Awesome
RESS
Rusty EcmaScript Scanner
A scanner/tokenizer for JS written in Rust
Usage
The primary way to interact with ress is through the Scanner
struct which implements Iterator
over the Item
struct. Item
has three fields token
for the Token
found, span
which represents the start and end of the byte position in the original string and location
which represents start and end character position with a line and column. It's definition looks like this.
Item {
token: Token::Punct(Punct::Bang),
span: Span {
start: 0,
end: 1,
},
location: SourceLocation {
start: Position {
line: 1,
column: 1,
},
end: Position {
line: 1,
column: 2,
}
}
}
Note: the EcmaScript spec allows for 4 new line characters, only two of which are normally rendered by modern text editors the location line numbers will count these unrendered lines.
Here is an example that check some JS text for the existence of a semicolon and panics if one is found.
extern crate ress;
use ress::{Scanner};
static JS: &str = include_str!("index.js");
fn main() {
let s = Scanner::new(JS);
for token in s {
let token = token.unwrap().token;
if token.matches_punct_str(";") {
panic!("A semi-colon!? Heathen!");
}
}
println!("Good show! Why use something that's optional?")
}
By far the most important part of Item
is the Token
enum, which will represent the 11 different types of token's supported by the ECMAScript specification.
ES Tokens
- Boolean Literal
- End of File
- Identifier
- Keyword
- Null Literal
- Numeric Literal
- Punctuation
- String Literal
- Regular Expression Literal
- Template String
- Comment
Keep in mind that keywords have been moving around a lot in JS between ES3 through ES2019 so you might find some items parsed as keywords in the ES2019 context that are not in the ES3 context, this should be dealt with at a higher level. A good example of this is yield
which is sometimes a keyword and sometimes an identifier, this package will always parse this as a Keyword. As of the writing of this readme ress
supports all tokens in the Stage 2 and Stage 3 ECMAScript Proposals with the exception of the #!
comments and number seperators.
For each of the token cases there is either a struct or enum to provide additional information with the exception of NullLiteral
and EoF
which should be self explanatory. The more complicated items do implement ToString
which should get you back to the original js text for that token. The Token
enum also provides a number of helper functions for building that picture without pulling the inner data our of the enum. Using the Punct
case as an example the helper functions look like this.
fn is_punct(&self) -> bool;
fn matches_punct(&self, p: Punct) -> bool;
fn matches_punct_str(&self, s: &str) -> bool;
A similar set of functions are available for each case.
Like all Iterators
the Scanner
has a next
method, It also has a look_ahead
method that will allow you to parse the next value without advancing. Using this method can be a convenient way to get the next token without performing a mutable borrow, however you will be incurring the cost of parsing that token twice. All Iterators
can be converted into a Peekable
Iterator with a peek
method, this will allow you to look ahead while only paying the cost once however peek
performs a mutable borrow which means it needs to be in a different scope than a call to next
.
// look_ahead
let js = "function() { return; }";
let mut s = Scanner::new(js);
let current = s.next();
let next = s.look_ahead();
let new_current = s.next();
assert_eq!(next, new_current);
// peekable (fails to compile)
let p = Scanner::new(js).peekable();
let current = s.next(); // <-- first mutable borrow
let next = p.peek(); // <-- second mutable borrow
For more intense lookahead scenarios Scanner
makes available the get_state
and set_state
methods. These methods will allow you to capture a snapshot of the current position and any context, and then later reset to that position and context.
let js = "function() {
return 0;
};";
let mut s = Scanner::new(js);
let start = s.get_state();
assert_eq!(s.next().unwrap().unwrap().token, Token::Keyword(Keyword::Function));
assert_eq!(s.next().unwrap().unwrap().token, Token::Punct(Punct::OpenParen));
assert_eq!(s.next().unwrap().unwrap().token, Token::Punct(Punct::CloseParen));
s.set_state(start);
assert_eq!(s.next().unwrap().unwrap().token, Token::Keyword(Keyword::Function));
Why?
Wouldn't it be nice to write new JS development tools in Rust? The clear-comments example is a proof of concept on how you might use this crate to do just that. This example will take in a JS file and output a version with all of the comments removed. An example of how you might see it in action is below (assuming you have a file called in.js in the project root).
$ cargo run --example clear-comments -- ./in.js ./out.js
Performance
The below stats are from running cargo +nightly bench
on a MBP (2.9 GHz i9-8850H & 16bg RAM).
Lib | Size | Time | +/- |
---|---|---|---|
Angular 1.5 | 1.16mb | 18.991 ms | 4.393 ms |
jquery | 271.75kb | 7.218 ms | 577.236 μs |
React | 59.09kb | 1.976 ms | 116.139 μs |
React-dom | 641.51kb | 16.880 ms | 3.614 ms |
Vue | 289.30kb | 9.675 ms | 1.402 ms |
If you are interested in getting an idea about performance without waiting for cargo bench
to complete you can run the following command.
$ cargo run --example major_libs