Getting Started with JSCodeshift

03/16/2023

Contents

jscodeshift is a powerful tool that allows you to perform automated code transformations on JavaScript code. It uses the AST (Abstract Syntax Tree) of your code to make changes to the code, allowing you to make complex changes to your codebase with ease. In this article, we’ll explore the basics of jscodeshift and how you can use it to transform your code.

Installation

To use jscodeshift, you’ll need to have Node.js installed on your machine. Once you have Node.js installed, you can install jscodeshift using the npm package manager:

npm install -g jscodeshift

Once jscodeshift is installed, you can use it from the command line by typing jscodeshift.

Basic usage

jscodeshift is a command-line tool that takes one or more JavaScript files as input and applies a set of transformations to them. To use jscodeshift, you’ll need to provide it with a transformation file that defines the changes you want to make to your code.

Here’s a basic example of how to use jscodeshift to transform a single file:

jscodeshift -t myTransformation.js myFile.js

In this example, myTransformation.js is the transformation file that defines the changes you want to make to myFile.js.

Transformation file

The transformation file is a JavaScript file that exports a function that takes two arguments: the AST of the file being transformed and some options. The function should return the updated AST.

Here’s an example transformation file:

module.exports = function(fileInfo, api, options) {
  const j = api.jscodeshift;

  return j(fileInfo.source)
    .find(j.Identifier)
    .replaceWith(function(node) {
      return j.identifier(node.name.toUpperCase());
    })
    .toSource();
};

This transformation file takes the AST of a JavaScript file and replaces all identifier names with their uppercase equivalent.

API

jscodeshift provides a powerful API for manipulating the AST of your code. The API is based on the popular AST explorer tool, which allows you to explore the AST of your code and see how it changes as you make edits.

The jscodeshift API is based on the concept of a “collection” of nodes. A collection is a set of nodes that you can perform operations on. Here are some of the most commonly used methods in the API:

  • j: This is the main entry point to the API. You use it to create a collection of nodes from a source string or an AST.
  • find(selector): This method allows you to select a subset of nodes in a collection based on a selector. The selector can be any valid CSS selector, such as Identifier[name=”foo”] to select all identifier nodes with the name “foo”.
  • replaceWith(replacement): This method allows you to replace nodes in a collection with a new node or set of nodes. The replacement argument can be a function that takes a node and returns a new node, or a collection of nodes.
  • toSource(): This method returns the updated source code based on the changes made to the collection.

Options

jscodeshift allows you to pass options to your transformation function using the -p flag. Options can be used to modify the behavior of your transformation function, such as specifying a configuration file or setting a variable.

Here’s an example of how to pass options to jscodeshift:

jscodeshift -t myTransformation.js myFile.js -p '{ "myOption": true }'

In this example, the myOption option is set to true and can be accessed within the transformation function like this:

module.exports = function(fileInfo, api, options) {
  const j = api.jscodeshift;
  const myOption = options.myOption || false;

  // use myOption in transformation logic

  return j(fileInfo.source)
    .toSource();
};

Wrapping up

In this article, we’ve explored the basics of using jscodeshift to transform JavaScript code. We’ve covered the installation process, the basic usage of jscodeshift, the structure of a transformation file, the jscodeshift API, and how to pass options to your transformation function.

jscodeshift is a powerful tool that can help you make complex changes to your codebase with ease. With a little bit of practice, you’ll be able to write custom transformation files that can automate tedious tasks, refactor code, and make your codebase more maintainable.

#JavaScript