Table of Contents

• Introduction
• What Are Interfaces in TypeScript?
  • Basic Syntax of Interfaces
  • Example: Defining an Interface
• Why Use Interfaces?
  • Structuring Code with Interfaces
  • Benefits of Using Interfaces
• Extending Interfaces
  • Syntax for Extending Interfaces
  • Example: Interface Extension
• Interface vs Type Alias
  • Key Differences
  • When to Use One Over the Other
• Implementing Interfaces in Classes
  • Syntax for Implementing Interfaces
  • Example: Implementing Interfaces in Classes
• Optional and Readonly Properties in Interfaces
  • Optional Properties
  • Readonly Properties
• Conclusion

Introduction

In TypeScript, interfaces are one of the most powerful tools for defining the structure of objects and enforcing type safety. An interface in TypeScript is like a contract that defines the shape of an object, specifying what properties and methods it should have. It plays a significant role in object-oriented programming (OOP) by providing a blueprint for classes and ensuring that objects adhere to a certain structure.

In this article, we will dive deep into TypeScript interfaces, explore their syntax and use cases, and show you how they help in building robust and type-safe applications. We will also compare interfaces with type aliases and cover some advanced topics like extending interfaces and implementing them in classes.

What Are Interfaces in TypeScript?

An interface in TypeScript is a way to define a contract for the structure of an object, including its properties and methods. Interfaces ensure that the object adheres to a specific shape, making it easier to reason about and maintain the code.

Basic Syntax of Interfaces

To define an interface in TypeScript, use the interface keyword followed by the interface name and the shape of the object it represents:

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

In this example, the Person interface defines the structure for an object with a name (string) and an age (number). Now, you can use this interface to type-check objects that conform to this structure.

Example: Defining an Interface

Here’s how you can use the Person interface to create an object:

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

The person object must have a name property of type string and an age property of type number, as defined in the Person interface. If you try to assign an object that doesn’t match this structure, TypeScript will raise a compile-time error.

Why Use Interfaces?

Interfaces are essential for several reasons, as they help you enforce structure, improve code readability, and maintain consistency across your codebase. Let’s explore the main reasons for using interfaces in TypeScript.

Structuring Code with Interfaces

Interfaces allow you to structure your code by clearly defining how objects should look. This is especially useful in larger projects, where different components or modules need to interact with each other. By enforcing consistent object shapes, interfaces help maintain a clean and predictable codebase.

For example, imagine you are working on a project that involves various data models. By defining interfaces for these models, you ensure that all related objects conform to a consistent structure:

interface Product {
  id: number;
  name: string;
  price: number;
}

interface Order {
  orderId: number;
  product: Product;
  quantity: number;
}

In this case, the Order interface relies on the Product interface, ensuring that all orders have a valid product with the necessary properties.

Benefits of Using Interfaces

1. Type safety: Interfaces help enforce the correct structure for objects and functions, reducing the chance of errors during runtime.
2. Reusability: Once you define an interface, it can be reused across your codebase to type-check multiple objects with the same shape.
3. Clarity: By using interfaces, you can clearly express the expected shape of data, making your code easier to understand for other developers.
4. Extensibility: Interfaces can be extended, allowing you to build on existing contracts without modifying the original structure.

Extending Interfaces

One of the powerful features of interfaces in TypeScript is the ability to extend them. Extending an interface means creating a new interface that inherits the properties and methods of an existing interface while adding new properties or methods. This helps in creating more specialized types while keeping the base contract intact.

Syntax for Extending Interfaces

To extend an interface, use the extends keyword:

interface Employee extends Person {
  jobTitle: string;
}

Here, the Employee interface extends the Person interface, meaning it inherits the name and age properties from Person and adds the jobTitle property.

Example: Interface Extension

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

interface Employee extends Person {
  jobTitle: string;
}

const employee: Employee = {
  name: "Alice",
  age: 28,
  jobTitle: "Software Engineer"
};

In this example, the Employee interface inherits the properties from Person and adds its own property, jobTitle. The employee object must satisfy the structure defined by the Employee interface.

