90 lines
3.4 KiB
Markdown
90 lines
3.4 KiB
Markdown
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# JS [](http://godoc.org/github.com/tdewolff/parse/js) [](http://gocover.io/github.com/tdewolff/parse/js)
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This package is a JS lexer (ECMA-262, edition 6.0) written in [Go][1]. It follows the specification at [ECMAScript Language Specification](http://www.ecma-international.org/ecma-262/6.0/). The lexer takes an io.Reader and converts it into tokens until the EOF.
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## Installation
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Run the following command
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go get github.com/tdewolff/parse/js
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or add the following import and run project with `go get`
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import "github.com/tdewolff/parse/js"
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## Lexer
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### Usage
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The following initializes a new Lexer with io.Reader `r`:
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``` go
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l := js.NewLexer(r)
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```
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To tokenize until EOF an error, use:
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``` go
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for {
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tt, text := l.Next()
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switch tt {
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case js.ErrorToken:
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// error or EOF set in l.Err()
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return
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// ...
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}
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}
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```
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All tokens (see [ECMAScript Language Specification](http://www.ecma-international.org/ecma-262/6.0/)):
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``` go
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ErrorToken TokenType = iota // extra token when errors occur
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UnknownToken // extra token when no token can be matched
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WhitespaceToken // space \t \v \f
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LineTerminatorToken // \r \n \r\n
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CommentToken
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IdentifierToken // also: null true false
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PunctuatorToken /* { } ( ) [ ] . ; , < > <= >= == != === !== + - * % ++ -- << >>
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>>> & | ^ ! ~ && || ? : = += -= *= %= <<= >>= >>>= &= |= ^= / /= => */
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NumericToken
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StringToken
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RegexpToken
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TemplateToken
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```
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### Quirks
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Because the ECMAScript specification for `PunctuatorToken` (of which the `/` and `/=` symbols) and `RegexpToken` depends on a parser state to differentiate between the two, the lexer (to remain modular) uses different rules. It aims to correctly disambiguate contexts and returns `RegexpToken` or `PunctuatorToken` where appropriate with only few exceptions which don't make much sense in runtime and so don't happen in a real-world code: function literal division (`x = function y(){} / z`) and object literal division (`x = {y:1} / z`).
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Another interesting case introduced by ES2015 is `yield` operator in function generators vs `yield` as an identifier in regular functions. This was done for backward compatibility, but is very hard to disambiguate correctly on a lexer level without essentially implementing entire parsing spec as a state machine and hurting performance, code readability and maintainability, so, instead, `yield` is just always assumed to be an operator. In combination with above paragraph, this means that, for example, `yield /x/i` will be always parsed as `yield`-ing regular expression and not as `yield` identifier divided by `x` and then `i`. There is no evidence though that this pattern occurs in any popular libraries.
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### Examples
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``` go
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package main
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import (
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"os"
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"github.com/tdewolff/parse/js"
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)
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// Tokenize JS from stdin.
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func main() {
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l := js.NewLexer(os.Stdin)
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for {
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tt, text := l.Next()
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switch tt {
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case js.ErrorToken:
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if l.Err() != io.EOF {
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fmt.Println("Error on line", l.Line(), ":", l.Err())
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}
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return
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case js.IdentifierToken:
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fmt.Println("Identifier", string(text))
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case js.NumericToken:
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fmt.Println("Numeric", string(text))
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// ...
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}
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}
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}
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```
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## License
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Released under the [MIT license](https://github.com/tdewolff/parse/blob/master/LICENSE.md).
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[1]: http://golang.org/ "Go Language"
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