What is pegjs?

PEG.js is a tool that helps you build custom parsers for JavaScript. A parser is a piece of software that takes raw text and understands its structure, turning it into data your application can work with. Instead of writing all the complex code to read and validate text from scratch, you write a simple set of rules describing what the text should look like, and this tool generates the parser code for you. You start by writing a grammar file, which is just a description of the patterns you expect in your input. For example, if you wanted to evaluate math expressions like "2*(3+4)", you would write rules explaining what a number looks like, how multiplication and addition work, and how parentheses group things together. You can also embed small snippets of JavaScript directly in your rules to compute values as the text is read. Once your rules are ready, the tool outputs a JavaScript object with a simple parse method. You pass it a string, and it returns the result. This is useful for anyone building a tool that needs to understand a custom language or format. If you are creating your own simple programming language, a configuration file reader, or a search query interpreter, this tool handles the heavy lifting of reading and validating the input. It also provides excellent error reporting out of the box. When someone types something that does not match your rules, the generated parser tells them exactly where the error happened and what it expected to find. The tool runs on Node.js or in the browser, and there is even an online version where you can type your rules, test inputs, and download the resulting parser immediately. The underlying technology is based on parsing expression grammar, which the project notes is more powerful than many traditional parsing approaches.