Acorn

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 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. Theinputparameter 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 or 5. This influences support for strict mode, the set of reserved words, and support for getters and setter. 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.

• 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.

• 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.

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

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 toparse(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 addedstartLocandendLocproperties when thelocations` option is enabled). This object will be reused (updated) for each token, so you can't count on it staying stable.

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

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 theparsefunction inacorn.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 onacorn.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.nodeshould be the AST node to walk, andvisitorsan object with properties whose names correspond to node types in the [Mozilla Parser API][mozapi]. 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.baseis a walker algorithm, andstate` 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.)

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.stateis the start state, andfunctionsshould 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 thecargument on it with(node, state)arguments. The optionalbaseargument provides the fallback walker functions for node types that aren't handled in thefunctions` object. If not given, the default walkers will be used.

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

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 predicatetest. startendendcan be eithernull(as wildcard) or a number.testmay 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.baseandstate` 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 likefindNodeAt`, but will match any node that exists 'around' (spanning) the given position.

findNodeAfter(node, pos, test, base, state)is similar tofindNodeAround`, 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: 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.