jscodeshift

What is jscodeshift?

jscodeshift is a toolkit for running codemods over multiple JavaScript or TypeScript files. It provides a runner as well as a wrapper around the recast AST toolkit, making it easier to write codemods that can transform code and assist in large-scale codebase refactors or migrations.

What are jscodeshift's main functionalities?

Transforming Syntax

This feature allows you to transform the syntax of JavaScript code. For example, the code sample provided changes all variable declarations to use 'const' instead of 'var' or 'let'.

const jscodeshift = require('jscodeshift');
const transform = (fileInfo, api) => {
  const j = api.jscodeshift;
  return j(fileInfo.source)
    .find(j.VariableDeclaration)
    .forEach(path => {
      j(path).replaceWith(
        j.variableDeclaration('const', path.node.declarations)
      );
    })
    .toSource();
};

Code Analysis

jscodeshift can be used to analyze code and extract information. In this example, the code collects all the identifiers from a JavaScript file.

const jscodeshift = require('jscodeshift');
const analyze = (fileInfo, api) => {
  const j = api.jscodeshift;
  const root = j(fileInfo.source);
  const identifiers = [];
  root.find(j.Identifier).forEach(path => {
    identifiers.push(path.node.name);
  });
  return identifiers;
};

Refactoring

The package can be used to perform refactoring tasks, such as renaming functions across a codebase. The provided code sample renames all function declarations to 'newFunctionName'.

const jscodeshift = require('jscodeshift');
const refactor = (fileInfo, api) => {
  const j = api.jscodeshift;
  return j(fileInfo.source)
    .find(j.FunctionDeclaration)
    .renameTo('newFunctionName')
    .toSource();
};

Other packages similar to jscodeshift

babel-codemod

babel-codemod rewrites JavaScript and TypeScript using Babel plugins. It is similar to jscodeshift but leverages the Babel ecosystem, which might be more familiar to some developers. It also supports the latest JavaScript features and TypeScript out of the box.

lebab

lebab is a tool that transforms legacy JavaScript code into modern, readable, and maintainable code using ES6+ syntax. It is similar to jscodeshift in its goals but is more opinionated and focused on upgrading to modern JavaScript syntax.

README

jscodeshift

jscodeshift is a toolkit for running codemods over multiple JS files. It provides:

• A runner, which executes the provided transform for each file passed to it. It also outputs a summary of how many files have (not) been transformed.
• A wrapper around recast, providing a different API. Recast is an AST-to-AST transform tool and also tries to preserve the style of original code as much as possible.

Install

Get jscodeshift from npm:

$ npm install -g jscodeshift

This will install the runner as jscodeshift.

Usage (CLI)

The CLI provides the following options:

$ jscodeshift --help

Usage: jscodeshift <path>... [options]

path     Files or directory to transform

Options:
   -t FILE, --transform FILE   Path to the transform file  [./transform.js]
   -c, --cpus                  (all by default) Determines the number of processes started.
   -v, --verbose               Show more information about the transform process  [0]
   -d, --dry                   Dry run (no changes are made to files)
   -p, --print                 Print output, useful for development
   --babel                     Apply Babel to transform files  [true]
   --extensions                File extensions the transform file should be applied to  [js]

This passes the source of all passed through the transform module specified with -t or --transform (defaults to transform.js in the current directory). The next section explains the structure of the transform module.

Transform module

The transform is simply a module that exports a function of the form:

module.exports = function(fileInfo, api, options) {
  // transform `fileInfo.source` here
  // ...
  // return changed source
  return source;
};

Arguments

fileInfo

Holds information about the currently processed file.

Property	Description
path	File path
source	File content

api

This object exposes the jscodeshift library and helper functions from the runner.

Property	Description
jscodeshift	A reference to the jscodeshift library
stats	A function to collect statistics during --dry runs

jscodeshift is a reference to the wrapper around recast and provides a jQuery-like API to navigate and transform the AST. Here is a quick example, a more detailed description can be found below.

/**
 * This replaces every occurence of variable "foo".
 */
module.exports = function(fileInfo, api) {
  return api.jscodeshift(fileInfo.source)
    .findVariableDeclarators('foo')
    .renameTo('bar')
    .toSource();
}

Note: This api is exposed for convenience, but you don't have to use it. You can use any tool to modify the source.

stats is a function that only works when the --dry options is set. It accepts a string, and will simply count how often it was called with that value.

