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270 lines
10 KiB
Markdown
270 lines
10 KiB
Markdown
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# Acorn
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A tiny, fast JavaScript parser written in JavaScript.
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## Community
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Acorn is open source software released under an
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[MIT license](https://github.com/acornjs/acorn/blob/master/acorn/LICENSE).
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You are welcome to
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[report bugs](https://github.com/acornjs/acorn/issues) or create pull
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requests on [github](https://github.com/acornjs/acorn). For questions
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and discussion, please use the
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[Tern discussion forum](https://discuss.ternjs.net).
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## Installation
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The easiest way to install acorn is from [`npm`](https://www.npmjs.com/):
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```sh
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npm install acorn
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```
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Alternately, you can download the source and build acorn yourself:
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```sh
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git clone https://github.com/acornjs/acorn.git
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cd acorn
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npm install
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```
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## Interface
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**parse**`(input, options)` is the main interface to the library. The
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`input` parameter is a string, `options` can be undefined or an object
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setting some of the options listed below. The return value will be an
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abstract syntax tree object as specified by the [ESTree
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spec](https://github.com/estree/estree).
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```javascript
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let acorn = require("acorn");
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console.log(acorn.parse("1 + 1"));
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```
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When encountering a syntax error, the parser will raise a
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`SyntaxError` object with a meaningful message. The error object will
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have a `pos` property that indicates the string offset at which the
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error occurred, and a `loc` object that contains a `{line, column}`
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object referring to that same position.
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Options can be provided by passing a second argument, which should be
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an object containing any of these fields:
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- **ecmaVersion**: Indicates the ECMAScript version to parse. Must be
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either 3, 5, 6 (2015), 7 (2016), 8 (2017), 9 (2018), 10 (2019) or 11
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(2020, partial support). This influences support for strict mode,
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the set of reserved words, and support for new syntax features.
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Default is 10.
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**NOTE**: Only 'stage 4' (finalized) ECMAScript features are being
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implemented by Acorn. Other proposed new features can be implemented
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through plugins.
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- **sourceType**: Indicate the mode the code should be parsed in. Can be
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either `"script"` or `"module"`. This influences global strict mode
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and parsing of `import` and `export` declarations.
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**NOTE**: If set to `"module"`, then static `import` / `export` syntax
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will be valid, even if `ecmaVersion` is less than 6.
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- **onInsertedSemicolon**: If given a callback, that callback will be
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called whenever a missing semicolon is inserted by the parser. The
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callback will be given the character offset of the point where the
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semicolon is inserted as argument, and if `locations` is on, also a
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`{line, column}` object representing this position.
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- **onTrailingComma**: Like `onInsertedSemicolon`, but for trailing
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commas.
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- **allowReserved**: If `false`, using a reserved word will generate
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an error. Defaults to `true` for `ecmaVersion` 3, `false` for higher
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versions. When given the value `"never"`, reserved words and
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keywords can also not be used as property names (as in Internet
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Explorer's old parser).
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- **allowReturnOutsideFunction**: By default, a return statement at
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the top level raises an error. Set this to `true` to accept such
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code.
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- **allowImportExportEverywhere**: By default, `import` and `export`
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declarations can only appear at a program's top level. Setting this
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option to `true` allows them anywhere where a statement is allowed.
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- **allowAwaitOutsideFunction**: By default, `await` expressions can
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only appear inside `async` functions. Setting this option to
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`true` allows to have top-level `await` expressions. They are
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still not allowed in non-`async` functions, though.
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- **allowHashBang**: When this is enabled (off by default), if the
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code starts with the characters `#!` (as in a shellscript), the
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first line will be treated as a comment.
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- **locations**: When `true`, each node has a `loc` object attached
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with `start` and `end` subobjects, each of which contains the
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one-based line and zero-based column numbers in `{line, column}`
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form. Default is `false`.
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- **onToken**: If a function is passed for this option, each found
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token will be passed in same format as tokens returned from
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`tokenizer().getToken()`.
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If array is passed, each found token is pushed to it.
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Note that you are not allowed to call the parser from the
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callback—that will corrupt its internal state.
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- **onComment**: If a function is passed for this option, whenever a
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comment is encountered the function will be called with the
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following parameters:
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- `block`: `true` if the comment is a block comment, false if it
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is a line comment.
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- `text`: The content of the comment.
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- `start`: Character offset of the start of the comment.
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- `end`: Character offset of the end of the comment.
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When the `locations` options is on, the `{line, column}` locations
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of the comment’s start and end are passed as two additional
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parameters.
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If array is passed for this option, each found comment is pushed
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to it as object in Esprima format:
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```javascript
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{
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"type": "Line" | "Block",
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"value": "comment text",
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"start": Number,
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"end": Number,
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// If `locations` option is on:
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"loc": {
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"start": {line: Number, column: Number}
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"end": {line: Number, column: Number}
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},
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// If `ranges` option is on:
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"range": [Number, Number]
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}
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```
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Note that you are not allowed to call the parser from the
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callback—that will corrupt its internal state.
