadata to a class, method, property, or parameter declaration at runtime. They are declared using the @ symbol followed by the decorator function name.

Here’s an example of a decorator that logs a message before and after a method is called:

function log(target: any, key: string, descriptor: PropertyDescriptor) {
const originalMethod = descriptor.value;
descriptor.value = function (...args: any[]) {
console.log(`Method ${key} called with arguments: ${args}`);
const result = originalMethod.apply(this, args);
console.log(`Method ${key} returned: ${result}`);
return result;
};
return descriptor;
}
class Example {
@log
greet(name: string): string {
return `Hello, ${name}!`;
}
}
const example = new Example();
example.greet("John");
// Output:
// Method greet called with arguments: John
// Method greet returned: Hello, John!

Decorators can also be used for class and property declaration, and they can take arguments. They are extensively used in frameworks like Angular for things like dependency injection and component decoration.

Modules and Namespaces

Modules

A module is a self-contained unit of code that encapsulates related functionality. It can be exported and imported in other modules, allowing for code reusability and organization.

In TypeScript, modules are usually defined in separate files. Each file can have one or more modules, and each module can contain functions, classes, variables, and other code. Modules can be imported and used by other modules using the import keyword.

Exporting and Importing Modules

To export a module, you can use the export keyword before the respective module’s declaration. This makes the module available for import in other modules.

export module MyModule {

// module code
}
export function myFunction() {
// function code
}

To import a module, you can use the import keyword followed by the module’s path. You can then access the exported elements using their names.

import { MyModule, myFunction } from "./myModule";

MyModule // access MyModule
myFunction() // call myFunction

Default Exports

A module can have a default export, which is the main entity that is exported from the module. A default export can be a function, class, object, or any value.

// myModule.ts

export default function myFunction() {
// function code
}
// main.ts
import myFunction from "./myModule";
myFunction() // call the default exported function

Namespaces

A namespace is a way to group related code declarations into a container. It helps avoid naming collisions by providing a hierarchical organization of identifiers.

In TypeScript, you can define a namespace using the namespace keyword. The exported members within a namespace can be accessed using the dot notation.

namespace MyNamespace {

export const myVariable = 123;
export function myFunction() {
// function code
}
}
const var = MyNamespace.myVariable; // accessing a variable
MyNamespace.myFunction(); // calling a function

Reference Import

You can also reference a module using /// <reference path="..." /> syntax. This is used when importing declaration files without using a module loader like import.

/// <reference path="myModule.d.ts" />

Error Handling and Debugging

Error Handling

• try/catch: Use the try/catch statement to catch errors and handle them gracefully. The code inside the try block is executed, and if an error occurs, the catch block is executed.
• throw: Use the throw statement to manually throw an error. This is useful when you want to explicitly handle specific error cases or create your own custom error messages.
• finally: Use the finally block to specify code that will be executed regardless of whether an error occurred or not. The code inside the finally block will always run, regardless of whether an exception was thrown.

Debugging

• console.log: The console.log() function is a common way to output values to the console for debugging purposes. It prints the specified message to the console.
• console.error: The console.error() function is used to output an error message to the console. This can be helpful for identifying and fixing errors in your code.
• debugger statement: Use the debugger statement to add a breakpoint to your code. When the code reaches the debugger statement, it will pause execution and allow you to inspect the current state of the program.
• source maps: Use source maps to debug TypeScript code in the browser. Source maps map the compiled JavaScript code back to the original TypeScript code, making it easier to debug and trace errors in the original source code.

Common Error Types

Here are some common error types that you may encounter while working with TypeScript:

1. Syntax Errors: These errors occur when you write invalid TypeScript code that does not conform to the syntax rules. Syntax errors are usually caught by the TypeScript compiler and displayed as error messages.
2. Type Errors: Type errors occur when you try to assign values of incompatible types or perform operations on incompatible types. TypeScript’s static type checking can help you catch these errors at compile-time.
3. Reference Errors: Reference errors occur when you try to access variables or properties that are not defined or do not exist. These errors are usually caught at runtime and can be debugged using the browser’s developer tools.
4. Logic Errors: Logic errors occur when your code does not produce the expected behavior or results. These errors are usually caused by incorrect logic or assumptions in your code and can be debugged using logging and stepping through the code.

