JavaScript Parsing Libraries & Tools
JavaScript is a crucial language for web development, and parsing JavaScript code to extract critical elements is a vital step in the process when working with APIs or web data.
At Rayobyte, we frequently discuss the need to scrape data from websites, which involves extracting raw data from websites to be analyzed and manipulated, providing key information.
Parsing is a component of the process, one that involves several steps. JavaScript parsers are tools or functions that enable this process.
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The goal of a JavaScript parser is to take raw data, often presented in strings, and format it in a way that JavaScript can then work with for other needs. This process of interpreting and transforming data is critical to many tasks.
If you need a bit more background on the benefits and uses, read our guide on what parsing is. Then, consider how you can do so using the JavaScript parser library and other tools available to you.
What Are JavaScript Parsers?
A JavaScript parser is a tool that lets web developers understand and then verify data, often using it to manipulate that data in some way. More specifically, JavaScript parsers read raw data or code and then turn it into structured data, often an AST, so that other tools can then work with it.
Parsers are, themselves, a set of instructions that handle the tasks desired. However, if you need specific information from text, you need to create a parser that will do this for you. There are several ways that you can parse language from Java.
The first option is to use an existing library. A library is a set of code written for a specific format, such as that written for XML or HTML. Libraries are ready-made, meaning you can use that information to create a parser for your task. This option is best when you have a basic parser need to fill, but this does not work in all situations.
The next option is to build your own parser, a custom process done by hand that gives you exact results. While this works well, it is the hardest and most time-consuming option, and may be challenging for some to do well. The third option is to use a tool or a library to generate a parser, and this tends to be the best option overall. It enables you to use an available library to create a parser that fits your needs in a lot less time.
When choosing which way to create a JavaScript parser, be sure to consider the way you plan to use it. Most of the time, these parsers are essential for handling specific types of data, such as JSON, XML, or HTML, or in some situations, can work well for custom AST structures. When choosing a JavaScript parser library to use to build your library, it is critical to consider which tools are best suited for your project needs.
JavaScript Parser Library Selection
The JavaScript parsing library selected needs to match the specific type of data you have. To provide some examples, consider the following:
JSON: If you are working with JSON formatted strings, you will need to turn those strings into a JavaScript object to make it useful. Doing so will enable developers to work with that data from external sources, including APIs. To do this with JSON formatted strings, you can use the JSON.parse() method – which is a built-in JavaScript method ideal for converting these data strings into usable objects.
An example of how to parse a string that is currently in the JSON format using this method, intending to return a JavaScript object, is this:
var obj = JSON.parse('{"firstName":"John", "lastName":"Doe"}');
Another important element about this method is that, when there is an error, it will show that. One of the best features about some JavaScript parsers is their ability to warn of an error, work around it following the strategies you tell it to, or to keep going. In situations where you are using the JSON.parse() method, it will throw a SyntaxError in any situation where the input string is not a valid JSON.
Also note that it accepts an optional reviver function that will transform the parsed data. Here is another example:
const s = '{"name": "Paul", "age": 55, "city": "Detroit"}';
const obj = JSON.parse(s);
console.log(obj);
In this situation, the output will be:
{ name: 'Paul', age: 55, city: 'Detroit' }
This means:
- S is a valid JSON string
- JSON.parse() converts it into an object obj.
- You can now access the object’s properties, including obj.name or obj.age
XML and HTML: It is quite common for the data you have to be in XML or HTML format. In these situations, you need to use a JavaScript parsing library that can handle those specific formats. One way to do that is with the DOMParser.
DOMParser is an interface that provides the direct ability to parse data in XML or HTML course code, taking it from a string and turning it into a DOM document. You can also use the same method to turn a DOM tree into an XML or HTML source. To do that, you need to use the XMLSerializer interface.
In some situations of HTML documents, you may want to replace a portion of the DOM with new DOM trees built from HTML. You can do that with this method by using the value of the Element.innerHTML and outerHTML properties. When you apply this technique, it can also be used to fetch HTML fragments from the DOM subtree associated with it. This allows for more elaborate details.
The DOMParser() will create a new DOMParser Object.
DOMParser.parseFromString() will parse a string using HTML parsers or the XML parser selected. It will then return that information in an HTMLDocument or in an XMLDocument.
