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JSTransform Build Status

A simple utility for pluggable JS syntax transforms using the esprima parser.

  • Makes it simple to write and plug-in syntax transformations
  • Makes it simple to coalesce multiple syntax transformations in a single pass of the AST
  • Gives complete control over the formatting of the output on a per-transformation basis
  • Supports source map generation
  • Comes pre-bundled with a small set of (optional) ES6 -> ES5 transforms

Note: If you're looking for a library for writing new greenfield JS transformations, consider looking at Babel or Recast instead of jstransform. We are still supporting jstransform (and intend to for a little while), but longer term we would like to direct efforts toward other open source projects that do a far better job of supporting a multi-pass JS transformation pipeline. This is important when attempting to apply many transformations to a source file. jstransform does a single pass resulting in performance benefits, but the tradeoff is that many transformations are much harder to write.


Advanced API

var jstranform = require('jstransform')

This is the API that jstransform has always supported. It gives very fine control over the transforms you use and what order they are run in. It also allows the use of custom transforms.

jstransform.transform(visitors, code, options={})

visitors Array of visitors. See the React JSX visitors as an example of what a visitor looks like.

code String of code to be transformed.

options Object with options that will be passed through to esprima and transforms.


This is the Syntax object re-exported from esprima. This is available because visitors will need access to this in order to effectively write transforms. By re-exporting we avoid the problem of conflicting versions of esprima being used.

Simple API

var simple = require('jstransform/simple')

The simple API was added to mirror the new command line interface. It works similarly to how react-tools worked - there is no need to know exactly which transforms to run. Instead transforms are selected automatically based on the options.

simple.transform(code, options={})

code String of code to be transformed.

options Object with options. Available options are:

option values default
react true: enable React transforms (JSX, displayName) false
es6 true: enable ES6 transforms false
es7 true: enable ES7 transforms false
harmony true: shortcut to enable ES6 & ES7 transforms false
utility true: enable utility transforms (trailing commas in objects, arrays) false
target es3: generate ES3 compatible code
es5: generate ES5 compatible code
stripTypes true: strips out Flow type annotations false
sourceMap true: generate and return a Source Map false
sourceMapInline true: append inline source map at the end of the transformed source false
sourceFilename the output filename for the source map "source.js"
es6module true: parses the file as an ES6 module false
nonStrictEs6module true: parses the file as an ES6 module, except disables implicit strict-mode (i.e. CommonJS modules et al are allowed) false

Returns: An Object with the following:

code: the transformed code

sourceMap: the source map object or null

simple.transformFile(file, options={}, callback)

file String of path to a file to transform. Will be passed directly to fs.readFile.

options See transform API.

callback Function to call with the result, where result is return value of transform.

callback(err, result)

simple.transformFileSync(file, options={})

The same as transformFile but the file is read synchronously.


JSTransform now ships with a CLI. It was taken from the react-tools CLI so should be very familiar.

% jstransform --help

  Usage: jstransform [options] <source directory> <output directory> [<module ID> [<module ID> ...]]


    -h, --help                               output usage information
    -V, --version                            output the version number
    -c, --config [file]                      JSON configuration file (no file or - means STDIN)
    -w, --watch                              Continually rebuild
    -x, --extension <js | coffee | ...>      File extension to assume when resolving module identifiers
    --relativize                             Rewrite all module identifiers to be relative
    --follow-requires                        Scan modules for required dependencies
    --use-provides-module                    Respect @providesModules pragma in files
    --cache-dir <directory>                  Alternate directory to use for disk cache
    --no-cache-dir                           Disable the disk cache
    --source-charset <utf8 | win1252 | ...>  Charset of source (default: utf8)
    --output-charset <utf8 | win1252 | ...>  Charset of output (default: utf8)
    --react                                  Turns on the React JSX and React displayName transforms
    --es6                                    Turns on available ES6 transforms
    --es7                                    Turns on available ES7 transforms
    --harmony                                Shorthand to enable all ES6 and ES7 transforms
    --utility                                Turns on available utility transforms
    --target [version]                       Specify your target version of ECMAScript. Valid values are "es3" and "es5". The default is "es5". "es3" will avoid uses of defineProperty and will quote reserved words. WARNING: "es5" is not properly supported, even with the use of es5shim, es5sham. If you need to support IE8, use "es3".
    --strip-types                            Strips out type annotations.
    --es6module                              Parses the file as a valid ES6 module. (Note that this means implicit strict mode)
    --non-strict-es6module                   Parses the file as an ES6 module, except disables implicit strict-mode. (This is useful if you're porting non-ES6 modules to ES6, but haven't yet verified that they are strict-mode safe yet)
    --source-map-inline                      Embed inline sourcemap in transformed source
    --source-filename                        Filename to use when generating the inline sourcemap. Will default to filename when processing files