Best Practices

Here are some best practices to follow when it comes to error handling and debugging in TypeScript:

• Use TypeScript’s type system: Take advantage of TypeScript’s static type checking to catch type errors at compile-time instead of runtime.
• Handle errors gracefully: Use try/catch blocks to catch and handle errors in a controlled manner, providing meaningful error messages to the user.
• Use specific error types: Create your own custom error types to handle specific error cases, allowing for more precise error handling and easier debugging.
• Use logging for debugging: Log relevant values and messages to the console using console.log() or console.error() to help identify and fix errors in your code.
• Use breakpoints: Place breakpoints in your code using the debugger statement to pause execution and inspect the program’s state at specific points.
• Use source maps: Enable source maps to debug TypeScript code in the browser, allowing you to trace and debug errors in the original TypeScript source code.

Conclusion

Error handling and debugging are crucial aspects of software development. By understanding different error handling techniques and debugging tools, you can effectively identify and fix errors in your TypeScript code, resulting in more robust and reliable applications.

Advanced TypeScript Features

Type Inference

TypeScript has a powerful type inference system that allows variables to be implicitly typed based on their initial value. This means that you usually don’t need to specify the type of a variable explicitly if TypeScript can infer it from the context. For example:

“`typescript

let num = 5; // Type inference: num is inferred as number

let str = “hello”; // Type inference: str is inferred as string

let bool = true; // Type inference: bool is inferred as boolean

“`

Type Guards

Type guards are a TypeScript feature that allows you to narrow down the type of a variable within a conditional block. This is useful when working with union types or unknown types. There are several ways to implement type guards in TypeScript, including using the `typeof` operator, the `instanceof` operator, or user-defined type predicates. For example:

“`typescript

function foo(arg: string | number) {

if (typeof arg === “string”) {

// arg is narrowed down to string type

console.log(arg.toUpperCase());

} else if (typeof arg === “number”) {

// arg is narrowed down to number type

console.log(arg.toFixed(2));

}

}

“`

Generics

Generics allow you to define reusable functions, classes, or interfaces that can work with a variety of types. They provide a way to create flexible and type-safe code. The type parameter is specified within angle brackets (`<>`) and can be used throughout the generic code. For example:

“`typescript

function identity(value: T): T {

return value;

}

let result = identity(42); // result is inferred as number

“`

Type Mappings

Type mappings, also known as mapped types, allow you to transform the properties of an existing type to create a new type. This is useful when you need to add, remove, or modify properties of an existing type. TypeScript provides several built-in utility types that can be used for type mappings, such as `Partial`, `Required`, and `Pick`. For example:

“`typescript

type Person = {

name: string;

age: number;

};

type PartialPerson = Partial; // All properties of Person become optional

type RequiredPerson = Required; // All optional properties become required

type NameOnly = Pick; // Only the “name” property is included

“`

Decorators

Decorators are a TypeScript feature that allows you to add metadata or modify the behavior of classes, properties, methods, or parameters at design time. Decorators are specified using the `@` symbol followed by the decorator name. They can be used to implement cross-cutting concerns such as logging, caching, or validation. For example:

“`typescript

function log(target: any, propertyKey: string | symbol, descriptor: PropertyDescriptor) {

const originalMethod = descriptor.value;

descriptor.value = function (…args: any[]) {

console.log(`Calling method ${propertyKey.toString()}`);

const result = originalMethod.apply(this, args);

console.log(`Method ${propertyKey.toString()} returned: ${result}`);

return result;

};

return descriptor;

}

class Calculator {

@log

add(a: number, b: number) {

return a + b;

}

}

const calculator = new Calculator();

console.log(calculator.add(2, 3)); // Output: Calling method add, Method add returned: 5

“`

Conditional Types

Conditional types are a TypeScript feature that allows you to create types that depend on a condition. They use a conditional expression to determine the resulting type based on the types of the input. This is useful when you want to create generic types that behave differently depending on the type of the input. For example:

