Acorn

Build Status

A tiny, fast JavaScript parser, written completely in JavaScript.

Installation

The easiest way to install acorn is with npm.

npm install acorn

Alternately, download the source.

git clone https://github.com/marijnh/acorn.git

Components

When run in a CommonJS (node.js) or AMD environment, exported values
appear in the interfaces exposed by the individual files, as usual.
When loaded in the browser (Acorn works in any JS-enabled browser more
recent than IE5) without any kind of module management, a single
global object acorn will be defined, and all the exported properties
will be added to that.

acorn.js

This file contains the actual parser (and is what you get when you
require("acorn") in node.js).

parse(input, options) is used to parse a JavaScript program.
The input parameter is a string, options can be undefined or an
object setting some of the options listed below. The return value will
be an abstract syntax tree object as specified by the
Mozilla Parser API.

When encountering a syntax error, the parser will raise a
SyntaxError object with a meaningful message. The error object will
have a pos property that indicates the character offset at which the
error occurred, and a loc object that contains a {line, column}
object referring to that same position.

• ecmaVersion: Indicates the ECMAScript version to parse. Must be
  either 3, 5, or 6. This influences support for strict mode, the set
  of reserved words, and support for new syntax features. Default is 5.

• strictSemicolons: If true, prevents the parser from doing
  automatic semicolon insertion, and statements that do not end with
  a semicolon will generate an error. Defaults to false.

• allowTrailingCommas: If false, the parser will not allow
  trailing commas in array and object literals. Default is true.

• forbidReserved: If true, using a reserved word will generate
  an error. Defaults to false. When given the value "everywhere",
  reserved words and keywords can also not be used as property names
  (as in Internet Explorer's old parser).

• allowReturnOutsideFunction: By default, a return statement at
  the top level raises an error. Set this to true to accept such
  code.

• locations: When true, each node has a loc object attached
  with start and end subobjects, each of which contains the
  one-based line and zero-based column numbers in {line, column}
  form. Default is false.

• onToken: If a function is passed for this option, each found
  token will be passed in same format as tokenize() returns.

  If array is passed, each found token is pushed to it.

  Note that you are not allowed to call the parser from the
  callback—that will corrupt its internal state.

• onComment: If a function is passed for this option, whenever a
  comment is encountered the function will be called with the
  following parameters:

  • block: true if the comment is a block comment, false if it
    is a line comment.
  • text: The content of the comment.
  • start: Character offset of the start of the comment.
  • end: Character offset of the end of the comment.

  When the locations options is on, the {line, column} locations
  of the comment’s start and end are passed as two additional
  parameters.

  If array is passed for this option, each found comment is pushed
  to it as object in Esprima format:

  {
    "type": "Line" | "Block",
    "value": "comment text",
    "range": ...,
    "loc": ...
  }
  

  Note that you are not allowed to call the parser from the
  callback—that will corrupt its internal state.

• ranges: Nodes have their start and end characters offsets
  recorded in start and end properties (directly on the node,
  rather than the loc object, which holds line/column data. To also
  add a semi-standardized "range" property holding a
  [start, end] array with the same numbers, set the ranges option
  to true.

• program: It is possible to parse multiple files into a single
  AST by passing the tree produced by parsing the first file as the
  program option in subsequent parses. This will add the toplevel
  forms of the parsed file to the "Program" (top) node of an existing
  parse tree.

• sourceFile: When the locations option is true, you can pass
  this option to add a source attribute in every node’s loc
  object. Note that the contents of this option are not examined or
  processed in any way; you are free to use whatever format you
  choose.

• directSourceFile: Like sourceFile, but a sourceFile property
  will be added directly to the nodes, rather than the loc object.

parseExpressionAt(input, offset, options) will parse a single
expression in a string, and return its AST. It will not complain if
there is more of the string left after the expression.

getLineInfo(input, offset) can be used to get a {line, column} object for a given program string and character offset.

tokenize(input, options) exports a primitive interface to
Acorn's tokenizer. The function takes an input string and options
similar to parse (though only some options are meaningful here), and
returns a function that can be called repeatedly to read a single
token, and returns a {start, end, type, value} object (with added
loc property when the locations option is enabled and range
property when the ranges option is enabled).

tokTypes holds an object mapping names to the token type objects
that end up in the type properties of tokens.

