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For nearly 25 years this best seller has been the go-to guide for JavaScript programmers. The seventh edition is fully updated to cover the version of JavaScript, and new chapters cover classes, modules The Third Edition also now includes Node. Ajax: The Definitive Guide. Is Ajax a new technology, or the same old stuff web developers have been using for years? Both, actually. This book demonstrates not only how tried-and-true web standards make Ajax possible, but how these older technologies allow you to give sites a decidedly modern Web 2.

Ajax: The Definitive Guide explains how to use standards lik Learn and master the new features in the new Eclipse Jakarta Faces formerly JavaServer Faces or JSF web framework in this definitive guide written by two of the driving forces of the Faces project and the co-creators of the OmniFaces library. Authors Bauke Scholtz and Arjan Tijms take you through real-world examples that demonstrate how JavaScript: The Definitive Guide has been the bible for JavaScript programmers-a programmer's guide and comprehensive reference to the core language and to the client-side JavaScript APIs defined by web browsers.

Many chapters have been completely rewritten to bring them in line with today's best web development practices. New chapters in this edition document jQuery and server side JavaScript. It's recommended for experienced programmers who want to learn the programming language of the Web, and for current JavaScript programmers who want to master it. David Flanagan 8 books. Holdener III Is Ajax a new technology, or the same old stuff web developers have been using for years?

Table of contents Product information. Core JavaScript 2. Client-Side JavaScript Core JavaScript Reference I. UTC Date. E Math. LN10 Math. LN2 Math.

LOG10E Math. LOG2E Math. PI Math. SQRT2 Math. NaN Number. ISBN:

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Decrement -- The -- operator expects an lvalue operand. It converts the value of the operand to a number, subtracts 1, and assigns the decremented value back to the operand. When used before the operand, it decrements and returns the decremented value. When used after the operand, it decrements the operand but returns the undecremented value. When used after its operand, no line break is allowed between the operand and the operator.

A bit is set in the result if the corresponding bit is set in one or both of the operands. For example, 0x 0x00FF evaluates to 0x12FF. Exclusive OR means that either operand one is true or operand two is true, but not both. It operates by reversing all bits in the operand. Relational expressions always evaluate to a boolean value, and that value is often used to control the flow of program execution in if, while, and for statements see Chapter 5. Both operators accept operands of any type, and both return true if their operands are the same and false if they are different.

Be sure you understand the differences between these assignment, equality, and strict equality operators, and be careful to use the correct one when coding! This makes it easy to remember that! An object is equal to itself, but not to any other object. If two distinct objects have the same number of properties, with the same names and values, they are still not equal. Two arrays that have the same elements in the same order are not equal to each other.

The NaN value is never equal to any other value, including itself! To check whether a value x is NaN, use x! NaN is the only value of x for which this expression will be true. If one value is 0 and the other is -0, they are also equal. If the strings differ in length or content, they are not equal. Two strings may have the same meaning and the same visual appearance, but still be encoded using different sequences of bit values. See String. If they refer to different objects they are not equal, even if both objects have identical properties.

If they are strictly equal, they are equal. If they are not strictly equal, they are not equal. Use the following rules and type conversions to check for equality: — If one value is null and the other is undefined, they are equal. If either value is false, convert it to 0 and try the comparison again. An object is converted to a primitive value by either its toString method or its valueOf method. The built-in classes of core JavaScript attempt valueOf conversion before toString conversion, except for the Date class, which performs toString conversion.

Objects that are not part of core JavaScript may convert themselves to primitive values in an implementation-defined way. The operands of these comparison operators may be of any type. Comparison can be performed only on numbers and strings, however, so operands that are not numbers or strings are converted. Otherwise, the return value of its toString method is used.

Infinity is larger than any number other than itself, and -Infinity is smaller than any number other than itself. If either operand is or converts to NaN, then the comparison operator always returns false.

Remember that JavaScript strings are sequences of bit integer values, and that string comparison is just a numerical comparison of the values in the two strings. The numerical encoding order defined by Unicode may not match the traditional collation order used in any particular language or locale. The boolean value true is first converted to the number 1, and the comparison is done again. Next, the string "1" is converted to the number 1. Since both values are now the same, the comparison returns true.

This rule can cause confusing results if you do not expect it. Result is 3. Result is "12". Numeric comparison. Result is false. String comparison. Result is true. It expects a right-side operand that is an object.

