Web Data Interchange Formats

CSC 342 - Web Technologies

Web Data Exchange

  • Data exchange is the process of transforming structured data from one format to another to facilitate data sharing between programs

  • A data exchange language is a language that is capable of expressing general purpose data

  • We will look at two data exchange languages that are commonly used to exchange data on the web:

    • Extensible Markup Language (XML)

    • JavaScript Object Notation (JSON)

Extensible Markup Language (XML)

  • XML is a markup language that uses a textual data format to represent arbitrary data structures

  • Schema systems exist for XML that enable XML based languages to be defined and validated

  • The file extension for XML files is .xml

  • The MIME type for XML text is application/xml

XML vs HTML

  • XML was designed as a data exchange format

  • HMTL was designed to display data to the user

  • XML tags are not predefined

  • XML was designed to be extensible

Basic XML Syntax Rules

  • An XML document begins with an optional declaration, for example:

    <?xml version="1.0" encoding="UTF-8"?>

  • XML documents must have a root element that is the parent of all other elements

  • XML elements must have a closing tag

  • XML tags are case sensitive

  • XML elements must be properly nested

  • XML attributes must be quoted

XML Elements

  • An XML element is a logical document component with the following basic syntax:

    <element attribute="value">content</element>

  • Tags begin with < and end with >

    • start-tag: <element>

    • end-tag: </element>

    • empty-element-tag: <element />

  • Attribute values must be in quoted (single or double quotes)

XML Element Naming Rules

  • Element names are case sensitive

  • Element names must start with a letter or an underscore

  • Element names cannot start with the letters xml (in any combination of upper and lower case)

  • Element names can contain letters, digits, hyphens, underscores, and periods

  • Element names cannot contain spaces

XML Entities

  • There are five pre-defined entity references

    • &lt; less than (<)

    • &gt; greater than (>)

    • &amp; ampersand (&)

    • &apos; apostophe (’)

    • &quot; quotation mark (")

XML Comments

  • Comments in XML begin with <!– and end with –>

    <!-- This is a comment -->

  • Two dashes – are not allowed in a comment

  • Comments cannot be nested as a consequence of the previous rule

XML Namespaces

  • XML elements are not predefined, so there is a chance that two different XML documents use the same element name

  • XML Namespaces are a way to handle element name conflicts

  • A namespace declaration has the following syntax
    xmlns:prefix="URI"

  • Example:

    <p:ul xmlns:p="https://example.com/p">
  <p:li>Item 1</p:li>
  <p:li>Item 2</p:li>
</p:list>

Accessing XML with JavaScript

  • The XML DOM defines properties and methods for accessing and editing XML

  • XML text data can be converted into an XML DOM object using the DOMParser object

    var parser = new DOMParser();
x = parser.parseFromString(t, "text/xml");
// t is variable containing an XML string

Valid XML Documents

  • A well formed XML document follows the XML syntax rules

  • A valid XML document must be well formed and conform to a document type definition

  • There are two main schema systems that can be used with XML

    • Document Type Definition (DTD)

    • XML Schema

  • The purpose of a schema is to define the structure of an XML document including:

    • The elements and attributes that can appear in a document

    • The number and order of child elements

    • The data types for elements and attributes

    • The default and fixed values for elements and attributes

JavaScript Object Notation (JSON)

  • JSON is designed to be a lightweight data exchange language

  • JSON is data is plain text

  • The file extension for JSON files is .json

  • The MIME type for JSON text is application/json

JSON Syntax

  • JSON syntax is similar to the syntax of defining literal objects in JavaScript:

    • Data is in name/value pairs of the form "name":value

    • Data is separated by commas

    • Curly braces hold objects

    • Square braces hold arrays

JSON Data Types

  • Number: a signed decimal number

  • String: a sequence of zero or more unicode characters delimited by by double quotes

  • Boolean: a value of true or false

  • Array: an ordered list of zero or more values separated by commas and delimited by square brackets

  • Object: an unordered collection of name/value pairs where pairs are separated by commas and delimited by curly braces

  • Null: the empty value indicated by the word null

Accessing JSON with JavaScript

  • JSON.parse(): convert a JSON string into a JavaScript type

  • JSON.stringify(): convert a JavaScript object into JSON text

    • JavaScript Date objects are converted into strings

    • JavaScript functions are removed since they are not valid

XML vs JSON

  • Similarities

    • “self describing” (human readable)

    • hierarchical (nested values)

    • can be parsed by many programming languages

    • can be fetched with XMLHttpRequest

  • Differences

    • JSON does not use end tags

    • XML is more difficult to parse than JSON

    • JSON has an array type

    • JSON cannot be validated by a schema (yet)