async function hash_hmac(type, message, key, base64) {
    const getUtf8Bytes = str =>
        new Uint8Array(
        [...unescape(encodeURIComponent(str))].map(c => c.charCodeAt(0))
    );

    const keyBytes = getUtf8Bytes(key);
    const messageBytes = getUtf8Bytes(message);

    const cryptoKey = await crypto.subtle.importKey(
        "raw", keyBytes, { name: "HMAC", hash: type },
        true, ["sign"]
    );

    const sig = await crypto.subtle.sign("HMAC", cryptoKey, messageBytes);

    const data = String.fromCharCode(...new Uint8Array(sig));

    return base64 ? btoa(data) : data;
}

(async function() {
    let key = "YdkQZp9Pq0OsKT5TlFzrgry7j1nw0XEmbNFm86zNU3+XFEmM/I+WxrAZE7yjFAD3iWJTQ10VN2+JK3fz4b3Viw==";
    let message = "1614117737467myJSON.json" + '{"json_data": "to-be-encoded"}';
    
    let hmac = await hash_hmac("SHA-256", message, atob(key), true);

    console.log(
        btoa("1614117737467;" + hmac)
    );
})();