深入浅出JavaScript实战指南

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"《JavaScript编程指南 Pragmatic Guide to JavaScript》是一本面向初学者和有经验的开发者的实用JavaScript教程，旨在帮助读者深入理解JavaScript语言并提升在实际项目中的应用能力。作者以其丰富的经验，提供了关于如何正确使用JavaScript的实用建议和背景知识，并对流行的JavaScript库的工作原理进行了揭秘。本书涵盖了从基础到高级的各种主题，包括表单验证、JSON处理、应用示例如数据融合和地理定位等，是提升JavaScript生产力的必备读物。" 在JavaScript编程领域，深入理解语言机制和最佳实践至关重要。《JavaScript编程指南 Pragmatic Guide to JavaScript》一书的核心价值在于其全面性和实用性。首先，它针对那些刚开始接触JavaScript或希望深化理解的开发者，提供了一个清晰的学习路径。书中不仅讲解了JavaScript的基础语法，如变量、函数、对象和作用域，还强调了语言的动态特性和原型继承等关键概念。此外，该书深入探讨了JavaScript在现代浏览器中的实际应用，如DOM操作、事件处理和异步编程。书中提到的“流行JavaScript库的工作原理”部分，可能涉及jQuery、React、Vue等库，帮助读者理解它们是如何优化和简化JavaScript开发的，这对于使用这些库进行前端开发的人来说极其有价值。书中还涵盖了诸如表单验证和JSON处理等常见任务，这些都是Web应用程序开发中不可或缺的部分。通过实例，读者可以学习如何有效地验证用户输入，确保数据安全和准确。JSON作为数据交换格式的广泛使用，使得掌握其处理技巧变得尤为重要。在高级主题方面，书中可能讨论了地理定位API，这是移动和Web应用中的热门功能，让读者能够构建基于位置的服务。同时，书中的“数据融合”可能是指如何将来自多个数据源的信息整合在一起，以创建更丰富、更个性化的用户体验。《JavaScript编程指南 Pragmatic Guide to JavaScript》是一本全面覆盖JavaScript基础知识和高级技术的书籍，得到了业内专家的高度评价。无论你是新手还是有经验的开发者，都能从中受益，提升自己的JavaScript编程技能，更好地应对现代Web开发的挑战。

P a r t I

Bread and Butter: Pure J a v a S c r i p t

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DYNAMICALLYSELECTINGA METHOD/PROPERTY 18

Dynamically Selecting a

Method/Property

Often you ﬁnd yourself wanting to call one of two methods (functions

associated with an object) depending on the situation. Or instead of

functions to call, this could be about reading, or writing, to one of two

possible properties (variables associated with an object). The code for this

would look something like what follows:

if (condition) {

myObj.method1(someArg);

} else {

myObj.method2(someArg);

}

JavaScript offers a simple syntax for dynamically selecting methods and

properties, all relying on the square brackets ([ ]) operator. Y o u see,

JavaScript has two interchangeable syntaxes for member access (that is a

common dynamic language trait):

obj[expressionResultingInMemberName] == obj.memberName

If you’ve e v e r plucked a v a l u e from an Array cell using its integer index,

you’ ve already used the square brackets operator for dynamic member

selection! This is because Arr ay objects have properties named after their

numerical indices (plus the length property). However, in this case,

JavaScript won’t let you use the dot operator (.) for direct access. m y A r r a y. 0

is invalid syntax (too bad, that wo u l d have made a good nerd trick).

Here’s why using the square brackets operator ([ ]) is more powerful than dot

notation: you can put anything in between the brackets to obtain the name

of the member (property or method) that you wa n t to access. Common cases

include literals, v a r i a b l e s holding the member name, name composition

e x p r e s s i o n s (mostly string concatenations), and quick if/then choices in the

form of a ternary operator (condition ? v a l u e I f T r u e : v a l u e I f F a l s e ). It’ll all be

turned into a string ﬁrst and then used to look up the member you want to use.

In JavaScript, functions are objects too and can be referenced like any other

v a l u e . When an e x p r e s s i o n results in a function, you can call it by using

parentheses, possibly with arguments, just like you woul d on a function

you’ re calling straight by its name.

Note that if the arguments you w a n t to pass to the method v a r y depending on

which technique you select, using parentheses may quickly become too

cluttered for easy reading. In that case, going for a regular if/else structure is

a wiser move.

Report erratum

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ACHIEVINGCODEPRIVACYWITHTHEMODULEP A T T E R N 20

Achieving Code Privacy with the

Module P a t t e r n

The more JavaScript there is in your codebase, the more your global scope

may get “polluted” with numerous functions and v a r i a b l e s that woul d

actually be better kept private to whatever set of code uses it. W i t h this

comes the risk of name collision, with one script unintentionally o ve r w r i t i n g

another’s identiﬁers. This leads to b u g s .

W e need to be able to create self-contained, opaque batches of Jav aScript

code, which wo u ld e x p o s e only selected identiﬁers, if any, to the outside

world. Indeed, this is a major requirement for “programming in the large,”

being able to bring in frameworks and libraries in any page without risking a

clash. This is what the module pattern is for.

The whole idea of the module pattern is to create a private scope for

v a r -declared identiﬁers and functions, a scope that only functions deﬁned

inside it can access. T o make some of these deﬁnitions accessible to the

outside worl d , our enclosing function has two choices. It may return an

object with these selected v a l u e s as properties (see the facing page); we just

need to assign that returned object to a v a r i a b l e in the outside scope. Another

way is to pass the enclosing function a scope object that it writes properties

to (to make these global, you’ d simply pass window).

In JavaScript, identiﬁers ﬁrst used with the v a r declaring keyword are local.

(They belong to the function currently deﬁned.) Identiﬁers ﬁrst used without

v a r are global. (They’re grafted onto the current default scope, which most

of the time means the global window object.)

In a few speciﬁc circumstances, the current default scope will not actually be

global, so there are ways to e x e c u t e code in a context where non-v a r

identiﬁers will not leak into the global namespace—but that is a bit outside

the scope of this task.

T e c h n i c a l l y , you do not have to name your “public properties” e x a c t l y like

your private identiﬁers. Indeed, you could deﬁne public methods on the ﬂy

in the returned object literal using anonymous functions. But such practices

would result in code that is more obscure (or misleading) to read

and—perhaps more importantly—to debug. As a rule of thumb, whenever

possible, try to deﬁne your functions using named function e x p r e s s i o n s :

function myFunctionName(...) { ... }

This makes for clearer code and helps a lot with the readability of the stack

traces when debugging your JavaScript.

Report erratum

this copy is (P1.0 printing, November 2010)

Download from Wow! eBook <www.wowebook.com>

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