# 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 lexer is
based on [`glexer`](https://hexdocs.pm/glexer), allowing you to convert JavaScript
source code into tokens. There is also an API which 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@2
```
```gleam
import just
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 = just.highlight_ansi(code)
io.println(highlighted)
// Render to html to show in the browser
let html = just.highlight_html(code)
io.println("<pre><code>" <> html <> "</code></pre>")
// Convert to "highlighting tokens" to highlight in some other way
let highlight_tokens = just.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.
