Use of Void in TypeScript

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

It is mainly 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'.

In the example above, the greet function has a return type of void, which means it does not return any value.

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

Here, the type of the variable result is void, as the printMessage function does not return any value.

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, typically 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.