babel-generator

What is babel-generator?

The babel-generator package is part of the Babel toolchain, which is used for converting ECMAScript 2015+ code into a backwards compatible version of JavaScript that can be run by older JavaScript engines. Babel-generator specifically is responsible for generating code from Babel's AST (Abstract Syntax Tree).

What are babel-generator's main functionalities?

Code Generation

This feature allows you to generate code from a Babel AST. In the example, the code is first transformed to an AST using Babel's transform function, and then the AST is passed to babel-generator to produce the final code output.

const generate = require('babel-generator').default;
const babel = require('@babel/core');

babel.transform('code', options, function(err, result) {
  const output = generate(result.ast);
  console.log(output.code);
});

Other packages similar to babel-generator

escodegen

Escodegen is another JavaScript code generator that takes an Abstract Syntax Tree (AST) and turns it into code. It is similar to babel-generator but does not require the use of Babel and its plugins to generate the AST. It can be used independently with other AST-producing parsers like Esprima.

recast

Recast provides both parsing and printing functionality, allowing you to read JavaScript code, manipulate it, and write it back out. While babel-generator focuses solely on code generation, Recast offers a more integrated approach to manipulating and regenerating code.

README

babel-generator

Turns an AST into code.

Install

$ npm install babel-generator

Usage

import {parse} from 'babylon';
import generate from 'babel-generator';

const code = 'class Example {}';
const ast = parse(code);

const output = generate(ast, { /* options */ }, code);

Options

Options for formatting output:

name	type	default	description
auxiliaryCommentBefore	string		Optional string to add as a block comment at the start of the output file
auxiliaryCommentAfter	string		Optional string to add as a block comment at the end of the output file
shouldPrintComment	function	opts.comments	Function that takes a comment (as a string) and returns true if the comment should be included in the output. By default, comments are included if opts.comments is true or if opts.minifed is false and the comment contains @preserve or @license
retainLines	boolean	false	Attempt to use the same line numbers in the output code as in the source code (helps preserve stack traces)
retainFunctionParens	boolean	false	Retain parens around function expressions (could be used to change engine parsing behavior)
comments	boolean	true	Should comments be included in output
compact	boolean or 'auto'	opts.minified	Set to true to avoid adding whitespace for formatting
minified	boolean	false	Should the output be minified
concise	boolean	false	Set to true to reduce whitespace (but not as much as opts.compact)
quotes	'single' or 'double'	autodetect based on ast.tokens	The type of quote to use in the output
filename	string		Used in warning messages
flowCommaSeparator	boolean	false	Set to true to use commas instead of semicolons as Flow property separators
jsonCompatibleStrings	boolean	false	Set to true to run jsesc with "json": true to print "\u00A9" vs. "©";
Options for source maps:

name	type	default	description
sourceMaps	boolean	false	Enable generating source maps
sourceMapTarget	string		The filename of the generated code that the source map will be associated with
sourceRoot	string		A root for all relative URLs in the source map
sourceFileName	string		The filename for the source code (i.e. the code in the code argument). This will only be used if code is a string.

AST from Multiple Sources

In most cases, Babel does a 1:1 transformation of input-file to output-file. However, you may be dealing with AST constructed from multiple sources - JS files, templates, etc. If this is the case, and you want the sourcemaps to reflect the correct sources, you'll need to make some changes to your code.

First, each node with a loc property (which indicates that node's original placement in the source document) must also include a loc.filename property, set to the source filename.

Second, you should pass an object to generate as the code parameter. Keys should be the source filenames, and values should be the source content.

Here's an example of what that might look like:

import {parse} from 'babylon';
import generate from 'babel-generator';

const a = 'var a = 1;';
const b = 'var b = 2;';
const astA = parse(a, { filename: 'a.js' });
const astB = parse(b, { filename: 'b.js' });
const ast = {
  type: 'Program',
  body: [].concat(astA.body, ast2.body)
};

const { code, map } = generate(ast, { /* options */ }, {
  'a.js': a,
  'b.js': b
});

// Sourcemap will point to both a.js and b.js where appropriate.