Guide JavaScript & DHTML Cookbook: Solutions and Example for Web Programmers

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JavaScript & DHTML Cookbook (2nd edition)

If, after the above modifications, you change the value of the prototype property, the private property values assigned individually do not reflect the prototype change. For example, changing the prototype status property to reflect the company-wide vacation period is accomplished as follows: coworker.

Javascript Modular Programming, for web applications (New) Part 1

For example, the following statement tests the equality of the local status property against the prototype status property: if employeeDB[2]. All objects accessible by JavaScript—custom objects, global language objects, and DOM objects in Mozilla and Opera 9 —are subject to prototype inheritance. When a prototype inheritance chain consists of two or more objects, you can also use scripts to access points higher up the chain. For example, an Array constructor inherits from the basic Object constructor. In other words, it is conceivable that a prototype property influencing an instance of an array could be defined for the Object or Array constructor prototype.

I mention it here in case you encounter this syntax in further research about simulating object-oriented techniques in JavaScript. The shortcut equivalents to these references are: myArray. It is possible in JavaScript to simulate what some programming languages describe as an interface or implements construction—a way of empowering one object with the 76 Chapter 3: Arrays and Objects properties and methods of another object without creating a subclass of the shared object. The approach to this implementation is not particularly intuitive, but it works nicely once you have it set up. In other words, invoking the member method of a teamMember object creates a new coworker object.

And in fact, the next statement of the teamMember constructor function invokes the member method, passing two of the incoming parameters to the coworker function. But all other pieces of the teamMember object are readily accessible and meaningful to your scripts. An unusual side effect to the connection between these nested objects is that the teamMember objects do not have the coworker constructor in their prototype chain.

Therefore, if you assign a property and value to the prototype of the coworker constructor, none of the teamMember objects gains that property.

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Then you can do something like the following: coworker. And, just like any prototype property or method, you can override the private value for a single instance without disturbing the default values of the other objects. In Mozilla and Opera 9, many of these language capabilities add potentially enormous power to the DOM you use every day. These browsers give scripters access to the constructors of every type of DOM object, thus allowing you to add prototype properties and methods to any class of DOM object. Go crazy! Solution Use the objectsArraysStrings.

The first, object2String , works through the properties of a custom object, and assembles a string in the same format used to generate the object in the curly-braced shortcut syntax. The sole parameter to the function is a reference to the custom object you wish to convert. The returned value is a string, including the curly braces surrounding the text. Each function relies on the other at times.

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For example, if you are converting an object to a string, and one of its properties is an array, the array portion is passed through array2String to get the desired format for that segment of the full string. Conversely, converting an array of objects requires calls to object2String to format the object portions. To reconstruct the object or array data type from the string, use one of the final two functions that applies to the outermost construction of the string.

The two functions perform the same operation, but the names are provided for each conversion type to improve readability of the code that invokes them. Despite warnings elsewhere in this book about performance degradation of the eval function, its use is necessary here. Let the type of the outermost data structure govern which of the two convert-tostring functions you use.

Even though your custom objects may be the most important part of your script data structure conceptually, they may be found within an array of those objects, as shown in Recipe 3. In this case, convert the entire array of objects to a single string by invoking array2String , and let it handle the object conversion along the way. Conversions between objects and strings are the foundation of JSON JavaScript Object Notation , one of the ways you can use Ajax technology to access server data from the current page.

See Recipe See Also Recipes 3. Solution Make use of what is known as encapsulation, an automatic feature of JavaScript objects. Each property or method name you define for an object is private to that object.

For example, if you define an object that contains three properties and two methods, the only name that is accessible to other script statements outside of the object is the identifier you use for the entire object. The five names you use for the properties and methods can be used anywhere else without fear of naming conflicts. Access to the internal properties and methods is via the standard objectName.

Discussion At issue here is the global i.

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Every global variable and, importantly, every non-nested named function has global scope. For example, if you import two external. So, too, with global variables that may be defined in multiple. When multiple authors work on a project, or if you use third-party script files, you may be unaware of the conflicts looming when all files are loaded into a single page. Thus, if you have a form containing a text input element whose ID is total, you cannot have any JavaScript global variable or function by that name.

The original library contains five functions, each of which occupies a global name spot: getExpDate, getCookieVal, getCookie, setCookie, and deleteCookie. The replacement library contains a single object, cookieMgr, which becomes the gateway to invoking the functions. Example shows the code for the revised cookie manager library, with changes shown in bold. Each function is coded as an anonymous function see Recipe 4. When a JavaScript custom object property name to the left of the colon is defined as an anonymous function to the right of the colon , the property becomes a method of the object.

Therefore, following the example from the Discussion section of Recipe 1. While you could refer to the object by name cookieMgr , that tactic limits you to always using that specific name for the object. Instead, use the this operator to point to the current object, as shown in the getCookie and deleteCookie methods. A couple of these recipes or your own variations on them may be part of your daily menu. Even simple subjects, such as JavaScript variables and functions, have numerous nuances that are easy to forget over time.

On another front, scripters without formal programming training tend to be rather loose in their attention to detail in the error department—something that can come back to bite you. On the other hand, the browser implementations of some of the details of exception handling are far from compatible. This chapter ends with some suggestions about improving script performance.

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Most scripts can scrape by with inefficiencies, but larger projects that deal with complex document trees and substantial amounts of hidden data delivered to the client must pay particular attention to performance. Discussion A JavaScript variable has no inherent limit to the amount of data it can hold. Maximum capacity is determined strictly by memory available to the browser application— information not accessible to your scripts.

Variable scope is an important concept to understand in JavaScript.

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Not only is a global variable accessible by all script statements in the current window or frame, but statements in other frames or windows served from the same domain and server can access those global variables by way of the window or frame reference. For example, a statement in a menu frame can reference a global variable named myVar in a frame named content as follows: parent.

Where you must exercise care is in defining a new variable inside a function with the var keyword. If you fail to use the keyword inside the function, the variable is treated as a global variable. Even though the keyword is optional for global variable declarations, it is good coding style to use var for globals as well. That way you can readily see where a variable is first used in a script. But in any case, this situation definitely calls for declaring the variables outside of the loops for another reason.

If you were to use the var keywords in the loop counter initialization statements e. Internally, the JavaScript interpreter creates a new variable space for each var keyword. Fortunately, the interpreter is also able to keep track of which variable repeatedly declared is the current one, but it places an unnecessary burden on resources.