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Description

eval()

is a global method that evaluates a string of JavaScript code. If

code

contains an ex-

pression,

eval

evaluates the expression and returns its value. (Some expressions, such as object

and function literals look like statements and must be enclosed in parentheses when passed

eval()

in order to resolve the ambiguity.) If

code

contains a JavaScript statement or state-

ments,

eval()

executes those statements and returns the value, if any, returned by the last

statement. If

code

does not return any value,

eval()

returns

undefined

. Finally, if

code

throws

an exception,

eval()

passes that exception on to the caller.

eval()

behaves different in ECMAScript 3 and ECMAScript 5, and in ECMAScript 5, it be-

haves differently in strict mode and non-strict mode, and a minor digression is necessary in

order to explain these differences. It is much easier to implement efficient interpreters when

a programming language defines

eval

as an operator instead of as a function. JavaScript’s

eval

is a function, but for the sake of efficiency, the language draws a distinction between

direct, operator-like calls to

eval()

and indirect calls. A direct call uses the identifier

eval

directly and, if you removed the parentheses, it would look like

eval

was an operator. Any

other invocation of

eval()

is an indirect call. If you assign the

eval()

function to a variable

with a different name and invoke it through that variable, it is an indirect call. Similarly, if

you invoke eval() as a method of the global object, it is an indirect call.

With that distinction between direct and indirect calls made, we can document the behavior

eval()

like this:

Direct call, ES3 and ES5 non-strict mode

eval()

evaluates

code

in the current lexical scope. If

code

contains variable or function

declarations they are defined in the local scope. This is the normal use-case for

eval()

Indirect call, ES3

The ECMAScript 3 specification allows interpreters to throw an EvalError for any indirect

call to

eval()

. Implementations of ES3 don’t generally do this in practice, but indirect

calls should be avoided.

Indirect call, ES5

In ECMAScript 5, indirect calls to

eval()

must not throw an EvalError, and instead must

evaluate

code

in the global scope, ignoring any local variables in the current lexical scope.

In ES5, we can assign

var geval = eval;

, then we can use

geval()

to evaluate

code

in the

global scope.

Direct or Indirect call, strict mode

In strict mode variable and function definitions in

code

are defined in a private scope that

lasts only for the duration of the

eval()

call. This means that direct calls to

eval()

strict mode cannot alter the lexical scope and indirect calls in strict mode cannot alter

the global scope. These rules apply if the invocation of

eval()

is in strict mode, or if

code

begins with a “use strict” directive.

eval()

provides a very powerful capability to the JavaScript language, but its use is infrequent

in real-world programs. Obvious uses are to write programs that act as recursive JavaScript

interpreters and to write programs that dynamically generate and evaluate JavaScript code.

Most JavaScript functions that expect string arguments convert whatever value they are passed

to a string before proceeding.

eval()

does not behave like this: if

code

is not a primitive string

eval()

Core JavaScript Reference | 773

Core JavaScript

Reference

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value, it is simply returned unchanged. Be careful, therefore, that you do not inadvertently

pass a String object to

eval()

when you intended to pass a primitive string value.

Example

eval("1+2"); // Returns 3

// This code uses client-side JavaScript methods to prompt the user to

// enter an expression and to display the results of evaluating it.

// See the client-side methods Window.alert() and Window.prompt() for details.

try {

alert("Result: " + eval(prompt("Enter an expression:","")));

}

catch(exception) {

alert(exception);

}

EvalError

thrown when eval() is used improperly

Object → Error → EvalError

Constructor

new EvalError()

new EvalError(message)

Arguments

message

An optional error message that provides details about the exception. If specified, this

argument is used as the value for the

message

property of the EvalError object.

Returns

A newly constructed EvalError object. If the

message

argument is specified, the Error object

uses it as the value of its

message

property; otherwise, it uses an implementation-defined

default string as the value of that property. When the

EvalError()

constructor is called as a

function without the

new

operator, it behaves just as it does when called with the

new

operator.

Properties

message

An error message that provides details about the exception. This property holds the string

passed to the constructor or an implementation-defined default string. See

Error.mes-

sage

for details.

name

A string that specifies the type of the exception. All EvalError objects inherit the value

“EvalError” for this property.

Description

An instance of the EvalError class may be thrown when the global function

eval()

is invoked

under any other name. See

eval()

for an explanation of the restrictions on how this function

may be invoked. See

Error

for details about throwing and catching exceptions.

EvalError

774 | Core JavaScript Reference

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See Also

Error

Error.message

Error.name

Function

a JavaScript function

Object → Function

Synopsis

functionfunctionname(argument_name_list) // Function definition statement

{

body

}

function (argument_name_list) {body} // Unnamed function literal

functionname(argument_value_list) // Function invocation

Constructor

new Function(argument_names..., body)

Arguments

argument_names...