It evaluates to true if the left-side value is the name of a property of the right-side object. The operator evaluates to true if the left-side object is an instance of the right-side class and evaluates to false otherwise.

If the left-side operand of instanceof is not an object, instanceof returns false. If the right-hand side is not a function, it throws a TypeError. To evaluate the expression o instanceof f, JavaScript evaluates f.

If it finds it, then o is an instance of f or of a superclass of f and the operator returns true. These operators are described in the subsections that follow.

If one or both of these operands is false, it returns false. The falsy values are false, 4. Thus, the right-side operand of instanceof should be a function. Here are examples: null, undefined, 0, -0, NaN, and "". All other values, including all objects, are truthy. If both operands are truthy, the operator returns a truthy value. Otherwise, one or both operands must be falsy, and the operator returns a falsy value.

This operator starts by evaluating its first operand, the expression on its left. On the other hand, if the value on the left is truthy, then the overall value of the expression depends on the value on the right-hand side.

If the value on the right is truthy, then the overall value must be truthy, and if the value on the right is falsy, then the overall value must be falsy. In the code above, the variable p is set to null, and the expression p. Whether those side effects occur depends on the value of the left-hand side. Despite the somewhat complex way that this operator actually works, it is most commonly used as a simple Boolean algebra operator that works on truthy and falsy values.

If one or both operands is truthy, it returns a truthy value. If both operands are falsy, it returns a falsy value. If that is not defined use a hard-coded constant. Its purpose is to invert the boolean value of its operand.

For example, if x is truthy! If x is falsy, then! This means that! As a unary operator,! It starts by evaluating its first operand, the expression on its left. If the value of this first operand is truthy, it returns that truthy value.

Otherwise, it evaluates its second operand, the expression on its right, and returns the value of that expression. It expects its right-side operand to be an arbitrary value of any type. The value of an assignment expression is the value of the right-side operand.

Although assignment expressions are usually quite simple, you may sometimes see the value of an assignment expression used as part of a larger expression. The assignment operator has right-to-left associativity, which means that when multiple assignment operators appear in an expression, they are evaluated from right to left. For numeric operands, it performs addition and assignment; for string operands, it performs concatenation and assignment.

Table lists them all. In the second it is evaluated twice. The two cases will differ only if a includes side effects such as a function call or an increment operator. If you find yourself using eval , you should think carefully about whether you really need to use it. The subsections below explain the basic use of eval and then explain two restricted versions of it that have less impact on the optimizer.

Is eval a Function or an Operator? The earliest versions of the language defined an eval function, and ever since then language designers and interpreter writers have been placing restrictions on it that make it more and more operator-like. Modern JavaScript interpreters perform a lot of code analysis and optimization. The problem with eval is that the code it evaluates is, in general, unanalyzable. Generally speaking, if a function calls eval , the interpreter cannot optimize that function. This issue could have been avoided if eval was an operator and a reserved word.

If you pass any value other than a string, it simply returns that value. If you pass a string, it attempts to parse the string as JavaScript code, throwing a SyntaxError if it fails.

If it successfully parses the string, then it evaluates the code and returns the value of the last expression or statement in the string or undefined if the last expression or statement had no value.

If the string throws an exception, the eval propagates that expression. The key thing about eval when invoked like this is that it uses the variable environment of the code that calls it.

That is, it looks up the values of variables and defines new variables and functions in the same way that local code does. If a function defines a local variable x and then calls eval "x" , it will obtain the value of the local variable. Note that the string of code you pass to eval must make syntactic sense on its own— you cannot use it to paste code fragments into a function.

It makes no sense to write eval "return;" , for example, because return is only legal within functions, and the fact that the evaluated string uses the same variable environment as the calling function does not make it part of that function.

Otherwise eval will throw a SyntaxError. As a workaround, however, interpreters simply do less optimization on any function that calls eval. But what should a JavaScript interpreter do, however, if a script defines an alias for eval and then calls that function by another name?

In order to simplify the job of JavaScript implementors, the ECMAScript 3 standard declared that interpreters did not have to allow this. In practice, most implementors did something else. When invoked by any other name, eval would evaluate the string as if it were top-level global code. Direct calls to eval use the variable environment of the calling context.

Any other call—an indirect call—uses the global object as its variable environment and cannot read, write, or define local variables or functions. As noted at the beginning of this section, it is rare to truly need to evaluate a string of code.

But if you do find it necessary, you are more likely to want to do a global eval than a local eval. Before IE9, IE differs from other browsers: it does not do a global eval when eval is invoked by a different name. But IE does define a global function named execScript that executes its string argument as if it were a top-level script.

