What's a typical use case for anonymous functions in JavaScript?

TL;DR

Anonymous function in Javascript is a function that does not have any name associated with it. They are typically used as arguments to other functions or assigned to variables.

// The filter method is passed an anonymous function.
arr.filter((x) => x > 1);

They are often used as arguments to other functions, known as higher-order functions, which can take functions as input and return a function as output. Anonymous functions can access variables from the outer scope, a concept known as closures, allowing them to "close over" and remember the environment in which they were created.

// Encapsulating Code
(function () {
  // Some code here.
})();

// Callbacks
setTimeout(function () {
  console.log('Hello world!');
}, 1000);

// Functional programming constructs
const arr = [1, 2, 3];
const double = arr.map(function (el) {
  return el * 2;
});
console.log(double); // [2, 4, 6]

Anonymous functions

Anonymous functions provide a more concise way to define functions, especially for simple operations or callbacks. Besides that, they can also be used in the following scenarios

Immediate execution

Anonymous functions are commonly used in Immediately Invoked Function Expressions (IIFEs) to encapsulate code within a local scope. This prevents variables declared within the function from leaking to the global scope and polluting the global namespace.

// This is an IIFE
(function () {
  var x = 10;
  console.log(x); // 10
})();

// x is not accessible here
console.log(typeof x); // undefined

In the above example, the IIFE creates a local scope for the variable x. As a result, x is not accessible outside the IIFE, thus preventing it from leaking into the global scope.

Callbacks

Anonymous functions can be used as callbacks that are used once and do not need to be used anywhere else. The code will seem more self-contained and readable when handlers are defined right inside the code calling them, rather than having to search elsewhere to find the function body.

setTimeout(() => {
  console.log('Hello world!');
}, 1000);

Higher-order functions

It is used as arguments to functional programming constructs like Higher-order functions or Lodash (similar to callbacks). Higher-order functions take other functions as arguments or return them as results. Anonymous functions are often used with higher-order functions like map(), filter(), and reduce().

const arr = [1, 2, 3];
const double = arr.map((el) => {
  return el * 2;
});
console.log(double); // [2, 4, 6]

Event Handling

In React, anonymous functions are widely used for defining callback functions inline for handling events and passing callbacks as props.

function App() {
  return <button onClick={() => console.log('Clicked!')}>Click Me</button>;
}

Follow-Up Questions

• How do anonymous functions differ from named functions?
• Can you explain the difference between arrow functions and anonymous functions?