A tiny, fast JavaScript parser written in JavaScript.
Acorn is open source software released under an MIT license.
You are welcome to report bugs or create pull requests on github.
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
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.
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 ofimport
andexport
declarations.NOTE: If set to
"module"
, then staticimport
/export
syntax will be valid, even ifecmaVersion
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 totrue
forecmaVersion
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
andexport
declarations can only appear at a program's top level. Setting this option totrue
allows them anywhere where a statement is allowed, and also allowsimport.meta
expressions to appear in scripts (whensourceType
is not"module"
). -
allowAwaitOutsideFunction: If
false
,await
expressions can only appear insideasync
functions. Defaults totrue
in modules forecmaVersion
2022 and later,false
for lower versions. Setting this option totrue
allows to have top-levelawait
expressions. They are still not allowed in non-async
functions, though. -
allowSuperOutsideMethod: By default,
super
outside a method raises an error. Set this totrue
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 whenecmaVersion
>= 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 aloc
object attached withstart
andend
subobjects, each of which contains the one-based line and zero-based column numbers in{line, column}
form. Default isfalse
. -
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:
{ "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
andend
properties (directly on the node, rather than theloc
object, which holds line/column data. To also add a semi-standardizedrange
property holding a[start, end]
array with the same numbers, set theranges
option totrue
. -
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 istrue
, you can pass this option to add asource
attribute in every node’sloc
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 asourceFile
property will be added (regardless of thelocation
option) directly to the nodes, rather than theloc
object. -
preserveParens: If this option is
true
, parenthesized expressions are represented by (non-standard)ParenthesizedExpression
nodes that have a singleexpression
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:
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.
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/>)", {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.
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-levelawait
expressions. See theallowAwaitOutsideFunction
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.