Just another polyfill for that `KeyboardEvent.key` property (lower-cased)

keyboard-event._key.js

(function() {

    // Key maps for the deprecated `KeyboardEvent.keyCode`
    var keys = {
        // control
        8: 'backspace',
        9: 'tab',
        13: 'enter',
        16: 'shift',
        17: 'control',
        18: 'alt',
        19: 'pause',
        20: 'capslock', // not working on `keypress`
        27: 'escape',
        33: 'pageup',
        34: 'pagedown',
        37: 'arrowleft',
        38: 'arrowup',
        39: 'arrowright',
        40: 'arrowdown',
        44: 'printscreen', // works only on `keyup` :(
        45: 'insert',
        46: 'delete',
        91: 'meta', // <https://bugzilla.mozilla.org/show_bug.cgi?id=1232918>
        93: 'contextmenu',
        // function
        112: 'f1',
        113: 'f2',
        114: 'f3',
        115: 'f4',
        116: 'f5',
        117: 'f6',
        118: 'f7',
        119: 'f8',
        120: 'f9',
        121: 'f10',
        122: 'f11',
        123: 'f12',
        // number
        48: ['0', ')'],
        49: ['1', '!'],
        50: ['2', '@'],
        51: ['3', '#'],
        52: ['4', '$'],
        53: ['5', '%'],
        54: ['6', '^'],
        55: ['7', '&'],
        56: ['8', '*'],
        57: ['9', '('],
        // alphabet
        65: 'a',
        66: 'b',
        67: 'c',
        68: 'd',
        69: 'e',
        70: 'f',
        71: 'g',
        72: 'h',
        73: 'i',
        74: 'j',
        75: 'k',
        76: 'l',
        77: 'm',
        78: 'n',
        79: 'o',
        80: 'p',
        81: 'q',
        82: 'r',
        83: 's',
        84: 't',
        85: 'u',
        86: 'v',
        87: 'w',
        88: 'x',
        89: 'y',
        90: 'z',
        // symbol
        32: ' ',
        59: [';', ':'],
        61: ['=', '+'],
        173: ['-', '_'],
        188: [',', '<'],
        190: ['.', '>'],
        191: ['/', '?'],
        192: ['`', '~'],
        219: ['[', '{'],
        220: ['\\', '|'],
        221: [']', '}'],
        222: ['\'', '"']
    };

    Object.defineProperty(KeyboardEvent.prototype, '_key', {
        get: function() {
            var key = this.key || keys[this.which || this.keyCode];
            if (typeof key === "object") {
                if (this.shiftKey) {
                    key = key[1] || key[0];
                } else {
                    key = key[0];
                }
            }
            return key.toLowerCase();
        }
    });

})();

Usage

document.onkeydown = function(e) {
    if (e.key) {
        alert(e.key); // native
    }
    alert(e._key); // custom
};