Acorn

A tiny, fast JavaScript parser written in JavaScript.

Community

Acorn is open source software released under an
MIT license.

You are welcome to
report bugs or create pull
requests on github. For questions
and discussion, please use the
Tern discussion forum.

Installation

The easiest way to install acorn is from npm:

npm install acorn

Alternately, you can download the source and build acorn yourself:

git clone https://github.com/acornjs/acorn.git
cd acorn
npm install

Interface

parse(input, options) is the main interface to the library. 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 ESTree
spec.

let acorn = require("acorn");
console.log(acorn.parse("1 + 1"));

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 string offset at which the
error occurred, and a loc object that contains a {line, column}
object referring to that same position.

Options can be provided by passing a second argument, which should be
an object containing any of these fields:

• ecmaVersion: Indicates the ECMAScript version to parse. Must be
  either 3, 5, 6 (2015), 7 (2016), 8 (2017), 9 (2018), 10 (2019) or 11
  (2020, partial support). This influences support for strict mode,
  the set of reserved words, and support for new syntax features.
  Default is 10.

NOTE: Only 'stage 4' (finalized) ECMAScript features are being
implemented by Acorn. Other proposed new features can be implemented
through plugins.

• sourceType: Indicate the mode the code should be parsed in. Can be
  either "script" or "module". This influences global strict mode
  and parsing of import and export declarations.

NOTE: If set to "module", then static import / export syntax
will be valid, even if ecmaVersion is less than 6.

• onInsertedSemicolon: If given a callback, that callback will be
  called whenever a missing semicolon is inserted by the parser. The
  callback will be given the character offset of the point where the
  semicolon is inserted as argument, and if locations is on, also a
  {line, column} object representing this position.

• onTrailingComma: Like onInsertedSemicolon, but for trailing
  commas.

• allowReserved: If false, using a reserved word will generate
  an error. Defaults to true for ecmaVersion 3, false for higher
  versions. When given the value "never", 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.

• allowImportExportEverywhere: By default, import and export
  declarations can only appear at a program's top level. Setting this
  option to true allows them anywhere where a statement is allowed.

• allowAwaitOutsideFunction: By default, await expressions can
  only appear inside async functions. Setting this option to
  true allows to have top-level await expressions. They are
  still not allowed in non-async functions, though.

• allowHashBang: When this is enabled (off by default), if the
  code starts with the characters #! (as in a shellscript), the
  first line will be treated as a comment.

• 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 tokens returned from
  tokenizer().getToken().

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:

javascript { "type": "Line" | "Block", "value": "comment text", "start": Number, "end": Number, // If `locations` option is on: "loc": { "start": {line: Number, column: Number} "end": {line: Number, column: Number} }, // If `ranges` option is on: "range": [Number, Number] }

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 (regardless of the location option) directly to the
  nodes, rather than the loc object.

• preserveParens: If this option is true, parenthesized expressions
  are represented by (non-standard) ParenthesizedExpression nodes
  that have a single expression property containing the expression
  inside parentheses.

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.

tokenizer(input, options) returns an object with a getToken
method that can be called repeatedly to get the next token, 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). When the token's type is tokTypes.eof, you
should stop calling the method, since it will keep returning that same
token forever.

In ES6 environment, returned result can be used as any other
protocol-compliant iterable:

for (let token of acorn.tokenizer(str)) {
  // iterate over the tokens
}

// transform code to array of tokens:
var tokens = [...acorn.tokenizer(str)];

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

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

The Parser class

Instances of the Parser class contain all the state and logic
that drives a parse. It has static methods parse,
parseExpressionAt, and tokenizer that match the top-level
functions by the same name.

When extending the parser with plugins, you need to call these methods
on the extended version of the class. To extend a parser with plugins,
you can use its static extend method.

var acorn = require("acorn");
var jsx = require("acorn-jsx");
var JSXParser = acorn.Parser.extend(jsx());
JSXParser.parse("foo(<bar/>)");

The extend method takes any number of plugin values, and returns a
new Parser class that includes the extra parser logic provided by
the plugins.

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|--ecma7|--ecma8|--ecma9|--ecma10: Sets the ECMAScript version
  to parse. Default is version 9.

• --module: Sets the parsing mode to "module". Is set to "script" otherwise.

• --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.

• --allow-hash-bang: If the code starts with the characters #! (as
  in a shellscript), the first line will be treated as a comment.

• --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.

Existing plugins

• acorn-jsx: Parse Facebook JSX syntax extensions

Plugins for ECMAScript proposals:

• acorn-stage3: Parse most stage 3 proposals, bundling:
• acorn-class-fields: Parse class fields proposal
• acorn-import-meta: Parse import.meta proposal
• acorn-private-methods: parse private methods, getters and setters proposaln