Interface vs Type Alias

While both interfaces and type aliases can be used to define object shapes, there are some important differences between them. Here’s a comparison to help you decide when to use each:

When to Use One Over the Other

• Use interfaces when you want to define object shapes, especially if you plan to extend or implement them in classes.
• Use type aliases when you need more flexibility, such as defining unions, intersections, or even primitive types.

Implementing Interfaces in Classes

Interfaces are often used in object-oriented programming to define contracts for classes. Classes can then implement these interfaces to ensure they adhere to a specific structure.

Syntax for Implementing Interfaces

To implement an interface in a class, use the implements keyword:

interface Animal {
  name: string;
  makeSound(): void;
}

class Dog implements Animal {
  name: string;
  
  constructor(name: string) {
    this.name = name;
  }

  makeSound() {
    console.log("Woof!");
  }
}

const dog = new Dog("Rex");
dog.makeSound();  // Output: Woof!

In this example, the Dog class implements the Animal interface. The class must provide the name property and the makeSound() method as specified by the interface.

Optional and Readonly Properties in Interfaces

TypeScript interfaces support two additional features: optional properties and readonly properties.

Optional Properties

Optional properties are properties that are not required to be present when creating an object. To mark a property as optional, use the ? symbol:

interface Product {
  id: number;
  name: string;
  price?: number;  // Optional property
}

const product: Product = {
  id: 1,
  name: "Laptop"
};

In this example, the price property is optional, so the product object can be created without it.

Readonly Properties

Readonly properties are properties that cannot be modified after the object is created. To make a property readonly, use the readonly keyword:

interface Product {
  readonly id: number;
  name: string;
}

const product: Product = {
  id: 1,
  name: "Laptop"
};

// product.id = 2;  // Error: Cannot assign to 'id' because it is a read-only property.

In this example, the id property is marked as readonly, so it cannot be modified after the object is created.

Conclusion

Interfaces are a fundamental feature in TypeScript, providing a way to define the structure of objects and ensuring type safety throughout your application. By using interfaces, you can create flexible, reusable, and type-safe code that adheres to clear contracts.

We’ve covered the basics of interfaces, their benefits, and advanced topics like extending interfaces and implementing them in classes. We’ve also highlighted the differences between interfaces and type aliases, helping you decide when to use each. By using interfaces effectively, you can build maintainable, scalable, and error-free TypeScript applications.

Table of Contents

• Introduction
• What Are Type Aliases?
  • Basic Syntax and Example
  • Practical Use Cases for Type Aliases
• Advanced Type Aliases
  • Union and Intersection Types with Type Aliases
  • Recursive Types and Type Aliases
• Pitfalls of Type Aliases
  • Type Aliases vs Interfaces
  • Potential Performance Issues
  • Overusing Type Aliases
• Conclusion

Introduction

Type aliases are one of TypeScript’s most versatile features. They provide a way to define custom names for types, enhancing code readability, reusability, and maintainability. By creating aliases, you can define complex types once and reuse them throughout your codebase.

However, while type aliases offer significant advantages, they also come with some potential pitfalls if misused. In this article, we’ll explore the power of type aliases, dive into advanced usage scenarios, and highlight some of the common mistakes developers can make when using them.

What Are Type Aliases?

A type alias in TypeScript is a mechanism for creating a new name for an existing type. The alias does not create a new type itself; it simply acts as a reference to an existing type. Type aliases can be used to represent primitive types, object shapes, functions, and even more complex types like unions and intersections.

Basic Syntax and Example

To define a type alias, use the type keyword followed by the alias name and the type it represents.

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

In this example, User is a type alias for an object that has name (a string) and age (a number).

