js: the JS language for rust-js

The js crate declares what JS has that Rust's std doesn't, as ReScript's standard library does: its promises, errors and regular expressions, its byte buffers, its JSON, and its global functions, its timers too, as ReScript's has them. What the browser adds is the webapi crate's; what std has, rust-js maps itself. It holds declarations only, so it's never compiled to JS. See ADR 0102.

use js::{decode_uri_component, encode_uri_component, settle, spawn};

let query = encode_uri_component("a b&c");             // encodeURIComponent("a b&c")
let text = decode_uri_component("%E0%A4%A");           // Err: a URIError
spawn(Box::new(async move {                            // runs, unawaited
    match settle(webapi::window::fetch(webapi::window, &url)).await {
        Ok(response) => { .. }                         // what fetch fulfils with
        Err(error) => { .. }                           // a network error: not a throw
    }
}));

• A promise is a Promise<T>, to .await (ADR 0029); settle(p) makes its .await a Result, and one a binding declares as Promise<Result<T, &JsError>> is one already (ADR 0035).
• JsError is what JS threw; js_error::to_string(e) is String(e), and js_error::is_error(e) and js_error::message(e) are an Error's.
• RegExp is there for what Rust would use regex for: reg_exp::new(r"^\d+$", ""), and reg_exp::replace(text, pattern, "$1"). A replacer closure, and matchAll, are a program's own bindings, typed for its pattern's groups.
• json::stringify(text) is a string's JSON, which is a JS string literal too. A Rust value's JSON is serde's (ADR 0077), and so is reading JSON: what JSON.parse gives is whatever the text holds.
• object::from_entries(entries) is a JS object of keys and values, as an API taking a dictionary wants, and object::is(a, b) is Object.is: whether two JS objects are one.
• set_timeout(f, ms) and set_interval(f, ms) run a closure later, or every so often, and clear_timeout and clear_interval of what they gave stop it: every JS runtime has them, browsers', workers' and Node's.
• number::to_fixed(x, digits) is x.toFixed(digits), JS's rounding: a tie away from zero, 2.5 to "3", where format!("{:.0}", 2.5) is "2".

builtins/build.sh -o "$PWD/target/libjs.rmeta"
./target/debug/rust-js app.rs -- --extern js=target/libjs.rmeta