Unlike eval , however, execScript always returns null. This means that in strict mode, evaluated code can query and set local variables, but it cannot define new variables or functions in the local scope.

You are not allowed to overwrite the eval function with a new value. This operator is sometimes written? Because this operator has three operands, the first goes before the? The first operand is evaluated and interpreted as a boolean. If the value of the first operand is truthy, then the second operand is evaluated, and its value is returned.

Otherwise, if the first operand is falsy, then the third operand is evaluated and its value is returned. Only one of the second and third operands is evaluated, never both. Its value is a string that specifies the type of the operand.

Although functions in JavaScript are a kind of object, the typeof operator considers functions to be sufficiently different that they have their own return value. Most browser vendors use native JavaScript function objects for the methods of their host objects. In IE9 these client-side methods are now true native function objects. When a property is deleted, the property ceases to exist. If it is not an lvalue, the operator takes no action and returns true.

Otherwise, delete attempts to delete the specified lvalue. Not all properties can be deleted, however: some built-in core and client-side properties are immune from deletion, and user-defined variables declared with the var statement cannot be deleted. Functions defined with the function statement and declared function parameters cannot be deleted either. Outside of strict mode, no exception occurs in these cases and delete simply returns false to indicate that the operand could not be deleted.

Would raise an exception in strict mode. Argument is not an lvalue: returns true Define a property of the a global object without var Try to delete it: returns true in non-strict mode 1. In JavaScript, memory deallocation is handled automatically by garbage collection, and you never have to worry about explicitly freeing up memory. Use 'delete this. This operator is unusual and infrequently used: it evaluates its operand, then discards the value and returns undefined.

Since the operand value is discarded, using the void operator makes sense only if the operand has side effects. The most common use for this operator is in a client-side javascript: URL, where it allows you to evaluate an expression for its side effects without the browser displaying the value of the evaluated expression. It evaluates its left operand, evaluates its right operand, and then returns the value of the right operand.

By that analogy, statements are JavaScript sentences or commands. Expressions are evaluated to produce a value, but statements are executed to make something happen. Expressions with side effects, such as assignments and function invocations, can stand alone as statements, and when used this way they are known as expression statements. A similar category of statements are the declaration statements that declare new variables and define new functions.

JavaScript programs are nothing more than a sequence of statements to execute. By default, the JavaScript interpreter executes these statements one after another in the order they are written. The sections that follow describe the various statements in JavaScript and explain their syntax. Table , at the end of the chapter, summarizes the syntax. A JavaScript program is simply a sequence of statements, separated from one another with semicolons, so once you are familiar with the statements of JavaScript, you can begin writing JavaScript programs.

This sort of statement was shown in Chapter 4. Assignment statements are one major category of expression statements. Thus, it is almost always used as a statement, rather than as part of a larger expression: delete o. For example: alert greeting ; window.

If a function does not have any side effects, there is no sense in calling it, unless it is part of a larger expression or an assignment statement. A statement block is simply a sequence of statements enclosed within curly braces. Just as expressions often contain subexpressions, many JavaScript statements contain substatements. Formally, JavaScript syntax usually allows a single substatement. For example, the while loop syntax includes a single statement that serves as the body of the loop.

Using a statement block, you can place any number of statements within this single allowed substatement. A compound statement allows you to use multiple statements where JavaScript syntax expects a single statement. The empty statement is the opposite: it allows you to include no statements where one is expected. The empty statement looks like this: ; The JavaScript interpreter takes no action when it executes an empty statement.

The empty statement is occasionally useful when you want to create a loop that has an empty body. JavaScript syntax requires a statement as a loop body, however, so an empty statement—just a bare semicolon—is used. Note that the accidental inclusion of a semicolon after the right parenthesis of a for loop, while loop, or if statement can cause frustrating bugs that are difficult to detect. This line does nothing When you intentionally use the empty statement, it is a good idea to comment your code in a way that makes it clear that you are doing it on purpose.

These statements define identifiers variable and function names that can be used elsewhere in your program and assign values to those identifiers.

First, it does not end with a semicolon. The primitive statements within the block end in semicolons, but the block itself does not. Second, the lines inside the block are indented relative to the curly braces that enclose them. This is optional, but it makes the code easier to read and understand. The subsections that follow explain the var statement and the function statement, but do not cover variables and functions comprehensively.

