wasm-bindgen/examples/performance/src/lib.rs

use std::time::{Duration, SystemTime, UNIX_EPOCH};

use wasm_bindgen::prelude::*;

// lifted from the `console_log` example
#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(js_namespace = console)]
    fn log(a: &str);
}

macro_rules! console_log {
    ($($t:tt)*) => (log(&format_args!($($t)*).to_string()))
}

#[wasm_bindgen(start)]
pub fn run() {
    let window = web_sys::window().expect("should have a window in this context");
    let performance = window
        .performance()
        .expect("performance should be available");

    console_log!("the current time (in ms) is {}", performance.now());

    let start = perf_to_system(performance.timing().request_start());
    let end = perf_to_system(performance.timing().response_end());

    console_log!("request started at {}", humantime::format_rfc3339(start));
    console_log!("request ended at {}", humantime::format_rfc3339(end));
}

fn perf_to_system(amt: f64) -> SystemTime {
    let secs = (amt as u64) / 1_000;
    let nanos = ((amt as u32) % 1_000) * 1_000_000;
    UNIX_EPOCH + Duration::new(secs, nanos)
}