Advanced API

Using a pre-bundled or existing transform:

 * Reads a source file that may (or may not) contain ES6 classes, transforms it
 * to ES5 compatible code using the pre-bundled ES6 class visitors, and prints 
 * out the result.
var es6ClassVisitors = require('jstransform/visitors/es6-class-visitors').visitorList;
var fs = require('fs');
var jstransform = require('jstransform');

var originalFileContents = fs.readFileSync('path/to/original/file.js', 'utf-8');

var transformedFileData = jstransform.transform(


Using multiple pre-bundled or existing transforms at once:

 * Reads a source file that may (or may not) contain ES6 classes *or* arrow
 * functions, transforms them to ES5 compatible code using the pre-bundled ES6 
 * visitors, and prints out the result.
var es6ArrowFuncVisitors = require('jstransform/visitors/es6-arrow-function-visitors').visitorList;
var es6ClassVisitors = require('jstransform/visitors/es6-class-visitors').visitorList;
var jstransform = require('jstransform');

// Normally you'd read this from the filesystem, but I'll just use a string here
// to simplify the example.
var originalFileContents = "var a = (param1) => param1; class FooClass {}";

var transformedFileData = jstransform.transform(

// var a = function(param1)  {return param1;}; function FooClass(){"use strict";}

Writing a simple custom transform:

 * Creates a custom transformation visitor that prefixes all calls to the
 * `eval()` function with a call to `alert()` saying how much of a clown you are
 * for using eval.
var jstransform = require('jstransform');
var utils = require('jstransform/src/utils');

var Syntax = jstransform.Syntax;

function visitEvalCallExpressions(traverse, node, path, state) {
  // Appends an alert() call to the output buffer *before* the visited node
  // (in this case the eval call) is appended to the output buffer
  utils.append('alert("...eval?...really?...");', state);

  // Now we copy the eval expression to the output buffer from the original
  // source
  utils.catchup(node.range[1], state);
visitEvalCallExpressions.test = function(node, path, state) {
  return node.type === Syntax.CallExpression
         && node.callee.type === Syntax.Identifier
         && === 'eval';

// Normally you'd read this from the filesystem, but I'll just use a string here
// to simplify the example.
var originalFileContents = "eval('foo');";

var transformedFileData = jstransform.transform(
  [visitEvalCallExpressions], // Multiple visitors may be applied at once, so an
                              // array is always expected for the first argument

// alert("...eval?...really?...");eval('foo');

Simple API

Reading a file and applying tranforms

var simple = require('jstransform/simple');
var fs = require('fs');

var originalCode = fs.readFileSync('path/to/file.js');

// Apply all available ES6 transforms
var transformed = simple.transform(originalCode, {es6: true});

// Apply ES6 and ES7, generating ES3 compatible code (for IE8)
transformed = simple.transform(originalCode, {harmony: true, target: 'es3'});

Migration Guide

Simple API

If you are coming from react-tools and using Node, the APIs are very similar. There are a couple important differences.

  1. JSX will not be tranformed by default! You must specify react: true in the options.
  2. The return value of transform is not the same. react-tools.transform only returned the resulting code. simple.transform always returns and object with a code property. However, if you were using react-tools.transformWithDetails, simple.transform is essentially the same.


These are virtually identical but again, there are some important difference.

  1. JSX will not be transformed by default! You must specify --react.
  2. We are using a different executable - jstransform - (jsx doesn't make sense here).
  3. There are a numbr of new options available. Namely --utility, --es6 and --es7 (--harmony is still available and will enable both).


A simple utility for pluggable JS syntax transforms using the esprima parser.







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