深入理解JavaScript：实践指南

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"Pragmatic Guide to JavaScript 是一本针对中高级JavaScript开发者的指南，深入探讨了JavaScript的常见应用场景和模式，提供了丰富的学习价值。读者评价中提到，这本书能够帮助读者理解流行的JavaScript库背后的原理，并给出实用的建议和背景知识，对于日常JavaScript任务有很好的参考价值。书中涵盖了从基础如表单验证和JSON处理到高级应用如混合应用和地理定位的各种主题，适合希望提高JavaScript在Web应用中生产力的开发者。" 在这本"Pragmatic Guide to JavaScript"中，作者将多年的经验凝聚成册，旨在帮助读者在实际编程中取得更大的进步。书中的内容不仅包括了JavaScript的基础知识，还深入到了现代浏览器环境下的高级技巧。例如，书中详细讲解了如何有效地进行表单验证，这是Web开发中必不可少的一环，确保用户输入的数据准确无误。此外，书中还涵盖了JSON（JavaScript Object Notation）的处理，这是一个广泛用于数据交换的轻量级格式，对于前后端通信至关重要。书中还探讨了JavaScript库的内部工作原理，这对于想要自定义或优化现有库的开发者来说极其有用。通过理解这些库的工作方式，开发者可以更高效地利用它们，或者创建自己的工具。比如，书中可能涉及了像jQuery、React或Angular这样的流行库，解析它们的核心功能和最佳实践。此外，"Pragmatic Guide to JavaScript"还讨论了现代Web应用中的热点话题，如地理位置服务（geolocation），这使得开发者能够构建更具交互性和个性化的应用。同时，书中可能还介绍了如何将Web应用与其他在线服务（如地图或社交媒体）融合，实现数据和功能的混合，也就是常说的Web应用“混搭”。这本书是针对有一定JavaScript基础的开发者，希望他们能够通过深入学习和理解JavaScript的内在机制，提升到更高的水平。无论你是独立开发者还是团队领导者，无论是对JavaScript有深厚兴趣还是在工作中需要提升JavaScript技能，"Pragmatic Guide to JavaScript"都是一个值得拥有的宝贵资源。

P a r t I

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

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 Array 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 uld 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 would on a function

you’ re calling straight by its name.

Note that if the arguments you wa 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.

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 ov e 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 JavaScript

code, which wo uld 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” ex 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.

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