astring

You can extend Astring by providing a custom generator in the options object. A generator is an object mapping node names to functions. Each function receives two arguments:

1. node: The AST node to generate code from.
2. state: An object exposing a write(string) method to append generated code.

To inherit existing behavior, use Object.assign({}, astring.GENERATOR, { ... }) to create your custom generator.

Example: Adding support for AwaitExpression

var customGenerator = Object.assign({}, astring.GENERATOR, {
  AwaitExpression: function (node, state) {
    state.write('await ')
    var argument = node.argument
    if (argument != null) {
      this[argument.type](argument, state)
    }
  },
})

var ast = {
  type: 'AwaitExpression',
  argument: {
    type: 'CallExpression',
    callee: {
      type: 'Identifier',
      name: 'callable',
    },
    arguments: [],
  },
}

var code = astring.generate(ast, {
  generator: customGenerator,
})
console.log(code === 'await callable();\n' ? 'It works!' : 'Something went wrong!')

// Make sure the astring module is imported and that `Object.assign` is defined

// Create a custom generator that inherits from Astring's base generator
var customGenerator = Object.assign({}, astring.GENERATOR, {
  AwaitExpression: function (node, state) {
    state.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: 'AwaitExpression',
  argument: {
    type: 'CallExpression',
    callee: {
      type: 'Identifier',
      name: 'callable',
    },
    arguments: [],
  },
}
// Format it
var code = astring.generate(ast, {
  generator: customGenerator,
})
// Check it
console.log(
  code === 'await callable();\n' ? 'It works!' : 'Something went wrong!',
)

To generate source maps, provide a sourceMap generator instance in the options object. Note that the AST nodes must include location information (e.g., via locations: true in your parser) for the mapping to work.

Example

var code = 'function add(a, b) { return a + b; }\n'
var ast = acorn.parse(code, {
  ecmaVersion: 6,
  sourceType: 'module',
  locations: true,
})

var map = new sourceMap.SourceMapGenerator({
  file: 'script.js',
})

var formattedCode = generate(ast, {
  sourceMap: map,
})

console.log(map.toString())

var code = 'function add(a, b) { return a + b; }\n'
var ast = acorn.parse(code, {
  ecmaVersion: 6,
  sourceType: 'module',
  // Locations are needed in order for the source map generator to work
  locations: true,
})
// Create empty source map generator
var map = new sourceMap.SourceMapGenerator({
  // Source file name must be set and will be used for mappings
  file: 'script.js',
})
var formattedCode = generate(ast, {
  // Enable source maps
  sourceMap: map,
})
// Display generated source map
console.log(map.toString())

If you provide an output stream in the options object, generate() will write the rendered code directly to that stream and return the stream instance.

Example (Node.js)

var code = 'let answer = 4 + 7 * 5 + 3;\n'
var ast = acorn.parse(code, { ecmaVersion: 6 })

// Format it and write the result to stdout
var stream = astring.generate(ast, {
  output: process.stdout,
})

// The returned value is the output stream
console.log('Does stream equal process.stdout?', stream === process.stdout)

// Make sure acorn and astring modules are imported

// 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 = astring.generate(ast, {
  output: process.stdout,
})
// The returned value is the output stream
console.log('Does stream equal process.stdout?', stream === process.stdout)

Astring supports comment generation if comments are attached to the AST nodes. You can use a tool like Astravel to attach comments parsed from the source code to their corresponding nodes.

Example

var code =
  [
    '// Compute the answer to everything',
    'let answer = 4 + 7 * 5 + 3;',
    '// Display it',
    'console.log(answer);',
  ].join('\n') + '\n'

var comments = []
var ast = acorn.parse(code, {
  ecmaVersion: 6,
  locations: true,
  onComment: comments,
})

// Attach comments to AST nodes
astravel.attachComments(ast, comments)

// Format it with comments enabled
var formattedCode = astring.generate(ast, {
  comments: true,
})

console.log(code === formattedCode ? 'It works!' : 'Something went wrong…')

// Make sure acorn, astravel and astring modules are imported

// 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 = astring.generate(ast, {
  comments: true,
})
// Check it
console.log(code === formattedCode ? 'It works!' : 'Something went wrong…')

The generate(node, options) function returns a string representing the rendered code of the provided ESTree-compliant AST node. If an output stream is provided in the options, it writes to that stream and returns it.

Options

• indent: string to use for indentation (defaults to " ")
• 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 GENERATOR)
• sourceMap: source map generator (defaults to null)
• expressionsPrecedence: custom map of node types and their precedence level (defaults to EXPRESSIONS_PRECEDENCE)

Example: Basic Generation

// Using acorn to parse code into an AST
var code = 'let answer = 4 + 7 * 5 + 3;\n'
var ast = acorn.parse(code, { ecmaVersion: 6 })

// Format it into a code string
var formattedCode = astring.generate(ast)
console.log(formattedCode)

var code = 'let answer = 4 + 7 * 5 + 3;\n'
var ast = acorn.parse(code, { ecmaVersion: 6 })
var formattedCode = astring.generate(ast)
console.log(code === formattedCode ? 'It works!' : 'Something went wrong…')