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.

Installation

The easiest way to install acorn is from npm:

``sh
npm install acorn
`

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

`sh
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 must be 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.

`javascript
let acorn = require("acorn");
console.log(acorn.parse("1 + 1", {ecmaVersion: 2020}));
`

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 are provided by in a second argument, which should be an
object containing any of these fields (only ecmaVersion is
required):

- ecmaVersion: Indicates the ECMAScript version to parse. Can be a
number, either in year (2022) or plain version number (6) form,
or "latest" (the latest the library supports). This influences
support for strict mode, the set of reserved words, and support for
new syntax features.

NOTE: Only 'stage 4' (finalized) ECMAScript features are being
implemented by Acorn. Other proposed new features must 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,
and also allows import.meta expressions to appear in scripts
(when sourceType is not "module").

- allowAwaitOutsideFunction: If false, await expressions can
only appear inside async functions. Defaults to true in modules
for ecmaVersion 2022 and later, false for lower versions.
Setting this option to true allows to have top-level await
expressions. They are still not allowed in non-async functions,
though.

- allowSuperOutsideMethod: By default, super outside a method
raises an error. Set this to true to accept such code.

- allowHashBang: When this is enabled, if the code starts with the
characters #! (as in a shellscript), the first line will be
treated as a comment. Defaults to true when ecmaVersion >= 2023.

- checkPrivateFields: By default, the parser will verify that
private properties are only used in places where they are valid and
have been declared. Set this to false to turn such checks off.

- 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.

Note that tokenizing JavaScript without parsing it is, in modern
versions of the language, not really possible due to the way syntax is
overloaded in ways that can only be disambiguated by the parse
context. This package applies a bunch of heuristics to try and do a
reasonable job, but you are advised to use parse with the onToken
option instead of this.

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

`javascript
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.

$3

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.

`javascript
var acorn = require("acorn");
var jsx = require("acorn-jsx");
var JSXParser = acorn.Parser.extend(jsx());
JSXParser.parse("foo()", {ecmaVersion: 2020});
`

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.

- --allow-await-outside-function: Allows top-level await expressions.
See the allowAwaitOutsideFunction option for more information.

- --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