JavaScript对象 notation 入门指南

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本文档是一本关于JavaScript对象表示法（JavaScript Object Notation, JSON）的详细介绍教程，由Lindsay Bassett撰写，英文原名为《Introduction to JavaScript Object Notation》。这本书在2015年由O'Reilly Media出版，ISBN号码为978-1-491-92948-3，是针对Web开发人员的一本早期发行版本。 JSON是一种轻量级的数据交换格式，它允许数据以键值对的形式存储和传输，非常适合于客户端与服务器之间的数据交互，特别是在JavaScript环境中，由于其简洁易读的特性，被广泛应用于API接口、配置文件、数据存储等多个场景。本书可能包含以下知识点： 1. **JSON基础**：介绍JSON的语法结构，包括键(key)和值(value)的概念，以及如何创建、数组和嵌套对象的表示方式。 2. **解析和序列化**：讲解如何使用JavaScript内置的`JSON.parse()`方法将JSON字符串转换为JavaScript对象，反之如何使用`JSON.stringify()`方法将JavaScript对象转换为JSON字符串。 3. **浏览器支持**：讨论浏览器对JSON的内置支持情况，以及在不支持JSON的旧版浏览器中的兼容处理策略。 4. **跨域应用**：介绍如何处理跨域问题，当从不同源请求JSON数据时，如何利用CORS（跨源资源共享）技术来解决安全限制。 5. **实用示例**：书中可能会提供大量实际项目中的应用场景，如用户登录状态管理、API调用响应数据解析等。 6. **错误处理和最佳实践**：讨论常见的JSON解析错误，如语法错误、类型转换错误，以及如何编写健壮的JSON代码以避免这些问题。 7. **深入理解**：探讨JSON在前后端分离架构、微服务架构中的角色，以及与XML、YAML等其他数据交换格式的区别和优势。 8. **版本更新和参考资料**：2015年8月首次发行的书籍，还提供了在线版本链接以及修订历史，读者可以查看最新的更新内容和错误修正。这本《Introduction to JavaScript Object Notation》旨在帮助读者掌握JSON的基础知识和实战技巧，以便在Web开发中高效地处理数据传输和操作。如果你是一名Web开发者或对JavaScript编程感兴趣，此书无疑是一个宝贵的参考资料。

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world’s languages and employ a team of translators. This would be expensive and

time consuming. This is an intelligent world, however, so the islands all agreed on a

single language with a standard format for communicating their trade data. Each

island employs just a single translator that understands the one data format of the

trade reports brought by the carrier seagulls.

The real world of technology is much like the imagined island world example. There

is a vast ocean, full of islands that have different languages, customs, and architecture.

The ability for these unique systems to communicate is integral to many businesses

and organizations. If each of these systems needed a translator for all the many ways

other systems structure their data, then communications would consume an unrea‐

sonable amount of time and resources. Instead, the systems agree on a single format

for data and employ a single translator.

JSON is a data interchange format that many systems have agreed on using for com‐

municating data. You may hear it referred to as a “data exchange format,” or simply a

“data format.” In this book, I will refer to JSON as a data interchange format because

the definition of “interchange” reminds us that the data format is intended for two or

more entities exchanging it.

Many, but not all systems have agreed on JSON for communicating data. There are

data interchange formats, such as Extensible Markup Language (XML), that were

around before JSON was even thought about. The real world is not quite as simple as

the island example. Many systems have and still use other formats, such as XML, or

more tabular, delimited formats such as comma-separated values (CSV). The deci‐

sion by each island in the real world for which data format to accept for communica‐

tion often has to do with how the data format relates to the customs, language, and

architecture of the island.

In the island world example, each of the hundreds of islands had its own language.

The data in the paper report that the carrier seagulls carried was in an agreed upon

format that was independent of language. This way, a single translator of the trade

reports data could be employed by each island. The same is true of JSON, except the

data is carried across networks in zeros and ones instead of by seagulls. The translator

isn’t a human, it is a parser employed by the system consuming the data so it can be

read within the system it is entering.

JSON Is Programming Language Independent

JSON stands for JavaScript Object Notation. The name of this data interchange for‐

mat may mislead people into thinking they will need to learn JavaScript to under‐

stand and use JSON. There would be some value in learning JavaScript before

learning JSON, as it was born out of a subset of JavaScript, but if you will not be using

JavaScript anytime soon it would be unnecessary. You may remain dedicated to the

2 | Chapter 1: What Is JSON?

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language or languages of your own island, for the spirit of a data interchange format

is to be independent of language.

JSON is based on JavaScript object literals. A detailed explanation into the “how” of

this is better suited for our discussion on syntax (Chapter 2) and data types (Chap‐

ter 3). For this chapter, the “why” is important. If a data interchange format is meant

to be language independent, then it may seem contradictory to have a data format

that is not only derived from a single language, but advertises it in its name: Java‐

Script Object Notation. Why, then?

If we return to the island example, imagine for a moment what the meeting to select

the data format would have been like. When the representatives from each of the

hundreds of islands arrived at this meeting, and looked to create a single data format,

the first thing they would want to find is common ground.

