Functions
Generally speaking, a function is a "subprogram" that can be called by code external (or internal in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the function body. Values can be passed to a function, and the function will return a value.
In JavaScript, functions are first-class objects, because they can have properties and
methods just like any other object. What distinguishes them from other objects is that
functions can be called. In brief, they are
Function
objects.
For more examples and explanations, see the JavaScript guide about functions.
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'
*/
var 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 (function
statement)
There is a special syntax for declaring functions (see function statement for details):
function name([param[, param[, ... param]]]) {
statements
}
name
-
The function name.
param
-
The name of an argument to be passed to the function.
statements
-
The statements comprising the body of the function.
The function expression (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
}
name
-
The function name. Can be omitted, in which case the function becomes known as an anonymous function.
param
-
The name of an argument to be passed to the function.
statements
-
The statements comprising the body of the function.
Here is an example of an anonymous function expression (the
name
is not used):
var myFunction = function() {
statements
}
It is also possible to provide a name inside the definition in order to create a named function expression:
var 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 declaration (function*
statement)
There is a special syntax for generator function declarations (see
function* statement
for details):
function* name([param[, param[, ... param]]]) {
statements
}
name
-
The function name.
param
-
The name of an argument to be passed to the function.
statements
-
The statements comprising the body of the function.
The generator function expression (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
}
name
-
The function name. Can be omitted, in which case the function becomes known as an anonymous function.
param
-
The name of an argument to be passed to the function.
statements
-
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
-
The name of an argument. Zero arguments need to be indicated with
()
. For only one argument, the parentheses are not required. (likefoo => 1
) statements
orexpression
-
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)
arg1, arg2, ... argN
-
Zero or more names to be used by the function as formal parameters. Each must be a proper JavaScript identifier.
functionBody
-
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)
arg1, arg2, ... argN
-
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
". functionBody
-
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.
-
arguments
: An array-like object containing the arguments passed to the currently executing function. -
arguments.callee
: The currently executing function. -
arguments.caller
: The function that invoked the currently executing function. -
arguments.length
: 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.
Method definition syntax
Starting with ECMAScript 2015, you are able to define own methods in a shorter syntax, similar to the getters and setters. See method definitions for more information.
var obj = {
foo() {},
bar() {}
};
Constructor vs. declaration vs. expression
Compare the following:
A function defined with the Function
constructor assigned to the
variable multiply
:
var 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
:
var multiply = function(x, y) {
return x * y;
};
A function expression of a function named func_name
assigned to
the variable multiply
:
var multiply = function func_name(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
undefined
if the function name was previously declared via a
var
statement). For example:
var y = function x() {};
alert(x); // throws an error
The function name also appears when the function is serialized via
Function
's 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 4th 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:
A function defined by 'new Function'
does not have a function name.
However, the serialized form of the function shows as if it has the name "anonymous."
For example, alert(new Function())
outputs:
function anonymous() {
}
Since the function actually does not have a name, anonymous
is not a
variable that can be accessed within the function. For example, the following would
result in an error:
var foo = new Function("alert(anonymous);");
foo();
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(); // alerts FOO!
function foo() {
alert('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 p = 5;
function myFunc() {
var p = 9;
function decl() {
console.log(p);
}
var expr = function() {
console.log(p);
};
var 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:
var foo = (new Function("var bar = \'FOO!\';\nreturn(function() {\n\talert(bar);\n});"))();
foo(); // The segment "function() {\n\talert(bar);\n}" 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:
var 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
var y = 20; // 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:
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) {
var numStr = num.toString(); // Initialize return value as string
var numZeros = totalLen - numStr.length; // Calculate no. of zeros
for (var i = 1; i <= numZeros; i++) {
numStr = "0" + numStr;
}
return numStr;
}
The following statements call the padZeros function.
var 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 ('function' === typeof window.noFunc) {
// 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.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-function-definitions |
Browser compatibility
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