And see Chapter 8 for complete details on functions. When var is used in top-level code, it declares global variables, visible throughout the JavaScript program. Unlike other global properties, however, properties created with var cannot be deleted. Initialization, however, occurs at the location of the var statement, and the value of the variable is undefined before that point in the code. These variables are hoisted, just like variables declared outside of a loop.

It can also be used in statement form. The function name is followed by a comma-separated list of parameter names in parentheses. These identifiers can be used within the body of the function to refer to the argument values passed when the function is invoked.

The body of the function is composed of any number of JavaScript statements, contained within curly braces. These statements are not executed when the function is defined.

Instead, they are associated with the new function object for execution when the function is invoked. Note that the curly braces are a required part of the function statement.

Unlike statement blocks used with while loops and other statements, a function body requires curly braces, even if the body consists of only a single statement. If o already has an own noninherited property named x, then the assignment simply changes the value of this existing property. Otherwise, the assignment creates a new property named x on the object o. If o previously inherited the property x, that inherited property is now hidden by the newly created own property with the same name.

Property assignment examines the prototype chain to determine whether the assignment is allowed. If o inherits a read-only property named x, for example, then the assignment is not allowed. If the assignment is allowed, however, it always creates or sets a property in the original object and never modifies the prototype chain.

This section explains the things that can go wrong when you query or set a property. It is not an error to query a property that does not exist. If the property x is not found as an own property or an inherited property of o, the property access expression o. The null and undefined values have no properties, and it is an error to query properties of these values.

Attempting to set a property on null or undefined also causes a TypeError, of course. Attempts to set properties on other values do not always succeed, either: some properties are read-only and cannot be set, and some objects do not allow the addition of new properties.

In strict mode, any failed attempt to set a property throws a TypeError exception. The rules that specify when a property assignment succeeds and when it fails are intuitive but difficult to express concisely. See the defineProperty method, however, for an exception that allows configurable read-only properties to be set. If p does not already exist on o, and if there is no setter method to call, then p must be added to o.

But if o is not extensible, then no new properties can be defined on it. Its single operand should be a property access expression. Surprisingly, delete does not operate on the value of the property but on the property itself: delete book. The delete operator only deletes own properties, not inherited ones. To delete an inherited property, you must delete it from the prototype object in which it is defined. Doing this affects every object that inherits from that prototype.

A delete expression evaluates to true if the delete succeeded or if the delete had no effect such as deleting a nonexistent property. Though it will remove configurable properties of nonextensible objects. Certain properties of built-in objects are nonconfigurable, as are properties of the global object created by variable declaration and function declaration. In strict mode, attempting to delete a nonconfigurable property causes a TypeError.

You can do this with the in operator, with the hasOwnProperty and propertyIsEnumerable methods, or simply by querying the property. The in operator expects a property name as a string on its left side and an object on its right. It returns true only if the named property is an own property and its enumerable attribute is true. Certain built-in properties are not enumerable. It runs the body of the loop once for each enumerable property own or inherited of the specified object, assigning the name of the property to the loop variable.

Built-in methods that objects inherit are not enumerable, but the properties that your code adds to objects are enumerable unless you use one of the functions described later to make them nonenumerable. The extend function, in particular, is one that is commonly included in JavaScript utility libraries. The implementation of extend shown here is correct but does not compensate for a well-known bug in Internet Explorer.

The first is Object. It works just like the keys utility function shown in Example It works like Object. The return value of this method becomes the value of the property access expression. When a program sets the value of an accessor property, JavaScript invokes the setter method, passing the value of the right-hand side of the assignment.

The return value of the setter method is ignored. Accessor properties do not have a writable attribute as data properties do. If it has only a getter method, it is a read-only property. And if it has only a setter method, it is a write-only property something that is not possible with data properties and attempts to read it always evaluate to undefined. Note that no colon is used to separate the name of the property from the functions that access that property, but that a comma is still required after the function body to separate the method from the next method or data property.

As an example, consider the following object that represents a 2D Cartesian point. Properties defined by getters and setters are sometimes known as accessor properties to distinguish them from data properties that have a simple value. JavaScript invokes these functions as methods of the object on which they are defined, which means that within the body of the function this refers to the point object.

So the getter method for the r property can refer to the x and y properties as this. Accessor properties are inherited, just as data properties are, so you can use the object p defined above as a prototype for other points. The next section shows how to add accessor properties to existing objects. In ECMAScript 3, there is no way to set these attributes: all properties created by ECMAScript 3 programs are writable, enumerable, and configurable, and there is no way to change this.