Now you can use the User alias anywhere in your code:

const user: User = {
  name: "Alice",
  age: 30
};

Practical Use Cases for Type Aliases

Type aliases are commonly used in situations where defining complex object types, functions, or union types is necessary. Below are some common use cases:

1. Defining object shapes:

type Product = {
  id: number;
  name: string;
  price: number;
};

2. Defining function signatures:

type Multiply = (a: number, b: number) => number;

const multiply: Multiply = (a, b) => a * b;

3. Using union types:

type Response = "success" | "error";

const response: Response = "success"; // Can be either "success" or "error"

4. Intersection types:

type Address = {
  street: string;
  city: string;
};

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

type Employee = Person & Address;

const employee: Employee = {
  name: "John",
  age: 28,
  street: "123 Main St",
  city: "Somewhere"
};

Advanced Type Aliases

Type aliases can become more powerful when combined with other TypeScript features, such as union types, intersection types, and even recursive types. Let’s explore these advanced scenarios.

Union and Intersection Types with Type Aliases

Type aliases allow you to define complex combinations of types, such as unions and intersections, which are incredibly useful for modeling real-world data.

type SuccessResponse = {
  status: "success";
  data: string;
};

type ErrorResponse = {
  status: "error";
  error: string;
};

type ApiResponse = SuccessResponse | ErrorResponse;

In this example, ApiResponse can be either a SuccessResponse or an ErrorResponse. Using type aliases with union types allows you to handle multiple possibilities for return values in a concise and type-safe manner.

Recursive Types and Type Aliases

One of the more advanced features of type aliases is their ability to define recursive types, where a type references itself. This is particularly useful for modeling tree structures, like linked lists or nested data.

type TreeNode = {
  value: number;
  left?: TreeNode;
  right?: TreeNode;
};

const root: TreeNode = {
  value: 10,
  left: {
    value: 5
  },
  right: {
    value: 20
  }
};

In this case, TreeNode is a recursive type that represents a binary tree. The left and right properties are of type TreeNode, meaning each node can have other nodes as children.

Pitfalls of Type Aliases

While type aliases provide tremendous power, they also come with potential pitfalls that can lead to issues down the road. Let’s take a look at some common mistakes developers make when using type aliases.

Type Aliases vs Interfaces

Both type aliases and interfaces can be used to define object shapes in TypeScript. However, there are some subtle differences between the two.

• Interfaces can be extended and merged, while type aliases cannot.
• Type aliases are more flexible in defining union and intersection types, while interfaces cannot directly express these.

While both have their strengths, you should consider the specific use case when deciding between them. Type aliases are ideal for defining complex combinations of types (like unions or intersections), while interfaces are often better suited for defining object shapes and can be extended or merged across different parts of the code.

Potential Performance Issues

Type aliases do not create new types at runtime. However, using very complex or deeply nested types can still impact performance during development, particularly in large codebases. TypeScript may struggle to resolve overly complex type aliases, leading to slower type-checking processes.

For example, recursive types (such as those modeling trees or graphs) may become very difficult to work with when the depth of recursion increases. You can often solve this by using interfaces instead of deeply nested type aliases, or by simplifying the types.

Overusing Type Aliases

One common pitfall is overusing type aliases when they are unnecessary. It’s tempting to create a type alias for every possible type, but doing so can make your code harder to read and understand. In some cases, using type aliases can introduce unnecessary complexity.

For example:

type StringType = string;
type NumberType = number;

In this case, the type aliases are redundant and don’t offer any meaningful abstraction. Stick to using type aliases where they truly add value—such as when dealing with complex types, unions, or intersections.

Conclusion

Type aliases in TypeScript provide a powerful mechanism for defining custom types that can improve code readability, maintainability, and reusability. From defining simple object shapes to combining complex types with unions and intersections, type aliases offer great flexibility in managing types.

However, like any powerful tool, they come with potential pitfalls. Understanding the nuances of type aliases—such as the differences between type aliases and interfaces, the potential for performance issues, and the temptation to overuse them—will help you write cleaner, more maintainable code.

As a best practice, always consider the specific scenario and whether a type alias is the best tool for the job. Type aliases are most valuable when used thoughtfully, and when combined with other TypeScript features like unions, intersections, and recursive types, they become even more powerful.