Any number of string arguments, each naming one or more arguments of the Function

object being created.

body

A string that specifies the body of the function. It may contain any number of JavaScript

statements, separated with semicolons, and may refer to any of the argument names

specified by previous arguments to the constructor.

Returns

A newly created Function object. Invoking the function executes the JavaScript code specified

body

Throws

SyntaxError

Indicates that there was a JavaScript syntax error in the

body

argument or in one of the

argument_names

arguments.

Properties

arguments[]

An array of arguments that were passed to the function. Deprecated.

caller

A reference to the Function object that invoked this one, or

null

if the function was

invoked from top-level code. Deprecated.

length

The number of named arguments specified when the function was declared.

Function

Core JavaScript Reference | 775

Core JavaScript

Reference

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prototype

An object which, for a constructor function, defines properties and methods shared by

all objects created with that constructor function.

Methods

apply()

Invokes a function as a method of a specified object, passing a specified array of argu-

ments.

bind()

Return a new function that invokes this one as a method of the specified object with the

optionally specified arguments.

call()

Invokes a function as a method of a specified object, passing the specified arguments.

toString()

Returns a string representation of the function.

Description

A function is a fundamental datatype in JavaScript. Chapter 8 explains how to define and use

functions, and Chapter 9 covers the related topics of methods, constructors, and the

proto

type

property of functions. See those chapters for complete details. Note that although func-

tion objects may be created with the

Function()

constructor described here, this is not effi-

cient, and the preferred way to define functions, in most cases, is with a function definition

statement or a function literal.

In JavaScript 1.1 and later, the body of a function is automatically given a local variable, named

arguments

, that refers to an Arguments object. This object is an array of the values passed as

arguments to the function. Don’t confuse this with the deprecated

arguments[]

property listed

earlier. See the

Arguments

reference page for details.

See Also

Arguments

; Chapter 8, Chapter 9

Function.apply()

invoke a function as a method of an object

Synopsis

function.apply(thisobj, args)

Arguments

thisobj

The object to which

function

is to be applied. In the body of the function,

thisobj

be-

comes the value of the

this

keyword. If this argument is

null

, the global object is used.

Function.apply()

776 | Core JavaScript Reference

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args

An array of values to be passed as arguments to

function

Returns

Whatever value is returned by the invocation of

function

Throws

TypeError

If this method is invoked on an object that is not a function or if this method is invoked

with an

args

argument that is not an array or an Arguments object.

Description

apply()

invokes the specified

function

as if it were a method of

thisobj

, passing it the argu-

ments contained in the

args

array. It returns the value returned by the function invocation.

Within the body of the function, the

this

keyword refers to the

thisobj

object.

The

args

argument must be an array or an Arguments object. Use

Function.call()

instead if

you want to specify the arguments to pass to the function individually instead of as array

elements.

Example

// Apply the default Object.toString() method to an object that

// overrides it with its own version of the method. Note no arguments.

Object.prototype.toString.apply(o);

// Invoke the Math.max() method with apply to find the largest

// element in an array. Note that first argument doesn't matter

// in this case.

var data = [1,2,3,4,5,6,7,8];

Math.max.apply(null, data);

See Also

Function.call()

Function.arguments[]

deprecated

arguments passed to a function

Synopsis

function.arguments[i]

function.arguments.length

Description

The

arguments

property of a Function object is an array of the arguments that are passed to a

function. It is defined only while the function is executing.

arguments.length

specifies the

number of elements in the array.

This property is deprecated in favor of the Arguments object; it should never be used in new

JavaScript code.

Function.arguments[]

Core JavaScript Reference | 777

Core JavaScript

Reference

See Also

Arguments

Function.bind()

ECMAScript 5

return a function that invokes this as a method

Synopsis

function.bind(o)

function.bind(o, args...)

Arguments

The object to which this function should be bound.

args...

Zero or more argument values that will also be bound.

Returns

A new function which invokes this function as a method of

and passes it the arguments

args

Description

The

bind()

method returns a new function which invokes this function as a method of the

object

. The arguments passed to this function consist of the

args

passed to

bind()

followed

by whatever values are passed to the new function.

Example

Suppose that

is a function and we call the

bind()

method like this:

var g = f.bind(o, 1, 2);

Now

is a new function and the invocation

g(3)

is equivalent to:

f.call(o, 1, 2, 3);

See Also

Function.apply()

Function.call()

, §8.7.4

Function.call()

invoke a function as a method of an object

Synopsis

function.call(thisobj, args...)

