An anonymous function (also known as a "lambda" or "block") is a function without a name or identifier.

An example of an anonymous function in JavaScript

1function () { ... }
2// can also be achieved using ES6 syntax (arrow functions)
3() => { ... }

An example of an anonymous function in Python

1lambda num: num + num

These functions cannot be referenced in code since they have no name, meaning they can only be used where they’re defined. They’re often used for a one-time operation or short-term use at runtime.

Anonymous functions can also assist in cleaner code if the name of a function is not relevant (for example, when a function is only used once, or a limited number of times).

Any code you can write with anonymous functions could also be written with normal, named functions; it’s mostly a matter of style.

1function toStatus(item) {
2  return item.status;
3}
4const statuses = items.map(toStatus);

Why use anonymous functions?

Anonymous functions may help make your code syntactically lighter, simpler and potentially cleaner, especially if you’re only using the function once.

An example of this might be when using callbacks (where a function calls a function passed in arguments), and you may not want to define a whole function for a callback.

1>>> numbers = [1, 2, 3, 4, 5, 6]
2>>> list(map(lambda num: num*num, numbers))
3[1, 4, 9, 16, 25, 36]

The function above maps each element in an array and multiplies it by itself.

I don’t really need to make an entire function definition just to multiply a number by itself, it’s a pretty simple operation.

Instead, we pass in an anonymous function to the map function which has one argument, num (this can be called anything).

1lambda num: num*num

Then, we just multiply num by itself. This function will run for each item in the list.

Higher-order functions

Anonymous functions can be used in "higher-order functions”.

A higher-order function (HOF) is a function, which either takes a function as one or more of its arguments, or that takes in another function as an argument, or returns another function.

It sounds complicated, but you’ve likely used a map function for arrays, where that function needs another function as an input to know how to map the items in the array.

1// The anonymous function is around the brackets after "map"
2// For each item in the "items" array, it will take in the item as an input and return "item.status"
3const statuses = items.map((item) => item.status);

Immediately Invoked Function Expressions (IIFEs)

In JavaScript, you can use create Immediately Invoked Function Expressions (IIFEs) to create a function definition and call it at the same time:

• Red: the function definition (to run console.log)
• Blue: a set of brackets to seperate the function from the call operator (so it's syntactically valid – a right curly bracket cannot appear next to a left bracket)
• Green: the call operator.

At runtime, what’s seen is a function inside the green brackets, with a call operator right next to it. Whatever is inside the green brackets is “called” – in this case, the function definition in red executes.

IIFEs are typically used to create a scope, or quickly create and execute a function. It’s often used to quickly create an asynchronous environment to use async/await:

1(async () => {
2  // we have access to `await` here, meaning we can do
3  // asynchronous operations
4  await fetch(/* ... */);
5})();