js

JS

This package is a JS lexer (ECMA-262, edition 6.0) written in Go. It follows the specification at ECMAScript Language Specification. The lexer takes an io.Reader and converts it into tokens until the EOF.

Installation

Run the following command

go get github.com/tdewolff/parse/js

or add the following import and run project with go get

import "github.com/tdewolff/parse/js"

Lexer

Usage

The following initializes a new Lexer with io.Reader r:

l := js.NewLexer(r)

To tokenize until EOF an error, use:

for {
	tt, text := l.Next()
	switch tt {
	case js.ErrorToken:
		// error or EOF set in l.Err()
		return
	// ...
	}
}

All tokens (see ECMAScript Language Specification):

ErrorToken          TokenType = iota // extra token when errors occur
UnknownToken                         // extra token when no token can be matched
WhitespaceToken                      // space \t \v \f
LineTerminatorToken                  // \r \n \r\n
CommentToken
IdentifierToken // also: null true false
PunctuatorToken /* { } ( ) [ ] . ; , < > <= >= == != === !==  + - * % ++ -- << >>
   >>> & | ^ ! ~ && || ? : = += -= *= %= <<= >>= >>>= &= |= ^= / /= => */
NumericToken
StringToken
RegexpToken
TemplateToken

Quirks

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).

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.

Examples

package main

import (
	"os"

	"github.com/tdewolff/parse/js"
)

// Tokenize JS from stdin.
func main() {
	l := js.NewLexer(os.Stdin)
	for {
		tt, text := l.Next()
		switch tt {
		case js.ErrorToken:
			if l.Err() != io.EOF {
				fmt.Println("Error on line", l.Line(), ":", l.Err())
			}
			return
		case js.IdentifierToken:
			fmt.Println("Identifier", string(text))
		case js.NumericToken:
			fmt.Println("Numeric", string(text))
		// ...
		}
	}
}

License

Released under the MIT license.