For the purposes of this section, we are going to consider getter and setter methods of an accessor property to be property attributes. Thus, we can say that a property has a name and four attributes. The four attributes of a data property are value, writable, enumerable, and configurable. So the four attributes of an accessor property are get, set, enumerable, and configurable.

The ECMAScript 5 methods for querying and setting the attributes of a property use an object called a property descriptor to represent the set of four attributes. A property descriptor object has properties with the same names as the attributes of the property it describes. Thus, the property descriptor object of a data property has properties named value, writable, enumerable, and configurable. And the descriptor for an accessor property has get and set properties instead of value and writable.

The writa ble, enumerable, and configurable properties are boolean values, and the get and set properties are function values, of course.

To obtain the property descriptor for a named property of a specified object, call Object. To query the attributes of inherited properties, you must explicitly traverse the prototype chain see Object. Note that this method alters an existing own property or creates a new own property, but it will not alter an inherited property. If you want to create or modify more than one property at a time, use Object.

The first argument is the object that is to be modified. The second argument is an object that maps the names of the properties to be created or modified to the property descriptors for those properties. It relies on the fact that Object. The other reasons that these methods might throw TypeError have to do with the attributes themselves. The writable attribute governs attempts to change the value attribute. And the configurable attribute governs attempts to change the other attributes and also specifies whether a property can be deleted.

The rules are not completely straightforward, however. It is possible to change the value of a nonwritable property if that property is configurable, for example. Also, it is possible to change a property from writable to nonwritable even if that property is nonconfigurable. Here are the complete rules. Calls to Object. You can change the value of a property that is configurable but nonwritable, however because that would be the same as making it writable, then changing the value, then converting it back to nonwritable.

Example included an extend function that copied properties from one object to another. That function simply copied the name and value of the properties and ignored their attributes. Furthermore, it did not copy the getter and setter methods of accessor properties, but simply converted them into static data properties.

Example shows a new version of extend that uses Object. Rather than being written as a function, this version is defined as a new Object method and is added as a nonenumerable property to Object. We learned there that the first argument to that method is the prototype object for the newly created object. This method also accepts a second optional argument, which is the same as the second argument to Object. If you pass a set of property descriptors to Object.

Most JavaScript implementations with the major exception of the IE web browser supported the object literal get and set syntax even before the adoption of ECMAScript 5. These implementations support a nonstandard legacy API for querying and setting getters and setters. This API consists of four methods available on all objects.

The names of each of these methods begin and end with double underscores to indicate that they are nonstandard methods. These nonstandard methods are not documented in the reference section. The subsections that follow explain what these attributes do and where possible how to query and set them. The prototype attribute is set when an object is created.

Objects created with new use the value of the prototype property of their constructor function as their prototype. And objects created with Object. There is no equivalent function in ECMAScript 3, but it is often possible to determine the prototype of an object o using the expression o. Objects created with a new expression usually inherit a constructor property that refers to the constructor function used to create the object.

And, as described above, constructor functions have a prototype property that specifies the prototype for objects created using that constructor. Note that objects created by object literals or by Object. Thus, constructor. To determine whether one object is the prototype of or is part of the prototype chain of another object, use the isPrototypeOf method.

To find out if p is the prototype of o write p. Create an object with that prototype. The default toString method inherited from Object. The tricky part is that many objects inherit other, more useful toString methods, and to invoke the correct version of toString , we must do so indirectly, using the Function. Example defines a function that returns the class of any object you pass it. Numbers, strings, and booleans behave like objects when the toString method is invoked on them, and the function includes special cases for null and undefined.

Objects created through built-in constructors such as Array and Date have class attributes that match the names of their constructors. Host objects typically have meaningful class attributes as well, though this is implementation-dependent.

Objects created through object literals or by Object. In ECMAScript 3, all built-in and user-defined objects are implicitly extensible, and the extensibility of host objects is implementation defined.

In ECMAScript 5, all built-in and user-defined objects are extensible unless they have been converted to be nonextensible, and the extensibility of host objects is again implementation defined.

ECMAScript 5 defines functions for querying and setting the extensibility of an object. To determine whether an object is extensible, pass it to Object. To make an object nonextensible, pass it to Object.

Note that there is no way to make an object extensible again once you have made it nonextensible. Also note that calling preventExtensions only affects the extensibility of the object itself. If new properties are added to the prototype of a nonextensible object, the nonextensible object will inherit those new properties. The extensible object attribute is often used in conjunction with the configurable and writable property attributes, and ECMAScript 5 defines functions that make it easy to set these attributes together.