At the end, the CLI will report those values. This can be useful while developing the transform, e.g. to find out how often a certain construct appears in the source(s).

options

Contains all options that have been passed to runner. This allows you to pass additional options to the transform. For example, if the CLI is called with

$ jscodeshift -t myTransforms fileA fileB --foo=bar

options would contain {foo: 'bar'}. jscodeshift uses nomnom to parse command line options.

Return value

The return value of the function determines the status of the transformation:

• If a string is returned and it is different from passed source, the transform is considered to be successful.
• If a string is returned but it's the same as the source, the transform is considered to be unsuccessful.
• If nothing is returned, the file is not supposed to be transformed (which is ok).

The CLI provides a summary of the transformation at the end. You can get more detailed information by setting the -v option to 1 or 2.

You can collect even more stats via the stats function as explained above.

Example

$ jscodeshift -t myTransform.js src
Processing 10 files...
Spawning 2 workers with 5 files each...
All workers done.
Results: 0 errors 2 unmodifed 3 skipped 5 ok

The jscodeshift API

As already mentioned, jscodeshift also provides a wrapper around recast. In order to properly use the jscodeshift API, one has to understand the basic building blocks of recast (and ASTs) as well.

Core Concepts

AST nodes

An AST node is a plain JavaScript object with a specific set of fields, in accordance with the Mozilla Parser API. The primary way to identify nodes is via their type.

For example, string literals are represented via Literal nodes, which have the structure

// "foo"
{
  type: 'Literal',
  value: 'foo',
  raw: '"foo"'
}

It's OK to not know the structure of every AST node type. The (esprima) AST explorer is an online tool to inspect the AST for a given piece of JS code.

Path objects

Recast itself relies heavily on ast-types which defines methods to traverse the AST, access node fields and build new nodes. ast-types wraps every AST node into a path object. Paths contain meta-information and helper methods to process AST nodes.

For example, the child-parent relationship between two nodes is not explicitly defined. Given a plain AST node, it is not possible to traverse the tree up. Given a path object however, the parent can be traversed to via path.parent.

For more information about the path object API, please have a look at ast-types.

Builders

To make creating AST nodes a bit simpler and "safer", ast-types defines a couple of builder methods, which are also exposed on jscodeshift.

For example, the following creates an AST equivalent to foo(bar):

// inside a module transform
var j = jscodeshift;
// foo(bar);
var ast = j.callExpression(
  j.identifier('foo'),
  [j.identifier('bar')]
);

The signature of each builder function is best learned by having a look at the definition files.

Collections and Traversal

In order to transform the AST, you have to traverse it and find the nodes that need to be changed. jscodeshift is built around the idea of collections of paths and thus provides a different way of processing an AST than recast or ast-types.

A collection has methods to process the nodes inside a collection, often resulting in a new collection. This results in a fluent interface, which can make the transform more readable.

Collections are "typed" which means that the type of a collection is the "lowest" type all AST nodes in the collection have in common. That means you cannot call a method for a FunctionExpression collection on a Identifier collection.

Here is an example of how one would find/traverse all Identifier nodes with jscodeshift and with recast:

// recast
var ast = recast.parse(src);
recast.visit(ast, {
  visitIdentifier: function(path) {
    // do something with path
    return false;
  }
});

// jscodeshift
jscodeshift(src)
  .find(jscodeshift.Identifier)
  .forEach(function(path) {
    // do something with path
  });

To learn about the provided methods, have a look at the Collection.js and its extensions.

Extensibility

jscodeshift provides an API to extend collections. By moving common operators into helper functions (which can be stored separately in other modules), a transform can be made more readable.

There are two types of extensions: generic extensions and type-specific extensions. Generic extensions are applicable to all collections. As such, they typically don't access specific node data, but rather traverse the AST from the nodes in the collection. Type-specific extensions work only on specific node types and are not callable on differently typed collections.

Examples

// Adding a method to all Identifiers
jscodeshift.registerMethods({
	logNames: function() {
		return this.forEach(function(path) {
			console.log(path.node.name);
		});
	}
}, jscodeshift.Identifier);

// Adding a method to all collections
jscodeshift.registerMethods({
	findIdentifiers: function() {
		return this.find(jscodeshift.Identifier);
	}
});

jscodeshift(ast).findIdentifiers().logNames();
jscodeshift(ast).logNames(); // error, unless `ast` only consists of Identifier nodes