Astring

A tiny and fast JavaScript code generator from an ESTree-compliant AST.

Key features:

• Generates JavaScript code up to version 7 and finished proposals.
• Works on ESTree-compliant ASTs such as the ones produced by Acorn.
• Extendable with custom AST node handlers.
• Considerably faster than Esotope (up to 4×), Escodegen (up to 10×), and UglifyJS (up to 125×).
• Supports comment generation with Astravel.
• No dependencies and small footprint (≈ 16 KB minified, ≈ 4 KB gziped).

Installation

⚠️ Astring relies on String.prototype.repeat(amount). If the environment running Astring does not define this method, add string.prototype.repeat or babel-polyfill.

Install with the Node Package Manager:

npm install astring

Alternatively, checkout this repository and install the development dependencies to build the module file:

git clone https://github.com/davidbonnet/astring.git
cd astring
npm install

A browser-ready minified version of Astring is available at dist/astring.min.js.

Import

With JavaScript 6 modules:

import generate, { defaultGenerator } from 'astring';

With CommonJS:

const { default: generate, defaultGenerator } = require('astring');

When used in a browser environment, the module exposes a global variable astring. The main function is accessible through the default property:

<script src="astring.min.js" type="text/javascript"></script>
<script type="text/javascript">
    var defaultGenerator = astring.defaultGenerator;
    var generate = astring.default;
</script>

Usage

A live demo showing Astring in action is available.

The default export of the astring module is a function that takes two arguments: node and options. It returns a string representing the rendered code of the provided AST node. However, if an output stream is provided in the options, it writes to that stream and returns it.

The options are:

• indent: string to use for indentation (defaults to "\t")
• lineEnd: string to use for line endings (defaults to "\n")
• startingIndentLevel: indent level to start from (defaults to 0)
• comments: generate comments if true (defaults to false)
• output: output stream to write the rendered code to (defaults to null)
• generator: custom code generator (defaults to astring.defaultGenerator)

Examples below are written in JavaScript 5 with Astring imported à la CommonJS.

Example

This example uses Acorn, a blazingly fast JavaScript AST producer and therefore the perfect companion of Astring.

// Make sure acorn and astring modules are imported
var generate = astring.default;
// Set example code
var code = "let answer = 4 + 7 * 5 + 3;\n";
// Parse it into an AST
var ast = acorn.parse(code, { ecmaVersion: 6 });
// Format it into a code string
var formattedCode = generate(ast, {
    indent: '   ',
    lineEnd: '\n'
});
// Check it
console.log((code === formattedCode) ? 'It works !' : 'Something went wrong…');

Using writable streams

This example for Node shows how to use writable streams to get the rendered code.

// Make sure acorn and astring modules are imported
var generate = astring.default;
// Set example code
var code = "let answer = 4 + 7 * 5 + 3;\n";
// Parse it into an AST
var ast = acorn.parse(code, { ecmaVersion: 6 });
// Format it and write the result to stdout
var stream = generate(ast, {
    output: process.stdout
});
// The returned value is the output stream
console.log('stream is process.stdout?', stream === process.stdout);

Generating comments

Astring supports comment generation, provided they are stored on the AST nodes. To do so, this example uses Astravel, a fast AST traveller and modifier.

// Make sure acorn, astravel and astring modules are imported
var generate = astring.default;
// Set example code
var code = [
    "// Compute the answer to everything",
    "let answer = 4 + 7 * 5 + 3;",
    "// Display it",
    "console.log(answer);"
].join('\n') + '\n';
// Parse it into an AST and retrieve the list of comments
var comments = [];
var ast = acorn.parse(code, {
    ecmaVersion: 6,
    locations: true,
    onComment: comments
});
// Attach comments to AST nodes
astravel.attachComments(ast, comments);
// Format it into a code string
var formattedCode = generate(ast, {
    indent: '   ',
    lineEnd: '\n',
    comments: true
});
// Check it
console.log(code === formattedCode ? 'It works !' : 'Something went wrong…');

Extending

Astring can easily be extended by updating or passing a custom code generator. A code generator consists of a mapping of node names and functions that take two arguments: node and state. The node points to the node from which to generate the code and the state holds various values and objects, the most important one being the output code stream.

This example shows how to support the await keyword which is part of the asynchronous functions proposal. The corresponding AwaitExpression node is based on this suggested definition.

// Make sure the astring module is imported and that `Object.assign` is defined
var generate = astring.default;
// Create a custom generator that inherits from Astring's default generator
var customGenerator = Object.assign({}, astring.defaultGenerator, {
    AwaitExpression: function(node, state) {
        state.output.write('await ');
        var argument = node.argument;
        if (argument != null) {
            this[argument.type](argument, state);
        }
    }
});
// Obtain a custom AST somehow (note that this AST is not obtained from a valid code)
var ast = {
    type: "Program",
    body: [{
        type: "ExpressionStatement",
        expression: {
            type: "AwaitExpression",
            argument: {
                type: "CallExpression",
                callee: {
                    type: "Identifier",
                    name: "callable"
                },
                arguments: []	
            }
        }
    }],
    sourceType: "module"
};
// Format it
var code = generate(ast, {
    generator: customGenerator
});
// Check it
console.log(code === 'await callable();\n' ? 'It works!' : 'Something went wrong…');

Command line interface

The bin/astring utility can be used to convert a JSON-formatted ESTree compliant AST of a JavaScript code. It accepts the following arguments:

• -i, --indent: string to use as indentation (defaults to "\t")
• -l, --line-end: string to use for line endings (defaults to "\n")
• -s, --starting-indent-level: indent level to start from (defaults to 0)
• -h, --help: print a usage message and exit
• -v, --version: print package version and exit

The utility reads the AST from stdin or from a provided list of files, and prints out the resulting code.

Example

As in the previous example, these examples use Acorn to get the JSON-formatted AST. This command pipes the AST output by Acorn from a script.js file to Astring and writes the formatted JavaScript code into a result.js file:

acorn --ecma6 script.js | astring --indent "  " > result.js

This command does the same, but reads the AST from an intermediary file:

acorn --ecma6 script.js > ast.json
astring --indent "  " ast.json > result.js

Building

All building scripts are defined in the package.json file and rely on the Node Package Manager. All commands must be run from within the root repository folder.

Production

The source code of Astring is written in JavaScript 6 and located at src/astring.js. It is compiled down to a JavaScript 5 file located at dist/astring.js, with its source map at dist/astring.js.map using Babel. This is achieved by running:

npm run build

If you are already using a JavaScript 6 to 5 compiler for your project, or a JavaScript 6 compliant interpreter, you can include the src/astring.js file directly.

Development

If you are working on Astring, you can use Watchify to build automatically at each update the bundle (along with a source map for easy debugging) located at dist/astring.debug.js by running:

npm start

Tests

While making changes to Astring, make sure it passes the tests:

npm test

You can also get an HTML report of the coverage:

npm run coverage

You can also run tests on a large array of files:

npm run test-scripts

Benchmark

Also, make sure that the modifications don't alter the performance by running benchmarks that compare Astring against Escodegen and Esotope:

npm run benchmark

Code format

Finally, make sure that the code is well formatted:

eslint src/astring.js

Roadmap

Planned features and releases are outlined on the milestones page.