This means that new properties cannot be added to the object, and existing properties cannot be deleted or configured. Existing properties that are writable can still be set, however. There is no way to unseal a sealed object. You can use Object. If the object has accessor properties with setter methods, these are not affected and can still be invoked by assignment to the property.

Use Object. It is important to understand that Object. If you want to thoroughly lock down an object, you probably need to seal or freeze the objects in the prototype chain as well.

These functions use the JSON data interchange format. Objects, arrays, strings, finite numbers, true, false, and null are supported and can be serialized and restored. NaN, Infinity, and -Infinity are serialized to null. Date objects are serialized to ISO-formatted date strings see the Date.

Function, RegExp, and Error objects and the undefined value cannot be serialized or restored. If a property value cannot be serialized, that property is simply omitted from the stringified output. Both JSON. Complete documentation for these functions is in the reference section.

These inherited properties are primarily methods, and because they are universally available, they are of particular interest to JavaScript programmers. This section explains a handful of universal object methods that are defined on Object. JavaScript invokes this method of an object whenever it needs to convert the object to a string. For example, when an array is converted to a string, you obtain a list of the array elements, themselves each converted to a string, and when a function is converted to a string, you obtain the source code for the function.

These customized versions of the toString method are documented in the reference section. See Array. The purpose of this method is to return a localized string representation of the object. The Date and Number classes define customized versions of toLocaleString that attempt to format numbers, dates, and times according to local conventions.

Array defines a toLocaleString method that works like toString except that it formats array elements by calling their toLocale String methods instead of their toString methods. If this method exists on the object to be serialized, it is invoked, and the return value is serialized, instead of the original object.

See Date. JavaScript calls this method automatically if an object is used in a context where a primitive value is required. The default valueOf method does nothing interesting, but some of the built-in classes define their own valueOf method see Date. Each value is called an element, and each element has a numeric position in the array, known as its index.

JavaScript arrays are untyped: an array element may be of any type, and different elements of the same array may be of different types.

Array elements may even be objects or other arrays, which allows you to create complex data structures, such as arrays of objects and arrays of arrays. JavaScript arrays are dynamic: they grow or shrink as needed and there is no need to declare a fixed size for the array when you create it or to reallocate it when the size changes.

JavaScript arrays may be sparse: the elements need not have contiguous indexes and there may be gaps. Every JavaScript array has a length property.

For nonsparse arrays, this property specifies the number of elements in the array. For sparse arrays, length is larger than the index of all elements. JavaScript arrays are a specialized form of JavaScript object, and array indexes are really little more than property names that happen to be integers. Implementations typically optimize arrays so that access to numerically indexed array elements is generally significantly faster than access to regular object properties. Arrays inherit properties from Array.

Array literal syntax allows an optional trailing comma, so [,,] has only two elements, not three. Another way to create an array is with the Array constructor. This form of the Array constructor can be used to preallocate an array when you know in advance how many elements will be required.

Using an array literal is almost always simpler than this usage of the Array constructor. A reference to the array should appear to the left of the brackets. The square brackets used to access array elements work just like the square brackets used to access object properties. JavaScript converts the numeric array index you specify to a string—the index 1 becomes the string "1"—then uses that string as a property name. Above, for example, we created an array a with a single element.

We then assigned values at indexes 1, 2, and 3. The length property of the array changed as we did so: a. All indexes are property names, but only property names that are integers between 0 and —1 are indexes. All arrays are objects, and you can create properties of any name on them. If you use properties that are array indexes, however, arrays have the special behavior of updating their length property as needed. Note that you can index an array using numbers that are negative or that are not integers.

When you do this, the number is converted to a string, and that string is used as the property name. Since the name is not a non-negative integer, it is treated as a regular object property, not an array index.

Also, if you index an array with a string that happens to be a non-negative integer, it behaves as an array index, not an object property. The same is true if you use a floating-point number that is the same as an integer: a[ This is just as true for arrays as it is for objects: 7.

You can use this syntax to both read and write the value of an element of an array. No element at this index. No property with this name.

Since arrays are objects, they can inherit elements from their prototype. If an array does inherit elements or use getters and setters for elements, you should expect it to use a nonoptimized code path: the time to access an element of such an array would be similar to regular object property lookup times. Normally, the length property of an array specifies the number of elements in the array. If the array is sparse, the value of the length property is greater than the number of elements.

