Function

A function is a code snippet that can be called by other code or by itself, or a variable that refers to the function. When a function is called, arguments are passed to the function as input, and the function can optionally return a value. A function in JavaScript is also an object.

A function name is an identifier included as part of a function declaration or function expression. The function name's scope depends on whether the function name is a declaration or expression.

Different types of functions

An anonymous function is a function without a function name. Only function expressions can be anonymous, function declarations must have a name:

js
// Anonymous function created as a function expression
(function () {});

// Anonymous function created as an arrow function
() => {};

The following terms are not used in the ECMAScript language specification, they're jargon used to refer to different types of functions.

A named function is a function with a function name:

js
// Function declaration
function foo() {}

// Named function expression
(function bar() {});

// Arrow function
const baz = () => {};

An inner function is a function inside another function (square in this case). An outer function is a function containing a function (addSquares in this case):

js
function addSquares(a, b) {
  function square(x) {
    return x * x;
  }
  return square(a) + square(b);
}

// Arrow function
const addSquares2 = (a, b) => {
  const square = (x) => x * x;
  return square(a) + square(b);
};

A recursive function is a function that calls itself. See recursion.

js
function loop(x) {
  if (x >= 10) return;
  loop(x + 1);
}

// Arrow function
const loop2 = (x) => {
  if (x >= 10) return;
  loop2(x + 1);
};

An Immediately Invoked Function Expression (IIFE) is a function that is called directly after the function is loaded into the browser's compiler. The way to identify an IIFE is by locating the extra left and right parenthesis at the end of the function's definition.

Function expressions, named or anonymous, can be called immediately.

js
(function foo() {
  console.log("Hello Foo");
})();

(function food() {
  console.log("Hello Food");
})();

(() => console.log("hello world"))();

Declared functions can't be called immediately this way, because IIFEs must be function expressions.

js
function foo() {
  console.log('Hello Foo');
}();

If you'd like to know more about IIFEs, check out the following page on Wikipedia: Immediately Invoked Function Expression

See also