Functions

Functions are one of the fundamental building blocks in JavaScript. A function in JavaScript is similar to a procedure—a set of statements that performs a task or calculates a value, but for a procedure to qualify as a function, it should take some input and return an output where there is some obvious relationship between the input and the output. To use a function, you must define it somewhere in the scope from which you wish to call it.

See also the exhaustive reference chapter about JavaScript functions to get to know the details.

Defining functions

Function declarations

A function definition (also called a function declaration, or function statement) consists of the function keyword, followed by:

The name of the function.
A list of parameters to the function, enclosed in parentheses and separated by commas.
The JavaScript statements that define the function, enclosed in curly braces, { /* … */ }.

For example, the following code defines a function named square:

function square(number) {
  return number * number;
}

The function square takes one parameter, called number. The function consists of one statement that says to return the parameter of the function (that is, number) multiplied by itself. The return statement specifies the value returned by the function, which is number * number.

Parameters are essentially passed to functions by value — so if the code within the body of a function assigns a completely new value to a parameter that was passed to the function, the change is not reflected globally or in the code which called that function.

When you pass an object as a parameter, if the function changes the object's properties, that change is visible outside the function, as shown in the following example:

function myFunc(theObject) {
  theObject.make = "Toyota";
}

const myCar = {
  make: "Honda",
  model: "Accord",
  year: 1998,
};

console.log(myCar.make); // "Honda"
myFunc(myCar);
console.log(myCar.make); // "Toyota"

When you pass an array as a parameter, if the function changes any of the array's values, that change is visible outside the function, as shown in the following example:

function myFunc(theArr) {
  theArr[0] = 30;
}

const arr = [45];

console.log(arr[0]); // 45
myFunc(arr);
console.log(arr[0]); // 30

Function declarations and expressions can be nested, which forms a scope chain. For example:

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

See function scopes and closures for more information.

Function expressions

While the function declaration above is syntactically a statement, functions can also be created by a function expression.

Such a function can be anonymous; it does not have to have a name. For example, the function