ECMAScript for XML Specification(Ecma-357)(3)

From this description, it is clear that XSLT’s methods for accessing and manipulating data structures are completely different from those used to access and manipulate ECMAScript data structures. Consequently, the XSLT learning curve for ECMAScript programmers is quite steep. In addition to learning a new data model, ECMAScript programmers have to learn a declarative programming model, recursive descent processing model, new expression language, new XML language syntax, and a variety of new programming concepts (templates, patterns, priority rules, etc.). These differences also make XSLT code harder to read, write and maintain for the ECMAScript programmer. In addition, it is not possible to use familiar development environments, debuggers and testing tools with XSLT.

5.2.3 Object Mapping

Several have also tried to navigate and manipulate XML data by mapping it to and from native ECMAScript objects. The idea is to map XML data onto a set of ECMAScript objects, manipulate those objects directly, then map them back to XML. This allows ECMAScript programmers to reuse their knowledge of ECMAScript objects to manipulate XML data.

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欧洲计算机制造商协会(ECMA)标准文档

This is a great idea, but unfortunately it does not work for a wide range of XML processing tasks. Native ECMAScript objects do not preserve the order of the original XML data and order is significant for XML. Not only do XML developers need to preserve the order of XML data, but they also need to control and manipulate the order of XML data. In addition, XML data contains artifacts that are not easily represented by the ECMAScript object model, such as namespaces, attributes, comments, processing instructions and mixed element content.

5.3 The E4X Approach

ECMAScript for XML was envisioned to address these problems. E4X extends the ECMAScript object model with native support for XML data. It reuses familiar ECMAScript operators for creating, navigating and manipulating XML, such that anyone who has used ECMAScript is able to start using XML with little or no additional knowledge. The extensions include native XML data types, XML literals (i.e., initialisers) and a small set of new operators useful for common XML operations, such as searching and filtering.

E4X applications are smaller and more intuitive to ECMAScript developers than comparable XSLT or DOM applications. They are easier to read, write and maintain requiring less developer time, skill and specialized knowledge. The net result is reduced code complexity, tighter revision cycles and shorter time to market for Internet applications. In addition, E4X is a lighter weight technology enabling a wide range of mobile applications.

6 Design Principles

The following non-normative design principles are used to guide the development of E4X and encourage consistent design decisions. They are listed here to provide insight into the E4X design rational and to anchor discussions on desirable E4X traits

Simple: One of the most important objectives of E4X is to simplify common programming tasks. Simplicity should not be compromised for interesting or unique features that do not address common programming problems.

Consistent: The design of E4X should be internally consistent such that developers can anticipate its behaviour.

Familiar: Common operators available for manipulating ECMAScript objects should also be available for manipulating XML data. The semantics of the operators should not be surprising to those familiar with ECMAScript objects. Developers already familiar with ECMAScript objects should be able to begin using XML objects with minimal surprises.

Minimal: Where appropriate, E4X defines new operators for manipulating XML that are not currently available for manipulating ECMAScript objects. This set of operators should be kept to a minimum to avoid unnecessary complexity. It is a non-goal of E4X to provide, for example, the full functionality of XPath.

Loose Coupling: To the degree practical, E4X operators will enable applications to minimize their dependencies on external data formats. For example, E4X applications should be able to extract a value deeply nested within an XML structure, without specifying the full path to the data. Thus, changes in the containment hierarchy of the data will not require changes to the application.

Complementary: E4X should integrate well with other languages designed for manipulating XML, such as XPath, XSLT and XML Query. For example, E4X should be able to invoke complementary languages when additional expressive power is needed without compromising the simplicity of the E4X language itself.

7 Notational Conventions

This specification extends the notational conventions used in the ECMAScript Edition 3 specification. In particular, it extends the algorithm notation to improve the clarity, readability and maintainability of this specification. The new algorithm conventions are described in this section.

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欧洲计算机制造商协会(ECMA)标准文档

7.1 Algorithm Conventions

This section introduces the algorithm conventions this specification adds to those used to describe the semantics of ECMAScript Edition 3. These conventions are not part of the E4X language. They are used within this specification to describe the semantics of E4X operations.

7.1.1 Indentation Style

This specification extends the notation used in the ECMAScript Edition 3 specification by defining an algorithm indentation style. The new algorithm indention style is used in this specification to group related collections of steps together. This convention is useful for expressing a set of steps that are taken conditionally or repeatedly. For example, the following algorithm fragment uses indentation to describe a set of steps that are taken conditionally:

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