Function.bind()

778 | Core JavaScript Reference

Arguments

thisobj

The object on which

function

is to be invoked. In the body of the function,

thisobj

becomes the value of the

this

keyword. If this argument is

null

, the global object is used.

args...

Any number of arguments, which will be passed as arguments to

function

Returns

Whatever value is returned by the invocation of

function

Throws

TypeError

If this method is invoked on an object that is not a function.

Description

call()

invokes the specified

function

as if it were a method of

thisobj

, passing it any argu-

ments that follow

thisobj

in the argument list. The return value of

call()

is the value returned

by the function invocation. Within the body of the function, the

this

keyword refers to the

thisobj

object, or to the global object if

thisobj

null

Use

Function.apply()

instead if you want to specify the arguments to pass to the function in

an array.

Example

// Call the default Object.toString() method on an object that

// overrides it with its own version of the method. Note no arguments.

Object.prototype.toString.call(o);

See Also

Function.apply()

Function.caller

deprecated; not defined in strict mode

the function that called this one

Synopsis

function.caller

Description

In early versions of JavaScript, the

caller

property of a Function object is a reference to the

function that invoked the current one. If the function is invoked from the top level of a Java-

Script program,

caller

null

. This property may be used only from within the function (i.e.,

the

caller

property is only defined for a function while that function is executing).

Function.caller

is not part of the ECMAScript standard and is not required in conforming

implementations. It should not be used.

Function.caller

Core JavaScript Reference | 779

Core JavaScript

Reference

Function.length

the number of declared arguments

Synopsis

function.length

Description

The

length

property of a function specifies the number of named arguments declared when

the function was defined. The function may actually be invoked with more than or fewer than

this number of arguments. Don’t confuse this property of a Function object with the

length

property of the Arguments object, which specifies the number of arguments actually passed

to the function. See

Arguments.length

for an example.

See Also

Arguments.length

Function.prototype

the prototype for a class of objects

Synopsis

function.prototype

Description

The

prototype

property is used when a function is used as a constructor. It refers to an object

that serves as the prototype for an entire class of objects. Any object created by the constructor

inherits all properties of the object referred to by the

prototype

property.

See Chapter 9 for a full discussion of constructor functions, the

prototype

property, and the

definition of classes in JavaScript.

See Also

Chapter 9

Function.toString()

convert a function to a string

Synopsis

function.toString()

Returns

A string that represents the function.

Function.length

780 | Core JavaScript Reference

Throws

TypeError

If this method is invoked on an object that is not a Function.

Description

The

toString()

method of the Function object converts a function to a string in an imple-

mentation-dependent way. In most implementations, such as the implementations in Firefox

and IE, this method returns a string of valid JavaScript code—code that includes the

func

tion

keyword, argument list, the complete body of the function, and so on. In these imple-

mentations, the output of this

toString()

method is valid input for the global

eval()

function.

This behavior is not required by the specification, however, and should not be relied upon.

Global

the global object

Object → Global

Synopsis

this

Global Properties

The global object is not a class, so the following global properties have individual reference

entries under their own names. That is, you can find details on the

undefined

property listed

under the name

undefined

, not under

Global.undefined

. Note that all top-level variables are

also properties of the global object:

Infinity

A numeric value that represents positive infinity.

NaN

The not-a-number value.

undefined

The

undefined

value.

Global Functions

The global object is an object, not a class. The global functions listed here are not methods

of any object, and their reference entries appear under the function name. For example, you’ll

find details on the

parseInt()

function under

parseInt()

, not

Global.parseInt()

decodeURI()

Decodes a string escaped with

encodeURI()

decodeURIComponent()

Decodes a string escaped with

encodeURIComponent()

encodeURI

Encodes a URI by escaping certain characters.

Global

Core JavaScript Reference | 781

Core JavaScript

Reference

encodeURIComponent

Encodes a URI component by escaping certain characters.

escape()

Encodes a string by replacing certain characters with escape sequences.

eval()

Evaluates a string of JavaScript code and returns the result.

isFinite()

Tests whether a value is a finite number.

isNaN()

Tests whether a value is the not-a-number value.

parseFloat()

Parses a number from a string.

parseInt()

Parses an integer from a string.

unescape()

Decodes a string encoded with

escape()

Global Objects

In addition to the global properties and functions listed earlier, the global object also defines

properties that refer to all the other predefined JavaScript objects. Most of these properties

are constructor functions:

Array

The

Array()

constructor.

Boolean

The

Boolean()

constructor.

Date

The

Date()

constructor.

Error

The

Error()

constructor.

EvalError

The

EvalError()

constructor.

Function

The

Function()

constructor.

JSON

A reference to an object that defines JSON parsing and serialization functions.

Math

A reference to an object that defines mathematical functions.

Global

782 | Core JavaScript Reference

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