Note on using with Escodegen

Escodegen supports generating comments from AST, attached in
Esprima-specific format. In order to simulate same format in
Acorn, consider following example:

var comments = [], tokens = [];

var ast = acorn.parse('var x = 42; // answer', {
    // collect ranges for each node
    ranges: true,
    // collect comments in Esprima's format
    onComment: comments,
    // collect token ranges
    onToken: tokens
});

// attach comments using collected information
escodegen.attachComments(ast, comments, tokens);

// generate code
console.log(escodegen.generate(ast, {comment: true}));
// > 'var x = 42;    // answer'

Using Acorn in an environment with a Content Security Policy

Some contexts, such as Chrome Web Apps, disallow run-time code evaluation.
Acorn uses new Function to generate fast functions that test whether
a word is in a given set, and will trigger a security error when used
in a context with such a
Content Security Policy
(see #90 and
#123).

The bin/without_eval script can be used to generate a version of
acorn.js that has the generated code inlined, and can thus run
without evaluating anything. In versions of this library downloaded
from NPM, this script will be available as acorn_csp.js.

acorn_loose.js

This file implements an error-tolerant parser. It exposes a single
function.

parse_dammit(input, options) takes the same arguments and
returns the same syntax tree as the parse function in acorn.js,
but never raises an error, and will do its best to parse syntactically
invalid code in as meaningful a way as it can. It'll insert identifier
nodes with name "✖" as placeholders in places where it can't make
sense of the input. Depends on acorn.js, because it uses the same
tokenizer.

util/walk.js

Implements an abstract syntax tree walker. Will store its interface in
acorn.walk when used without a module system.

simple(node, visitors, base, state) does a 'simple' walk over
a tree. node should be the AST node to walk, and visitors an
object with properties whose names correspond to node types in the
Mozilla Parser API. The properties should contain functions
that will be called with the node object and, if applicable the state
at that point. The last two arguments are optional. base is a walker
algorithm, and state is a start state. The default walker will
simply visit all statements and expressions and not produce a
meaningful state. (An example of a use of state it to track scope at
each point in the tree.)

ancestor(node, visitors, base, state) does a 'simple' walk over
a tree, building up an array of ancestor nodes (including the current node)
and passing the array to callbacks in the state parameter.

recursive(node, state, functions, base) does a 'recursive'
walk, where the walker functions are responsible for continuing the
walk on the child nodes of their target node. state is the start
state, and functions should contain an object that maps node types
to walker functions. Such functions are called with (node, state, c)
arguments, and can cause the walk to continue on a sub-node by calling
the c argument on it with (node, state) arguments. The optional
base argument provides the fallback walker functions for node types
that aren't handled in the functions object. If not given, the
default walkers will be used.

make(functions, base) builds a new walker object by using the
walker functions in functions and filling in the missing ones by
taking defaults from base.

findNodeAt(node, start, end, test, base, state) tries to
locate a node in a tree at the given start and/or end offsets, which
satisfies the predicate test. start end end can be either null
(as wildcard) or a number. test may be a string (indicating a node
type) or a function that takes (nodeType, node) arguments and
returns a boolean indicating whether this node is interesting. base
and state are optional, and can be used to specify a custom walker.
Nodes are tested from inner to outer, so if two nodes match the
boundaries, the inner one will be preferred.

findNodeAround(node, pos, test, base, state) is a lot like
findNodeAt, but will match any node that exists 'around' (spanning)
the given position.

findNodeAfter(node, pos, test, base, state) is similar to
findNodeAround, but will match all nodes after the given position
(testing outer nodes before inner nodes).

Command line interface

The bin/acorn utility can be used to parse a file from the command
line. It accepts as arguments its input file and the following
options:

• --ecma3|--ecma5|--ecma6: Sets the ECMAScript version to parse. Default is
  version 5.

• --strictSemicolons: Prevents the parser from doing automatic
  semicolon insertion. Statements that do not end in semicolons will
  generate an error.

• --locations: Attaches a "loc" object to each node with "start" and
  "end" subobjects, each of which contains the one-based line and
  zero-based column numbers in {line, column} form.

• --compact: No whitespace is used in the AST output.

• --silent: Do not output the AST, just return the exit status.

• --help: Print the usage information and quit.

The utility spits out the syntax tree as JSON data.