Table of Contents

• Introduction
• What Are Union Types?
  • Syntax of Union Types
  • Example Code: Union Types
  • Practical Use Cases of Union Types
• What Are Intersection Types?
  • Syntax of Intersection Types
  • Example Code: Intersection Types
  • Practical Use Cases of Intersection Types
• Comparison Between Union and Intersection Types
• Conclusion

Introduction

TypeScript’s powerful type system allows developers to work with more advanced concepts that enhance code safety and maintainability. Among these advanced features, union types and intersection types are two of the most commonly used concepts for combining multiple types into a single type. Understanding how they work, and when to use them, is crucial for writing efficient and type-safe TypeScript code.

In this article, we will take a deep dive into union types and intersection types, explain their syntax, explore practical use cases, and compare them to help you decide when to use one over the other.

What Are Union Types?

Union types in TypeScript allow you to define a variable that can hold values of more than one type. You specify the possible types a variable can have by separating them with a vertical bar (|). A union type is useful when a variable can accept different types depending on the scenario, but you want to limit the options to a set of types.

Syntax of Union Types

The syntax for union types is simple: you use the pipe (|) operator to separate multiple types.

type UnionType = Type1 | Type2;

Example Code: Union Types

Let’s look at a practical example:

function getLength(input: string | number): number {
  if (typeof input === "string") {
    return input.length;  // TypeScript knows input is a string
  } else {
    return input.toString().length;  // TypeScript knows input is a number
  }
}

console.log(getLength("Hello, World!"));  // Output: 13
console.log(getLength(12345));  // Output: 5

In this example, the input parameter can either be a string or a number. Depending on the type of input, the function calculates the length differently: if it’s a string, it uses the .length property, and if it’s a number, it converts it to a string and calculates the length of that string.

Practical Use Cases of Union Types

Union types are commonly used in situations where a value can take on more than one type. Some practical use cases include:

• Handling multiple possible input types: Functions that can accept strings, numbers, or even objects.
• Handling error types: When a function can either return a success value or an error message, you can use union types to represent both possibilities.
• Working with optional parameters: Union types can be used to define an optional parameter (undefined as one of the types).

type Success = { success: true, data: string };
type Error = { success: false, error: string };

function fetchData(): Success | Error {
  // Simulate a random outcome (for example purposes)
  if (Math.random() > 0.5) {
    return { success: true, data: "Data fetched successfully" };
  } else {
    return { success: false, error: "Failed to fetch data" };
  }
}

What Are Intersection Types?

Intersection types in TypeScript allow you to combine multiple types into one. This means that the resulting type will have all the properties and methods of each of the combined types. Intersection types are useful when you want to ensure that a variable or parameter conforms to multiple types at once.

Syntax of Intersection Types

The syntax for intersection types is simple: you use the ampersand (&) operator to combine types.

type IntersectionType = Type1 & Type2;

Example Code: Intersection Types

Let’s see an example where we combine two types to form a new one:

type Person = { name: string, age: number };
type Employee = { jobTitle: string, salary: number };

type EmployeePerson = Person & Employee;

const employee: EmployeePerson = {
  name: "John Doe",
  age: 30,
  jobTitle: "Software Engineer",
  salary: 80000,
};

console.log(employee);

In this example, the EmployeePerson type is an intersection of the Person and Employee types. As a result, any object of type EmployeePerson must have both name and age (from Person) and jobTitle and salary (from Employee).

Practical Use Cases of Intersection Types

Intersection types are used in scenarios where a value needs to conform to multiple types simultaneously. Some practical use cases include:

• Combining multiple objects: You can combine types to create objects that conform to multiple contracts.
• Enhancing functionality: When a class or object needs to combine the behavior of two different interfaces, you can use intersection types.
• Mixin functionality: When working with object-oriented programming or functional programming, intersection types allow you to mix different features or behaviors into a single type.

interface CanFly {
  fly(): void;
}

interface CanSwim {
  swim(): void;
}

class FlyingSwimmingAnimal implements CanFly, CanSwim {
  fly() {
    console.log("Flying");
  }
  swim() {
    console.log("Swimming");
  }
}

type FlyingSwimmingAnimalType = CanFly & CanSwim;

const duck: FlyingSwimmingAnimalType = new FlyingSwimmingAnimal();
duck.fly();  // Output: Flying
duck.swim();  // Output: Swimming