Sparse arrays can be created with the Array constructor or simply by assigning to an array index larger than the current array length.

Arrays that are sufficiently sparse are typically implemented in a slower, more memoryefficient way than dense arrays are, and looking up elements in such an array will take about as much time as regular object property lookup. Note that when you omit value in an array literal, you are not creating a sparse array. The omitted element exists in the array and has the value undefined. This is subtly different than array elements that do not exist at all. Understanding sparse arrays is an important part of understanding the true nature of JavaScript arrays.

In practice, however, most JavaScript arrays you will work with will not be sparse. And, if you do have to work with a sparse array, your code will probably treat it just as it would treat a nonsparse array with undefined elements. For arrays that are dense i. Or, put another way, an array sparse or not will never have an element whose index is greater than or equal to its length.

In order to maintain this invariant, arrays have two special behaviors. Delete all elements. Length is 5, but no elements, like new Array 5 You can also set the length property of an array to a value larger than its current value. Doing this does not actually add any new elements to the array, it simply creates a sparse area at the end of the array. Make the length property readonly. Similarly, if you make an array element nonconfigurable, it cannot be deleted.

If it cannot be deleted, then the length property cannot be set to less than the index of the nonconfigurable element. You can also use the push method to add one or more values to the end of an array: 7. Note that using delete on an array element does not alter the length property and does not shift elements with higher indexes down to fill in the gap that is left by the deleted property.

If you delete an element from an array, the array becomes sparse. As we saw above, you can also delete elements from the end of an array simply by setting the length property to the new desired length. Arrays have a pop method it works with push that reduces the length of an array by 1 but also returns the value of the deleted element.

There is also a shift method which goes with unshift to remove an element from the beginning of an array. Unlike delete, the shift method shifts all elements down to an index one lower than their current index. Finally, splice is the general-purpose method for inserting, deleting, or replacing array elements. It alters the length property and shifts array elements to higher or lower indexes as needed. If this is not the case, you should test the array elements before using them.

This loop assigns enumerable property names including array indexes to the loop variable one at a time. Implementations typically iterate array elements in ascending order, but this is not guaranteed. In particular, if an array has both object properties and array elements, the property names may be returned in the order they were created, 7. ECMAScript 5 defines a number of new methods for iterating array elements by passing each one, in index order, to a function that you define.

To access a value in an array of arrays, simply use the [] operator twice. For example, suppose the variable matrix is an array of arrays of numbers. Every element in matrix[x] is an array of numbers. To access a particular number within this array, you would write matrix[x][y]. As usual, complete details can be found under Array in the client-side reference section. You can specify an optional string that separates the elements in the resulting string.

If no separator string is specified, a comma is used. To sort an array into some order other than alphabetical, you must pass a comparison function as an argument to sort. This function decides which of its two arguments should appear first in the sorted array. If the first argument should appear before the second, the comparison function should return a number less than zero. If the first argument should appear after the second in the sorted array, the function should return a number greater than zero.

And if the two values are equivalent i. Since the comparison functions are used only once, there is no need to give them names. If any of these arguments is itself an array, then it is the array elements that are concatenated, not the array itself. Note, however, that concat does not recursively flatten arrays of arrays. Its two arguments specify the start and end of the slice to be returned.

The returned array contains the element specified by the first argument and all subsequent elements up to, but not including, the element specified by the second argument. If only one argument is specified, the returned array contains all elements from the start position to the end of the array. If either argument is negative, it specifies an array element relative to the last element in the array.

An argument of -1, for example, specifies the last element in the array, and an argument of -3 specifies the third from last element of the array. Note that slice does not modify the array on which it is invoked. Unlike slice and concat , splice modifies the array on which it is invoked. Note that splice and slice have very similar names but perform substantially different operations. Elements of the array that come after the insertion or deletion point have their indexes increased or decreased as necessary so that they remain contiguous with the rest of the array.

The second argument specifies the number of elements that should be deleted from spliced out of the array. If this second argument is omitted, all array elements from the start element to the end of the array are removed. These arguments may be followed by any number of additional arguments that specify elements to be inserted into the array, starting at the position specified by the first argument.

The push method appends one or more new elements to the end of an array and returns the new length of the array. The pop method does the reverse: it deletes the last element of an array, decrements the array length, and returns the value that it removed. Note that both methods modify the array in place rather than produce a modified copy of the array. The combination of push and pop allows you to use a JavaScript array to implement a first-in, last-out stack.