When it comes to XML parsers with a string, it is possible that an error will occur. If a string does not represent well-formed XML, the XMLDocument returned by parseFromString will include the following:
<parserror>
This is the node that will describe the nature of the parsing error, allowing the web developer to take appropriate steps.
JavaScript Parsing Library and Tool Examples
JavaScript parsers are a critical component of your work, but how do you create a parser? To do so, you will need to use tools that will generate parsers that are specifically usable in JavaScript, including JavaScript libraries to build parsers.
The tools that you can use to create the code for a parser are called generators. Libraries that create parsers are called parser combinators. Generators are complex, and they can be difficult to navigate in terms of creating them yourself. Most are specific to a language.
Knowing this, consider what the structure of a parser is. Most will continue with a lexer and a proper parser. Not all parsers will have this two-component method, but most do. The lexer and parser will work in sequence with each other. The lexer scans the input from the data you have and then processes the matching tokens. The JavaScript parsers then scan the tokens and produce the parsing result. The lexers and parser use definitions called rules, which give them specific insight into what to do.
You are likely to see parsing trees and abstract syntax tree (AST) mentioned from time to time in this process. Both of these trees have a root that presents the whole piece of code that was parsed. There are also smaller subtrees representing portions of code that become smaller until they are a single component.
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The difference between a parsing tree and an AST (though they are often used interchangeably) is in the abstraction. The parse tree will have all of the tokens that were within the program, including all possible details and rules. AST, on the other hand, is a narrowed-down version that does not detract from the end result but creates a simpler outcome.
Another component of this process to consider in JavaScript parsers is the grammar. Grammar is the rule – the formal description of a language that is to be used to recognize the structure. Grammar lists the rules that will then define how each construct will be composed. A rule could reference another token type or other rules within it. Some of the most common grammars used are Backus-Naur Form (BNF) and Extended Backus-Naur Form.
Another important component to understand is left-recursive rules, which are an important feature in JavaScript parsers. What this means is that the rule could start with a reference to itself. It can also be indirect. The bottom line is that it provides instruction on how to read the information. Some parser generators will support direct left-recursive rules but not indirect ones.
Breaking Down JavaScript Parsers
Let’s break down the full outline of what JavaScript parsers are and how they function, including the libraries that you need to use. First, remember what they are. JavaScript parsers will understand and then check the code before transforming it into something that is more usable. Typically, this means turning it into a structure AST that another program will then work with. Parsers can check for format code, errors, and update the code to a different version.
Outputs from the parser will include parse trees or ASTs, and sometimes tokenized streams. Knowing this, you can then choose the JavaScript parsing library you want to use to create your parser. Numerous libraries exist other than those listed here, including Acorn and Esprima, for example.
What’s the End Result? How JavaScript Parsers Help You
Ultimately, a JavaScript parser will become critical to all modern web development. It ensures that the necessary structural data is available to use in a way that makes sense for the program you are using. This allows you to then perform operations on complex data formats with far more ease.
Thinking about this, consider a few examples of how you can apply the use of JavaScript parsers for various tasks. Here are some real-world use cases that allow you to get information.
Code Linting and formatting: This is one of the most common reasons to use a parser like this. If you use a tool like Prettier, it will use a parser to navigate through your code and find any concerns. If there are concerns in terms of formatting or inconsistencies, it will then fix them. With Prettier, for example, it understands the code and then modifies the format as needed to match specific style rules. This then makes it possible for the code to look the same no matter where it came from.
Static Analysis: Another common real-world use for JavaScript parsers is static analysis. Parsers are essential for tools that look specifically at code without running it to find errors or security risks. They can also confuse areas of code style. These tools need to ensure that the code is in a format that is safe to analyze and then work with.
Using JavaScript Parsers
Because JavaScript parsers are so important in modern web development, they enable developers to interact with structured data, it is critical to learn about them, including what a parser generator in JavaScript can do as well as how to choose the best JavaScript parser library.
By using JavaScript parsers, various applications that depend on JavaScript can be read in an efficient way and then processed quickly. This enables the display of dynamic content in a smooth and efficient process as well.
Rayobyte can help you with the process. While using JavaScript parsing libraries for web scraping is a common path, we also recommend using our web scraping API, which can speed up the process. Contact Rayobyte to learn more.
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