Comparison Between Union and Intersection Types

While union and intersection types may seem similar, they serve different purposes. Let’s compare them:

| Feature | Union Types (|) | Intersection Types (&) |

|—————–|—————————|—————————-|

| Definition | A type that can be one of several types | A type that must satisfy all specified types |

| Behavior | Accepts values from any of the types | Requires a value to have properties of all combined types |

| Use Case | When a value can be one of several types | When a value needs to conform to multiple types simultaneously |

| Syntax | type X = Type1 | Type2; | type Y = Type1 & Type2; |

| Example | let value: string | number; | let value: Person & Employee; |

Conclusion

Union types and intersection types are two powerful tools in TypeScript that allow developers to create more flexible and robust code.

• Union types allow a variable to be one of several types, making them ideal for cases where the value could be one of multiple possibilities. They are best used when you want to define flexible function parameters or return types.
• Intersection types combine multiple types into one, ensuring the value satisfies all of the types’ properties and behaviors. They are useful when you want to create more complex structures that need to conform to multiple interfaces or types.

Understanding when to use each type is key to writing maintainable and type-safe TypeScript code. By mastering union and intersection types, you’ll be able to handle more complex data structures with ease and improve the reliability of your codebase.

Table of Contents

• Introduction
• What Are Literal Types?
  • String Literal Types
  • Numeric Literal Types
  • Boolean Literal Types
  • Example Code
• Type Narrowing: The Concept
  • What is Type Narrowing?
  • Type Guards
  • Example Code: Type Narrowing with typeof and instanceof
• Practical Use Cases of Literal Types and Type Narrowing
• Conclusion

Introduction

TypeScript is a superset of JavaScript that adds strong typing, making your code more predictable and less error-prone. One of the key features of TypeScript is its ability to narrow types and use literal values as specific types, allowing for more precise type checking. Literal types and type narrowing are two closely related concepts that provide significant power in making your TypeScript code more reliable.

In this article, we will explore the concept of literal types, how they differ from basic types, and how TypeScript’s type narrowing mechanism works. We will also dive into practical examples and use cases for these features to help you write more maintainable and bug-free TypeScript code.

What Are Literal Types?

Literal types in TypeScript refer to specific, fixed values that can be assigned to variables or used in function signatures. Instead of using general types like string or number, TypeScript allows you to define types based on the exact value of a variable. This enables more precise type checking and leads to fewer bugs.

String Literal Types

String literal types allow you to define types that can only be one of a limited set of string values. This is useful when you want to restrict a variable to specific string options.

Example Code:

type Direction = "left" | "right" | "up" | "down";

function move(direction: Direction) {
  console.log(`Moving ${direction}`);
}

move("left");  // OK
move("right");  // OK
move("north");  // Error: Argument of type 'north' is not assignable to parameter of type 'Direction'.

In this example, the Direction type can only be one of the specified string literals: "left", "right", "up", or "down". Any other string, like "north", will result in a compile-time error.

Numeric Literal Types

Just like string literal types, numeric literal types allow you to restrict a variable to specific numeric values. This is especially useful when you want to ensure a variable can only hold a predefined number.

Example Code:

type Age = 18 | 21 | 30;

let myAge: Age;
myAge = 21;  // OK
myAge = 25;  // Error: Type '25' is not assignable to type 'Age'.

Here, the Age type can only be 18, 21, or 30. Assigning any other number results in an error.

Boolean Literal Types

Boolean literal types allow you to specify whether a variable can only be true or false. This is useful when you want to define strict boolean values for certain flags or conditions.

Example Code:

type IsActive = true | false;

let isUserActive: IsActive;
isUserActive = true;  // OK
isUserActive = false;  // OK
isUserActive = "yes";  // Error: Type '"yes"' is not assignable to type 'IsActive'.

In this case, IsActive can only be true or false. Other values, such as a string, will not be allowed.

