@babel/generator

What is @babel/generator?

The @babel/generator package is a part of the Babel toolchain that is responsible for generating code from an Abstract Syntax Tree (AST). It takes an AST as input and turns it into code with source maps, if desired. This is particularly useful when you want to transform code using Babel and then generate the transformed code.

What are @babel/generator's main functionalities?

Code Generation from AST

This feature allows you to generate code from an AST. The code sample demonstrates parsing a piece of code into an AST and then generating the code back from the AST.

const { parse } = require('@babel/parser');
const generate = require('@babel/generator').default;
const code = 'let a = 1;';
const ast = parse(code);
const output = generate(ast, {}, code);
console.log(output.code);

Source Map Support

This feature enables the generation of source maps along with the generated code. The code sample shows how to generate a source map for the code that is being generated from an AST.

const { parse } = require('@babel/parser');
const generate = require('@babel/generator').default;
const code = 'let a = 1;';
const ast = parse(code);
const output = generate(ast, { sourceMaps: true }, code);
console.log(output.map);

Other packages similar to @babel/generator

escodegen

Escodegen is an ECMAScript code generator that takes an AST and converts it back into code. It is similar to @babel/generator but does not provide the same level of integration with the Babel ecosystem.

recast

Recast is a JavaScript AST tool that allows you to parse, visit, and transform the nodes of your AST, and then regenerate JavaScript code. It preserves the original formatting as much as possible and can be used in conjunction with Babel.

astring

Astring is a tiny and fast JavaScript code generator from an ESTree-compliant AST. It is smaller and may be faster than @babel/generator but lacks the extensive plugin system and integrations that Babel offers.

README

@babel/generator

Turns a Babylon AST into code.

Install

npm install --save-dev @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)
filename	string		Used in warning messages
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
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 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, { sourceFilename: 'a.js' });
const astB = parse(b, { sourceFilename: 'b.js' });
const ast = {
  type: 'Program',
  body: [].concat(astA.program.body, astB.program.body)
};

const { code, map } = generate(ast, { sourceMaps: true }, {
  'a.js': a,
  'b.js': b
});

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