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- # Acorn
-
- A tiny, fast JavaScript parser written in JavaScript.
-
- ## Community
-
- Acorn is open source software released under an
- [MIT license](https://github.com/acornjs/acorn/blob/master/acorn/LICENSE).
-
- You are welcome to
- [report bugs](https://github.com/acornjs/acorn/issues) or create pull
- requests on [github](https://github.com/acornjs/acorn).
-
- ## Installation
-
- The easiest way to install acorn is from [`npm`](https://www.npmjs.com/):
-
- ```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](https://github.com/estree/estree).
-
- ```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](https://bugzilla.mozilla.org/show_bug.cgi?id=745678)
- `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.
-
- ### 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.
-
- ```javascript
- 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.
-
- ## 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`](https://github.com/RReverser/acorn-jsx): Parse [Facebook JSX syntax extensions](https://github.com/facebook/jsx)
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