What part of the function definition contains the statements?

Description

Every function in JavaScript is a Function object. See Function for information on properties and methods of Function objects.

To return a value other than the default, a function must have a return statement that specifies the value to return. A function without a return statement will return a default value. In the case of a constructor called with the new keyword, the default value is the value of its this parameter. For all other functions, the default return value is undefined.

The parameters of a function call are the function's arguments. Arguments may be passed by value (in the case of primitive values) or by reference (in the case of objects). This means that if a function reassigns a primitive type parameter, the value won't change outside the function. In the case of an object type parameter, if its properties are mutated, the change will impact outside of the function. See the following example:

/* Declare the function 'myFunc' */
function myFunc(theObject) {
  theObject.brand = "Toyota";
}

/*
 * Declare variable 'mycar';
 * create and initialize a new Object;
 * assign reference to it to 'mycar'
 */
const mycar = {
  brand: "Honda",
  model: "Accord",
  year: 1998
};

/* Logs 'Honda' */
console.log(mycar.brand);

/* Pass object reference to the function */
myFunc(mycar);

/*
 * Logs 'Toyota' as the value of the 'brand' property
 * of the object, as changed to by the function.
 */
console.log(mycar.brand);

The this keyword does not refer to the currently executing function, so you must refer to Function objects by name, even within the function body.

Defining functions

There are several ways to define functions:

The function declaration

There is a special syntax for declaring functions (see function statement for details):

function name([param[, param[, ... param]]]) {
  statements
}

The function name.

The name of an argument to be passed to the function.

The statements comprising the body of the function.

The function expression

A function expression is similar to and has the same syntax as a function declaration (see function expression for details). A function expression may be a part of a larger expression. One can define "named" function expressions (where the name of the expression might be used in the call stack for example) or "anonymous" function expressions. Function expressions are not hoisted onto the beginning of the scope, therefore they cannot be used before they appear in the code.

function [name]([param[, param[, ... param]]]) {
  statements
}

The function name. Can be omitted, in which case the function becomes known as an anonymous function.

The name of an argument to be passed to the function.

The statements comprising the body of the function.

Here is an example of an anonymous function expression (the name is not used):

const myFunction = function () {
  statements
}

It is also possible to provide a name inside the definition in order to create a named function expression:

const myFunction = function namedFunction() {
  statements
}

One of the benefits of creating a named function expression is that in case we encountered an error, the stack trace will contain the name of the function, making it easier to find the origin of the error.

As we can see, both examples do not start with the function keyword. Statements involving functions which do not start with function are function expressions.

When functions are used only once, a common pattern is an IIFE (Immediately Invoked Function Expression).

(function () {
  statements
})();

IIFE are function expressions that are invoked as soon as the function is declared.

The generator function (function*) declaration

There is a special syntax for generator function declarations (see function* statement for details):

function* name([param[, param[, ... param]]]) {
  statements
}

The function name.

The name of an argument to be passed to the function.

The statements comprising the body of the function.

The generator function (function*) expression

A generator function expression is similar to and has the same syntax as a generator function declaration (see function* expression for details):

function* [name]([param[, param[, ... param]]]) {
  statements
}

The function name. Can be omitted, in which case the function becomes known as an anonymous function.

The name of an argument to be passed to the function.

The statements comprising the body of the function.

The arrow function expression (=>)

An arrow function expression has a shorter syntax and lexically binds its this value (see arrow functions for details):

([param[, param]]) => {
  statements
}

param => expression

(param) => expression

The name of an argument. Zero arguments need to be indicated with (). For exactly one argument, the parentheses are not required. (like foo => 1)

Multiple statements need to be enclosed in brackets. A single expression requires no brackets. The expression is also the implicit return value of the function.

The Function constructor

Note: Using the Function constructor to create functions is not recommended since it needs the function body as a string which may prevent some JS engine optimizations and can also cause other problems.

