JavaScript Anonymous Functions

Last Updated : 06 Jun, 2025

An anonymous function is simply a function that does not have a name. Unlike named functions, which are declared with a name for easy reference, anonymous functions are usually created for specific tasks and are often assigned to variables or used as arguments for other functions.

In JavaScript, you normally use the function keyword followed by a name to declare a function. However, in an anonymous function, the name is omitted. These functions are often used in situations where you don’t need to reuse the function outside its immediate context.

Note: You'll get a syntax error if you don't use parentheses (). The parentheses are needed to treat the anonymous function as an expression, which returns a function object.

Syntax

The below-enlightened syntax illustrates the declaration of an anonymous function using the normal declaration:

function() {
    // Function Body
 }

We may also declare an anonymous function using the arrow function technique which is shown below:

( () => {
    // Function Body...
} )();

The below examples demonstrate anonymous functions.

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Example 1: Defining an anonymous function that prints a message to the console. The function is then stored in the greet variable. We can call the function by invoking greet().

JavaScript

const greet = function () {
	console.log("Welcome to GeeksforGeeks!");
};
    
greet();

Example 2: Passing arguments to the anonymous function.

JavaScript

const greet = function( str ) {
	console.log("Welcome to ", str);
};
    
greet("GeeksforGeeks!");

As JavaScript supports Higher-Order Functions, we can also pass anonymous functions as parameters into another function.

Example 3: Passing an anonymous function as a callback function to the setTimeout()method. This executes this anonymous function 2000ms later.

JavaScript

setTimeout(function () {
	console.log("Welcome to GeeksforGeeks!");
}, 2000);

Self-Executing Anonymous Functions

Another common use of anonymous functions is to create self-executing functions (also known as IIFE - Immediately Invoked Function Expressions). These functions run immediately after they are defined.

Example 4: Creating a self-executing function.

JavaScript

(function () {
	console.log("Welcome to GeeksforGeeks!");
})();

Arrow functions

ES6 introduced a new and shorter way of declaring an anonymous function, which is known as Arrow Functions.In an Arrow function, everything remains the same, except here we don't need the function keyword also. Here, we define the function by a single parenthesis and then '=>' followed by the function body.

Example 5: This is an example of anonymous function with arrow function.

JavaScript

const greet = () => {
	console.log("Welcome to GeeksforGeeks!");
}
    
greet();

If we have only a single statement in the function body, we can even remove the curly braces.

Example 6:In this example, a simple arrow function with a single expression is used.

JavaScript

const greet = () => console.log("Welcome to GeeksforGeeks!");

greet();

Output

Welcome to GeeksforGeeks!

Example 7: illustrates a self-executing function (IIFE), which immediately invokes itself after being defined:

JavaScript

(() => {
	console.log("GeeksforGeeks");
})();

Conclusion

Anonymous functions are a powerful feature in JavaScript that help keep code clean and concise. They are great for one-time tasks like callbacks or event handlers. With the introduction of arrow functions in ES6, writing anonymous functions has become even easier and more readable. While they may not always be reusable, their simplicity and flexibility make them an essential tool in modern JavaScript programming.