The languages of each island may have been unique, but there were things the island‐

ers found they had in common. Most of the languages were spoken primarily with

the human voice and included a written form of the language represented by charac‐

ters of some sort. Additionally, facial expressions and hand movements were also

present. There were a few troublesome islands where the people communicated by

other means, such as hitting sticks together or winking, but the majority of the

islands found common ground with their written and spoken forms of language.

In the real world, there are hundreds of programming languages. Some are more

popular and commonly used than others, but the language landscape is diverse.

When college students major in computer science in preparation for a career in pro‐

gramming, they do not study all the programming languages. Students usually begin

with one language, and the language itself is not so important as learning the univer‐

sally accepted programming concepts. Once students gain an understanding of these

concepts, they can more easily learn other programming languages through their

ability to recognize the common features and functionalities.

If we set aside the word “JavaScript” from the name “JavaScript Object Notation,” we

would be left with “Object Notation.” In fact, let’s forget JavaScript all together. We

could then say we are using an object notation data interchange format. “Object” is a

common programming concept, in particular to object-oriented programming

(OOP). Most computer science students studying programming will learn the con‐

cept of objects.

Without diving into an explanation of objects, let’s settle our attention on the word

“Notation.” Notation implies a system of characters for representing data such as

numbers or elements. With or without an understanding of objects in programming,

it is not a stretch to see the value of having a notation to describe something that is

common across programming languages.

JSON Is Programming Language Independent | 3

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Returning again to the island example, the islanders themselves found a notation that

represented a common tie among the majority of languages. Most of the islanders

had a similar way of representing numbers with tallies, and it was agreed they could

understand a series of symbols for representing real-world objects such as wheat or

fabric. Even the island that communicated by winking found this format acceptable.

Despite the agreement among the vast majority of islands, there were still a few

islands, such as the island that communicated by hitting sticks together, that did not

find the format understandable. A good data interchange format covers the majority,

but there are usually outliers. When we talk about this coverage, a term often thrown

around is portability. Portability, or the compatibility in transferring information

between platforms and systems, is the very goal of a data interchange format.

Circling back to notation, the notation of JSON may originate from JavaScript, but

the notation itself is the important part. Not only is JSON language independent, it

represents data in a way that speaks to common elements of many programming lan‐

guages. With the way that data is represented, such as numbers and words, even the

programming languages that aren’t object-oriented can find this format acceptable.

Key Terms and Concepts

This chapter covered the following key terms:

JSON

JavaScript Object Notation

Notation

A system of characters for representing data such as numbers or elements

Data interchange format

Text used to exchange data between platforms or systems

Portability

Transferring information between platforms in a way that is compatible with

both systems

We also discussed these key concepts:

•

JSON is a data interchange format

• JSON is programming language independent (JavaScript is not required to use it)

• JSON is based on the object literal notation of JavaScript (emphasis on the word

“notation”)

• JSON represents data in a way that is friendly to universal programming con‐

cepts

4 | Chapter 1: What Is JSON?

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In the "x = 5” example, 5 is a number literal. A number is a data type. Other types of

data are strings (made up of characters), boolean (true or false), null (nothing), col‐

lections of values, and objects. Representing a number value in a way that we can see

is simple, and we use a number character. Representing a boolean value is also simple

and we can use true/false or 0/1. If you are familiar with the concept of objects, you

will understand that representing an object is no easy or simple matter. If you aren’t

familiar with the concept of objects, that is OK too.

In programming, the concept of an object is similar to how you would describe a

real-world object, such as your shoes. You could describe your shoes with attributes

or properties such as color, style, brand, and the type of insole. Some of these

attribute values could be a number, such as shoe size, and others could be a boolean

(true/false) such as “has laces.” Example 2-1shows an example.

Example 2-1. Using JSON to describe the shoes I’m wearing right now

{

"brand": "Crocs",

"color": "pink",

"size": 9,

"hasLaces": false

}

Don’t be concerned just yet about the syntax in the shoe example; we will arrive there

later in this chapter. The main point of the shoe example is that you (even a human)

can literally read the attributes of my shoe. The data type for my JSON shoe example

is object. We can see the literal value of the object which exposes the properties or

attributes in a way which we can see (and read). These attributes or properties of the

shoe object are represented as name-value pairs.

JSON is based on JavaScript object literals. The key phrase here is “based on.” In Java‐

Script (and most programming languages with objects), the object can include a

function. So not only could I represent the properties of my shoe with the JavaScript

object, but I could create a function called “walk.”

However, data interchange is about data, so JSON does not include the functions of

JavaScript object literals. The way that JSON is based on JavaScript object literals is

purely in the syntactic representation of the object literal and its properties. This rep‐

resentation of properties is achieved with name-value pairs.

Name-Value Pairs

The concept of name-value pairs is widespread in computing. They are called by

other names as well: key-value pairs, attribute-value pairs, and field-value pairs. In

this book, we will refer to them as name-value pairs.

6 | Chapter 2: JSON Syntax

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