As all other objects, Function objects can be created using the new operator:

new Function (arg1, arg2, ... argN, functionBody)

Zero or more names to be used by the function as formal parameters. Each must be a proper JavaScript identifier.

A string containing the JavaScript statements comprising the function body.

Invoking the Function constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

The GeneratorFunction constructor

Note: GeneratorFunction is not a global object, but could be obtained from generator function instance (see GeneratorFunction for more detail).

Note: Using the GeneratorFunction constructor to create functions is not recommended since it needs the function body as a string which may prevent some JS engine optimizations and can also cause other problems.

As all other objects, GeneratorFunction objects can be created using the new operator:

new GeneratorFunction (arg1, arg2, ... argN, functionBody)

Zero or more names to be used by the function as formal argument names. Each must be a string that conforms to the rules for a valid JavaScript identifier or a list of such strings separated with a comma; for example "x", "theValue", or "a,b".

A string containing the JavaScript statements comprising the function definition.

Invoking the GeneratorFunction constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

Function parameters

Default parameters

Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed. For more details, see default parameters.

Rest parameters

The rest parameter syntax allows representing an indefinite number of arguments as an array. For more details, see rest parameters.

The arguments object

You can refer to a function's arguments within the function by using the arguments object. See arguments.

An array-like object containing the arguments passed to the currently executing function.

The currently executing function.

The number of arguments passed to the function.

Defining method functions

Getter and setter functions

You can define getters (accessor methods) and setters (mutator methods) on any standard built-in object or user-defined object that supports the addition of new properties. The syntax for defining getters and setters uses the object literal syntax.

Binds an object property to a function that will be called when that property is looked up.

Binds an object property to a function to be called when there is an attempt to set that property.

Method definition syntax

In object literals, you are able to define own methods in a shorter syntax, similar to the getters and setters. See method definitions for more information.

const obj = {
  foo() {},
  bar() {},
};

Constructor vs. declaration vs. expression

Compare the following:

A function defined with the Function constructor assigned to the variable multiply:

const multiply = new Function('x', 'y', 'return x * y');

A function declaration of a function named multiply:

function multiply(x, y) {
  return x * y;
} // there is no semicolon here

A function expression of an anonymous function assigned to the variable multiply:

const multiply = function (x, y) {
  return x * y;
};

A function expression of a function named funcName assigned to the variable multiply:

const multiply = function funcName(x, y) {
  return x * y;
};

Differences

All do approximately the same thing, with a few subtle differences:

There is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or get another value, if the same name is declared elsewhere). For example:

const y = function x() {};
console.log(x); // ReferenceError: x is not defined

The function name also appears when the function is serialized via its toString() method.

On the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared.

As the const y = function x() {} example shows, the function name can be different from the variable the function is assigned to. They have no relation to each other. A function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in, as well as in their own body.

A function defined by new Function will dynamically have its source assembled, which is observable when you serialize it. For example, console.log(new Function().toString()) gives:

function anonymous(
) {

}

This is the actual source used to compile the function. However, although the Function() constructor will create the function with name anonymous, this name is not added to the scope of the body. The body only ever has access to global variables. For example, the following would result in an error:

new Function("alert(anonymous);")();

Unlike functions defined by function expressions or by the Function constructor, a function defined by a function declaration can be used before the function declaration itself. For example:

foo(); // Logs "FOO!"
function foo() {
  console.log('FOO!');
}

A function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a Function constructor does not inherit any scope other than the global scope (which all functions inherit).