Instead of being inserted into the array one at a time, arguments are inserted all at once as with the splice method. This means that they appear in the resulting array in the same order in which they appeared in the argument list.

Had the elements been inserted one at a time, their order would have been reversed. For an array, this method converts each of its elements to a string calling the toString methods of its elements, if necessary and outputs a comma-separated list of those strings.

Note that the output does not include square brackets or any other sort of delimiter around the array value. For example: [1,2,3]. It converts each array element to a string by calling the toLocaleString method of the element, and then it concat- enates the resulting strings using a locale-specific and implementation-defined separator string.

The subsections below describe these methods. Before we cover the details, however, it is worth making some generalizations about these ECMAScript 5 array methods. First, most of the methods accept a function as their first argument and invoke that function once for each element or some elements of the array.

If the array is sparse, the function you pass is not invoked for nonexistent elements. In most cases, the function you supply is invoked with three arguments: the value of the array element, the index of the array element, and the array itself. Often, you only need the first of these argument values and can ignore the second and third values. Most of the ECMAScript 5 array methods that accept a function as their first argument accept an optional second argument. If specified, the function is invoked as if it is a method of this second argument.

That is, the second argument you pass becomes the value of the this keyword inside of the function you pass. The return value of the function you pass is important, but different methods handle the return value in different ways. If you pass a function to these methods, that function may modify the array, of course.

As described above, you pass the function as the first argument to forEach. That is, there is no equivalent of the break statement you can use with a regular for loop. If you need to terminate early, you must throw an exception, and place the call to forEach within a try block.

The following code defines a foreach function that calls the forEach method within such a try block. If the function passed to foreach throws foreach. For the map method, however, the function you pass should return a value.

Note that map returns a new array: it does not modify the array it is invoked on. If that array is sparse, the returned array will be sparse in the same way: it will have the same length and the same missing elements. The function you pass to it should be predicate: a function that returns true or false. The predicate is invoked just as for forEach and map. If the return value is true, or a value that converts to true, then the element passed to the predicate is a member of the subset and is added to the array that will become the return value.

Note that both every and some stop iterating array elements as soon as they know what value to return. Note also that by mathematical convention, every returns true and some returns false when invoked on an empty array. The first is the function that performs the reduction operation. The task of this reduction function is to somehow combine or reduce two values into a single value, and to return that reduced value. In the examples above, the functions combine two values by adding them, multiplying them, and choosing the largest.

The second optional argument is an initial value to pass to the function. Functions used with reduce are different than the functions used with forEach and map. The familiar value, index, and array values are passed as the second, third, and fourth arguments. The first argument is the accumulated result of the reduction so far. On the first call to the function, this first argument is the initial value you passed as the second argument to reduce.

On subsequent calls, it is the value returned by the previous invocation of the function. In the first example above, the reduction function is first called with arguments 0 and 1. It adds these and returns 1. It is then called again with arguments 1 and 2 and it returns 3. This final value, 15, becomes the return value of reduce.

You may have noticed that the third call to reduce above has only a single argument: there is no initial value specified. When you invoke reduce like this with no initial value, it uses the first element of the array as the initial value.

This means that the first call to the reduction function will have the first and second array elements as its first and second arguments. In the sum and product examples above, we could have omitted the initial value argument. Calling reduce on an empty array with no initial value argument causes a TypeError. If you call it with only one value—either an array with one element and no initial value or an empty array and an initial value—it simply returns that one value without ever calling the reduction function.

The optional initial value argument takes its place. See the Function. It is worth noting that the every and some methods described above perform a kind of array reduction operation. They differ from reduce , however, in that they terminate early when possible, and do not always visit every array element.

The examples shown so far have been numeric for simplicity, but reduce and reduce Right are not intended solely for mathematical computations. Consider the union function from Example The first argument is the value to search for. The second argument is optional: it specifies the array index at which to begin the search. If this argument is omitted, indexOf starts at the beginning and lastIndexOf starts at the end.

Negative values are allowed for the second argument and are treated as an offset from the end of the array, as they are for the splice method: a value of —1, for example, specifies the last element of the array. The following function searches an array for a specified value and returns an array of all matching indexes. This demonstrates how the second argument to indexOf can be used to find matches beyond the first. Given an unknown object, it is often useful to be able to determine whether it is an array or not.

Each has its own JavaScript environment, with its own global object. And each global object has its own set of constructor functions.

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