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- **ranges**: Nodes have their start and end characters offsets
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recorded in `start` and `end` properties (directly on the node,
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rather than the `loc` object, which holds line/column data. To also
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add a
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[semi-standardized](https://bugzilla.mozilla.org/show_bug.cgi?id=745678)
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`range` property holding a `[start, end]` array with the same
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numbers, set the `ranges` option to `true`.
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- **program**: It is possible to parse multiple files into a single
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AST by passing the tree produced by parsing the first file as the
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`program` option in subsequent parses. This will add the toplevel
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forms of the parsed file to the "Program" (top) node of an existing
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parse tree.
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- **sourceFile**: When the `locations` option is `true`, you can pass
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this option to add a `source` attribute in every node’s `loc`
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object. Note that the contents of this option are not examined or
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processed in any way; you are free to use whatever format you
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choose.
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- **directSourceFile**: Like `sourceFile`, but a `sourceFile` property
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will be added (regardless of the `location` option) directly to the
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nodes, rather than the `loc` object.
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- **preserveParens**: If this option is `true`, parenthesized expressions
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are represented by (non-standard) `ParenthesizedExpression` nodes
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that have a single `expression` property containing the expression
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inside parentheses.
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**parseExpressionAt**`(input, offset, options)` will parse a single
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expression in a string, and return its AST. It will not complain if
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there is more of the string left after the expression.
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**tokenizer**`(input, options)` returns an object with a `getToken`
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method that can be called repeatedly to get the next token, a `{start,
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end, type, value}` object (with added `loc` property when the
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`locations` option is enabled and `range` property when the `ranges`
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option is enabled). When the token's type is `tokTypes.eof`, you
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should stop calling the method, since it will keep returning that same
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token forever.
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In ES6 environment, returned result can be used as any other
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protocol-compliant iterable:
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```javascript
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for (let token of acorn.tokenizer(str)) {
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// iterate over the tokens
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}
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// transform code to array of tokens:
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var tokens = [...acorn.tokenizer(str)];
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```
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**tokTypes** holds an object mapping names to the token type objects
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that end up in the `type` properties of tokens.
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**getLineInfo**`(input, offset)` can be used to get a `{line,
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column}` object for a given program string and offset.
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### The `Parser` class
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Instances of the **`Parser`** class contain all the state and logic
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that drives a parse. It has static methods `parse`,
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`parseExpressionAt`, and `tokenizer` that match the top-level
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functions by the same name.
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When extending the parser with plugins, you need to call these methods
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on the extended version of the class. To extend a parser with plugins,
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you can use its static `extend` method.
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```javascript
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var acorn = require("acorn");
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var jsx = require("acorn-jsx");
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var JSXParser = acorn.Parser.extend(jsx());
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JSXParser.parse("foo(<bar/>)");
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```
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The `extend` method takes any number of plugin values, and returns a
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new `Parser` class that includes the extra parser logic provided by
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the plugins.
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## Command line interface
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The `bin/acorn` utility can be used to parse a file from the command
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line. It accepts as arguments its input file and the following
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options:
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- `--ecma3|--ecma5|--ecma6|--ecma7|--ecma8|--ecma9|--ecma10`: Sets the ECMAScript version
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to parse. Default is version 9.
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- `--module`: Sets the parsing mode to `"module"`. Is set to `"script"` otherwise.
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- `--locations`: Attaches a "loc" object to each node with "start" and
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"end" subobjects, each of which contains the one-based line and
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zero-based column numbers in `{line, column}` form.
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- `--allow-hash-bang`: If the code starts with the characters #! (as
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in a shellscript), the first line will be treated as a comment.
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- `--compact`: No whitespace is used in the AST output.
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- `--silent`: Do not output the AST, just return the exit status.
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- `--help`: Print the usage information and quit.
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The utility spits out the syntax tree as JSON data.
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## Existing plugins
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- [`acorn-jsx`](https://github.com/RReverser/acorn-jsx): Parse [Facebook JSX syntax extensions](https://github.com/facebook/jsx)
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Plugins for ECMAScript proposals:
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- [`acorn-stage3`](https://github.com/acornjs/acorn-stage3): Parse most stage 3 proposals, bundling:
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- [`acorn-class-fields`](https://github.com/acornjs/acorn-class-fields): Parse [class fields proposal](https://github.com/tc39/proposal-class-fields)
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- [`acorn-import-meta`](https://github.com/acornjs/acorn-import-meta): Parse [import.meta proposal](https://github.com/tc39/proposal-import-meta)
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- [`acorn-private-methods`](https://github.com/acornjs/acorn-private-methods): parse [private methods, getters and setters proposal](https://github.com/tc39/proposal-private-methods)n
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