const hexString = "68656c6c6f";
const byteArray = Buffer.from(hexString, 'hex');
console.log(byteArray);

由于浏览器不支持Buffer,使用下面的形式

const hexStringToByteArray = function (hexString) {
    const bytes = new Uint8Array(hexString.length / 2);

    for (let i = 0; i < hexString.length; i += 2) {
        bytes[i / 2] = parseInt(hexString.substr(i, 2), 16);
    }

    return bytes;
}

sleep一段时间

var sleep = async function (ms) {
            return new Promise(function (resolve) {
                setTimeout(resolve, ms);
            });
        };

使用

for (var i = 0; i < 10; i++) {
            console.log(i, Date.now());
            await sleep(2000);
        }

Math.random()