Use of Void in TypeScript

The void type in TypeScript represents the absence of a value. It is similar to the concept of void found in other programming languages (like C++ and C#).

It is primarily used in the context of functions to indicate that a function does not return any significant value.

When a function is designed to perform an action (sometimes called a side effect), such as modifying a state or printing a message to the console, it does not need to return a result, and the void type is specified.

function greet(): void {
    console.log("Hello, world!");
}

let result: void = greet(); // Correct, but unnecessary
// result = 1; // Error: Cannot assign type 'number' to type 'void'.

Variable Assignment

A variable of type void cannot be assigned. It can only receive undefined as a value.

function printMessage(message: string): void {
    console.log(message);
}

const result: void = printMessage("Message without return value");

console.log(result);  // undefined

Common Use Cases

Side Effect Functions

Functions that are used to perform actions but do not need to return a value, such as functions that print to the console or modify global variables, usually have a return type of void.

function logMessage(message: string): void {
    console.log(message);
}

logMessage("This is a log message."); // This is a log message.

Interfaces

In interfaces, methods that do not return a value are also defined with the void type.

interface Processor {
    start(): void;
    stop(): void;
}

class Computer implements Processor {
    start(): void {
        console.log("Computer started.");
    }

    stop(): void {
        console.log("Computer stopped.");
    }
}

let pc: Computer = new Computer();
pc.start(); // Computer started.
pc.stop(); // Computer stopped.