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);

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Readme

Source

@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.

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Last updated on 02 Apr 2018

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