Type Narrowing: The Concept

Type narrowing is a technique used in TypeScript to refine the type of a variable within a specific scope, often within conditional blocks like if, switch, or loops. TypeScript uses control flow analysis to automatically narrow the type of a variable when it’s possible to determine the exact type of the variable at a certain point in the program.

Narrowing helps TypeScript understand that a variable’s type can be refined in certain contexts, and this allows the developer to take advantage of more specific behaviors.

What is Type Narrowing?

Type narrowing is the process of refining a broader type into a more specific type. TypeScript can narrow types based on various conditions like checking the value of a variable, its type, or its properties.

For instance, if a variable can be either a string or null, TypeScript can narrow the type down to a string within a block of code that checks if the variable is not null.

Type Guards

Type guards are expressions or techniques that help TypeScript understand the type of a variable in a given scope. Type guards allow TypeScript to infer the type more precisely and narrow down the possible types in conditional branches.

There are several ways to narrow types in TypeScript:

1. typeof Operator: You can use the typeof operator to narrow the type of primitive values like string, number, and boolean.
2. instanceof Operator: The instanceof operator is used to narrow types based on object instances, especially for classes.
3. User-defined Type Guards: TypeScript allows you to create custom type guards using functions that assert the type of a variable.

Example Code: Type Narrowing with typeof and instanceof

Narrowing with typeof

function printValue(value: string | number) {
  if (typeof value === "string") {
    console.log(value.toUpperCase());  // TypeScript knows value is a string
  } else {
    console.log(value.toFixed(2));  // TypeScript knows value is a number
  }
}

printValue("hello");  // Output: "HELLO"
printValue(123.456);  // Output: "123.46"

In this example, the typeof operator is used to narrow the type of the value variable. Inside the if block, TypeScript knows that value must be a string, and inside the else block, TypeScript knows that value must be a number.

Narrowing with instanceof

class Dog {
  bark() {
    console.log("Woof!");
  }
}

class Cat {
  meow() {
    console.log("Meow!");
  }
}

function speak(animal: Dog | Cat) {
  if (animal instanceof Dog) {
    animal.bark();  // TypeScript knows animal is a Dog
  } else {
    animal.meow();  // TypeScript knows animal is a Cat
  }
}

const myDog = new Dog();
speak(myDog);  // Output: "Woof!"

Here, instanceof is used to check whether the animal variable is an instance of the Dog class, thereby narrowing the type and allowing the correct method to be called.

Practical Use Cases of Literal Types and Type Narrowing

• Control Flow with Enum Values: Using literal types, you can enforce that only specific values are accepted in functions or variables, preventing invalid inputs. When combined with type narrowing, it ensures that the logic behaves according to the exact value of the literal.

type Status = "pending" | "approved" | "rejected";

function handleStatus(status: Status) {
  if (status === "pending") {
    console.log("Waiting for approval...");
  } else if (status === "approved") {
    console.log("Action approved!");
  } else {
    console.log("Action rejected.");
  }
}

• Form Validation: Literal types can be used to define strict values for form input options, and type narrowing ensures that the correct validation rules are applied based on user selection.
• Event Handling: Literal types are commonly used in event-driven architectures to specify exact event types, and type narrowing ensures that the right data is passed to the corresponding handler.

Conclusion

Literal types and type narrowing are powerful features in TypeScript that help you write more type-safe and robust code. Literal types allow you to define specific, fixed values for variables, while type narrowing ensures that variables are refined to a more specific type based on certain conditions.

By using these features together, you can write code that is not only more maintainable and predictable but also less prone to runtime errors. Understanding when and how to use literal types and type narrowing is essential for writing TypeScript that scales and remains error-free.

Table of Contents

• Introduction
• What is the void Type?
  • Use Cases for void
  • Example Code
• What is the never Type?
  • Use Cases for never
  • Example Code
• Differences Between void and never
• When to Use void and never
• Best Practices for Using void and never
• Conclusion

Introduction

TypeScript provides several advanced features that make it a more robust and safer programming language than JavaScript. One such feature is the ability to define special return types for functions. Two of the most intriguing return types in TypeScript are void and never. These types might seem similar at first glance because they both represent the idea of “no value” in some capacity, but they are fundamentally different and serve distinct purposes.

