Proactively hide sensitive items before OS X keyboard shortcuts for taking screenshots are completed

secret.js

/**
 * secret.js
 * 
 * Hides senstive DOM elements on the page before a screenshot can
 * be taken on OS X using the Cmd + Shift + 3 or Cmd + Shift + 4 
 * keyboard shortcuts under *some* circumstances. Obviously this 
 * will only work if the browser has focus.
 * 
 * A note on Windows:
 * The last time I checked, the PrtScn key on Windows fired as a 
 * system event and triggered a screenshot before the key event 
 * reaches the browser.
 */

const CMD_KEY_LEFT = 91;
const CMD_KEY_RIGHT = 93;
const SHIFT_KEY = 16;

const sensitive_items = [
    document.getElementById('secret'),
    document.getElementById('another_secret')
];

var active_keys = [];

function disableSensitiveItems() {
    sensitive_items.forEach(function(el) {
        el.style.display = 'none';
    });
}

function enableSensitiveItems() {
    sensitive_items.forEach(function(el) {
        el.style.display = '';
    });
}

function _checkKeyCombo() {
    function isPressed(key) {
        return active_keys.indexOf(key) !== -1;
    }

    if (isPressed(SHIFT_KEY) && (isPressed(CMD_KEY_LEFT) || isPressed(CMD_KEY_RIGHT))) {
        disableSensitiveItems();
    }
}

function _handleKeyDown(e) {
    active_keys.push(e.keyCode);
    _checkKeyCombo();
}

function _handleKeyUp() {
    active_keys = [];
    enableSensitiveItems();
}

window.addEventListener('keydown', _handleKeyDown);
window.addEventListener('keyup', _handleKeyUp);