/*
 * Declare and initialize a variable 'p' (global)
 * and a function 'myFunc' (to change the scope) inside which
 * declare a variable with same name 'p' (current) and
 * define three functions using three different ways:-
 *     1. function declaration
 *     2. function expression
 *     3. function constructor
 * each of which will log 'p'
 */
// var declarations create properties on the global object
var p = 5;
function myFunc() {
  const p = 9;

  function decl() {
    console.log(p);
  }
  const expr = function () {
    console.log(p);
  };
  const cons = new Function('\tconsole.log(p);');

  decl();
  expr();
  cons();
}
myFunc();

/*
 * Logs:-
 * 9  - for 'decl' by function declaration (current scope)
 * 9  - for 'expr' by function expression (current scope)
 * 5  - for 'cons' by Function constructor (global scope)
 */

Functions defined by function expressions and function declarations are parsed only once, while those defined by the Function constructor are not. That is, the function body string passed to the Function constructor must be parsed each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than new Function(...). Therefore the Function constructor should generally be avoided whenever possible.

It should be noted, however, that function expressions and function declarations nested within the function generated by parsing a Function() constructor's string aren't parsed repeatedly. For example:

const foo = (new Function("const bar = 'FOO!'; return function() { alert(bar); };"))();
foo(); // The segment "function() { alert(bar); }" of the function body string is not re-parsed.

A function declaration is very easily (and often unintentionally) turned into a function expression. A function declaration ceases to be one when it either:

• becomes part of an expression
• is no longer a "source element" of a function or the script itself. A "source element" is a non-nested statement in the script or a function body:

let x = 0;               // source element
if (x === 0) {           // source element
  x = 10;                // not a source element
  function boo() {}      // not a source element
}
function foo() {         // source element
  let y = 2;             // source element
  function bar() {}      // source element
  while (y < 10) {       // source element
    function blah() {}   // not a source element
    y++;                 // not a source element
  }
}

Examples

// function declaration
function foo() {}

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

// function expression
x = function hello() {}

if (x) {
  // function expression
  function world() {}
}

// function declaration
function a() {
  // function declaration
  function b() {}
  if (0) {
    // function expression
    function c() {}
  }
}

Block-level functions

In strict mode, starting with ES2015, functions inside blocks are now scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode.

'use strict';

function f() {
  return 1;
}

{
  function f() {
    return 2;
  }
}

f() === 1; // true

// f() === 2 in non-strict mode

Block-level functions in non-strict code

In a word: Don't.

In non-strict code, function declarations inside blocks behave strangely. For example:

if (shouldDefineZero) {
  function zero() {     // DANGER: compatibility risk
    console.log("This is zero.");
  }
}

ES2015 says that if shouldDefineZero is false, then zero should never be defined, since the block never executes. However, it's a new part of the standard. Historically, this was left unspecified, and some browsers would define zero whether the block executed or not.

In strict mode, all browsers that support ES2015 handle this the same way: zero is defined only if shouldDefineZero is true, and only exists within that scope of the if-block.

A safer way to define functions conditionally is to assign a function expression to a variable:

// Using a var makes it available as a global variable,
// with closer behavior to a top-level function declaration
var zero;
if (shouldDefineZero) {
  zero = function () {
    console.log("This is zero.");
  };
}

Examples

Returning a formatted number

The following function returns a string containing the formatted representation of a number padded with leading zeros.

// This function returns a string padded with leading zeros
function padZeros(num, totalLen) {
  let numStr = num.toString();             // Initialize return value as string
  const numZeros = totalLen - numStr.length; // Calculate no. of zeros
  for (let i = 1; i <= numZeros; i++) {
    numStr = `0${numStr}`;
  }
  return numStr;
}

The following statements call the padZeros function.

let result;
result = padZeros(42, 4); // returns "0042"
result = padZeros(42, 2); // returns "42"
result = padZeros(5, 4);  // returns "0005"

Determining whether a function exists

You can determine whether a function exists by using the typeof operator. In the following example, a test is performed to determine if the window object has a property called noFunc that is a function. If so, it is used; otherwise, some other action is taken.

if (typeof window.noFunc === 'function') {
  // use noFunc()
} else {
  // do something else
}

Note that in the if test, a reference to noFunc is used—there are no brackets "()" after the function name so the actual function is not called.

See also