In this article, we will explore what the void and never types are, their use cases, differences, and when and how to use them effectively. By understanding these special function return types, you’ll be able to write cleaner, more precise TypeScript code.

What is the void Type?

The void type is used in TypeScript to represent the absence of a return value from a function. It indicates that the function doesn’t return anything. While it might seem similar to undefined, void is explicitly used for functions that don’t need to return anything, such as event handlers or callbacks.

In TypeScript, a function with a void return type will not return a value, and trying to do so will result in a compile-time error.

Use Cases for void

• Event Handlers and Callbacks: Functions that are invoked as callbacks or event handlers often don’t return a value, so their return type should be void.
• Non-returning Functions: Functions that perform actions but don’t need to return a value, such as logging or performing side effects, should have the void return type.

Example Code

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

greet("John");  // OK
greet("John") + 5;  // Error: Operator '+' cannot be applied to a void type

In this example, the greet function does not return anything, so its return type is void. If we try to use its result in an expression like greet("John") + 5, TypeScript will throw an error because the return type of the function is not a value that can be used in arithmetic.

What is the never Type?

The never type is a more specialized type in TypeScript. It represents the type of values that never occur. Functions that return never are functions that do not complete normally. These can include functions that throw an error or functions that enter an infinite loop.

In other words, a function with a never return type can never reach a normal completion state. It either throws an error or never returns at all (e.g., in an infinite loop).

Use Cases for never

• Throwing Errors: Functions that throw exceptions without returning any value are ideal candidates for the never return type.
• Infinite Loops: Functions that contain infinite loops, which never allow the function to finish or return, should use the never type.
• Exhaustiveness Checking: The never type can also be used in exhaustive checks, ensuring that all possible values are handled in a switch case or if-else block.

Example Code

function throwError(message: string): never {
  throw new Error(message);
}

function infiniteLoop(): never {
  while (true) {
    console.log("Running...");
  }
}

throwError("Something went wrong!");  // OK
infiniteLoop();  // OK

In this example, throwError is a function that throws an error, and since it doesn’t return a value, its return type is never. The infiniteLoop function runs indefinitely, so it also has a never return type.

Differences Between void and never

Although both void and never are used for functions that do not return a value, they have key differences:

When to Use void and never

Both void and never are essential for improving the type safety and clarity of your code, but they are used in different situations.

Use void When:

• You have a function that doesn’t return a value, like logging or event handling.
• You want to explicitly indicate that a function has no meaningful return value.

Use never When:

• A function is designed to throw an error and never return a value (e.g., error handling).
• A function contains an infinite loop or is otherwise not expected to terminate.
• You are performing exhaustive checks in switch-case statements or other control flow mechanisms to ensure all cases are covered.

Best Practices for Using void and never

• Use void for non-returning functions: Functions like callbacks, event handlers, and logging functions that don’t need to return a value should use the void return type.
• Use never for functions that throw errors or have infinite loops: For functions that are designed to throw exceptions or run forever, use the never return type to indicate that they don’t complete normally.
• Leverage never in exhaustive checks: When handling union types, especially in switch-case statements, use never to ensure that all possible cases are covered.

type Animal = { type: "dog"; bark: () => void } | { type: "cat"; meow: () => void };

function handleAnimal(animal: Animal): void {
  switch (animal.type) {
    case "dog":
      animal.bark();
      break;
    case "cat":
      animal.meow();
      break;
    default:
      // The 'never' type guarantees we don't miss any cases
      throw new Error("Unknown animal type!");
  }
}

Conclusion

The void and never types in TypeScript are powerful tools that allow you to explicitly define the return types of functions that don’t return values. While void is used for functions that don’t return a meaningful value, never is used for functions that either throw an error or never return because of an infinite loop.

Understanding when and how to use these types can improve the clarity, type safety, and maintainability of your TypeScript code. By using these special return types appropriately, you can ensure that your code behaves as expected and prevent common runtime errors.