“`typescript

type TypeName = T extends string ? “string” :

T extends number ? “number” :

T extends boolean ? “boolean” :

“unknown”;

let nameOfType: TypeName; // nameOfType is inferred as “number”

“`

Intersection and Union Types

Intersection types allow you to combine multiple types into a single type. The resulting type will have all the properties of the individual types. This is done using the `&` operator. Union types, on the other hand, allow a value to be of more than one type. The resulting type will be a combination of the individual types. This is done using the `|` operator. For example:

“`typescript

type Employee = {

name: string;

age: number;

};

type Student = {

name: string;

grade: number;

};

type Person = Employee & Student; // Person will have properties name, age, and grade

let value: string | number; // value can be either a string or a number

“`

Declaration Merging

Declaration merging is a TypeScript feature that allows you to extend the types defined in external libraries or modules. It is especially useful when working with JavaScript libraries that don’t provide type definitions. Declaration merging allows you to add or modify properties, methods, or types of the existing declarations. For example:

“`typescript

// Original definition from an external library

interface SomeLibrary {

foo: number;

}

// Declaration merging to add a new property

interface SomeLibrary {

bar: string;

}

let library: SomeLibrary;

library.foo = 42;

library.bar = “hello”;

“`

Feature	Description
Type Inference	Allows variables to be implicitly typed based on their initial value.
Type Guards	Narrows down the type of a variable within a conditional block.
Generics	Allows the creation of reusable functions, classes, or interfaces that work with a variety of types.
Type Mappings	Transforms the properties of an existing type to create a new type.
Decorators	Adds metadata or modifies the behavior of classes, properties, methods, or parameters.
Conditional Types	Creates types that depend on a condition, allowing for generic types that behave differently based on input types.
Intersection and Union Types	Combines multiple types into a single type or allows a value to be of more than one type.
Declaration Merging	Extends the types defined in external libraries or modules.

FAQ:

What is TypeScript Reference: Cheat Sheets?

TypeScript Reference: Cheat Sheets is a collection of cheat sheets that serve as a quick reference guide for TypeScript developers. These cheat sheets provide a concise overview of various TypeScript concepts, syntax, and features.

What are the advantages of using TypeScript Reference: Cheat Sheets?

Using TypeScript Reference: Cheat Sheets has several advantages. Firstly, they provide a quick and easy way to refresh your memory on TypeScript concepts and syntax. Secondly, the cheat sheets are concise and well-organized, making it easy to find the information you need. Additionally, they cover a wide range of topics, making them suitable for both beginners and experienced TypeScript developers.

How can I use TypeScript Reference: Cheat Sheets?

You can use TypeScript Reference: Cheat Sheets by downloading or printing them and keeping them handy while you’re coding in TypeScript. Whenever you need to quickly reference a TypeScript concept or syntax, you can consult the cheat sheets to find the information you need.

What topics are covered in TypeScript Reference: Cheat Sheets?

TypeScript Reference: Cheat Sheets cover a wide range of topics, including basic types and interfaces, functions, classes and inheritance, modules, decorators, generics, and more. They provide a comprehensive overview of the TypeScript language and its features.

Are the cheat sheets suitable for beginners?

Yes, the cheat sheets are suitable for beginners. They provide a concise and easy-to-understand overview of TypeScript concepts and syntax, making them ideal for those who are just starting to learn TypeScript. The cheat sheets can help beginners quickly grasp the fundamentals of TypeScript and start writing TypeScript code.

Where can I find TypeScript Reference: Cheat Sheets?

You can find TypeScript Reference: Cheat Sheets on the official TypeScript website. They are available for free download in PDF format. You can also access them online and view them in your web browser.

Can I contribute to TypeScript Reference: Cheat Sheets?

Yes, you can contribute to TypeScript Reference: Cheat Sheets. The cheat sheets are hosted on GitHub, and you can submit pull requests to make contributions. This allows you to add or update content, fix errors, or suggest improvements to the cheat sheets.