# JS [![API reference](https://img.shields.io/badge/godoc-reference-5272B4)](https://pkg.go.dev/github.com/tdewolff/minify/v2/parse/js?tab=doc)
This package is a JS lexer (ECMAScript 2020) written in [Go][1]. It follows the specification at [ECMAScript 2020 Language Specification](https://tc39.es/ecma262/). The lexer takes an io.Reader and converts it into tokens until the EOF.
## Installation
Run the following command
go get -u github.com/tdewolff/parse/v2/js
or add the following import and run project with `go get`
import "github.com/tdewolff/parse/v2/js"
## Lexer
### Usage
The following initializes a new Lexer with io.Reader `r`:
``` go
l := js.NewLexer(parse.NewInput(r))
```
To tokenize until EOF an error, use:
``` go
for {
tt, text := l.Next()
switch tt {
case js.ErrorToken:
// error or EOF set in l.Err()
return
// ...
}
}
```
### Regular Expressions
The ECMAScript specification for `PunctuatorToken` (of which the `/` and `/=` symbols) and `RegExpToken` depend on a parser state to differentiate between the two. The lexer will always parse the first token as `/` or `/=` operator, upon which the parser can rescan that token to scan a regular expression using `RegExp()`.
### Examples
``` go
package main
import (
"os"
"github.com/tdewolff/parse/v2/js"
)
// Tokenize JS from stdin.
func main() {
l := js.NewLexer(parse.NewInput(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))
// ...
}
}
}
```
## Parser
### Usage
The following parses a file and returns an abstract syntax tree (AST).
``` go
ast, err := js.NewParser(parse.NewInputString("if (state == 5) { console.log('In state five'); }"))
```
See [ast.go](https://github.com/tdewolff/parse/blob/master/js/ast.go) for all available data structures that can represent the abstact syntax tree.
## License
Released under the [MIT license](https://github.com/tdewolff/parse/blob/master/LICENSE.md).
[1]: http://golang.org/ "Go Language"