# Just
A JavaScript lexer and syntax highlighter for Gleam!
[](https://hex.pm/packages/just)
[](https://hexdocs.pm/just/)
Just is a JavaScript lexer and syntax highlighter written in Gleam.
The `just` module, based on [`glexer`](https://hexdocs.mp/glexer) exposes a
standard lexer API, allowing you to convert JavaScript source code into tokens.
The `just/highlight` module allows you to highlight javascript code using ansi
colours, html or a custom format. Heavily inspired by [`contour`](https://hexdocs.pm/contour).
```sh
gleam add just@1
```
```gleam
import just
import just/highlight
pub fn main() {
let code = "console.log('Hello, world!');"
let lexer = just.new(code) |> just.strict_mode
// Lex syntax tokens for parsing or other uses
let #(tokens, errors) = just.tokenise(lexer)
let assert [] = errors
parse_js(tokens)
// Highlight with ansi codes to print in the terminal
let highlighted = highlight.ansi(code)
io.println(highlighted)
// Render to html to show in the browser
let html = highlight.html(code)
io.println("<pre><code>" <> html <> "</code></pre>")
// Convert to "highlighting tokens" to highlight in some other way
let highlight_tokens = highlight.tokens(code)
highlight_tokens_some_other_way(highlight_tokens)
}
```
Further documentation can be found at <https://hexdocs.pm/just>.
### Missing features
The Just lexer should be able to accurately lex most valid JavaScript programs,
but there are a few things it is missing:
- Proper backtracking for Regular Expressions. Currently the lexer uses the previously
lexed token to determine whether to lex a `/` character as a division operator or a
regular expression. This means it can fail in some edge-cases.
- Lexing of full-unicode identifiers. JavaScript supports more than just ASCII characters
to make up its identifiers. Currently, Just doesn't support non-ASCII characters in
identifiers.
- Lexing identifier escape-sequences. Similarly, JavaScript allows unicode escape sequences
as part of identifiers (e.g. `let \u0065 = 10;`). This is currently not supported by Just.
