Testcase for https://github.com/Infocatcher/Private_Tab/blob/master/virtualKeyCodes.js

getVKChar.js

function getVKChar(vk) {
	// Firefox doesn't have string representation for some codes...
	// https://developer.mozilla.org/en-US/docs/DOM/KeyboardEvent#Virtual_key_codes
	if(/^VK_(?:NUMPAD)?([\d+A-Z])$/.test(vk))
		return RegExp.$1;
	switch(vk) {
		case "VK_SPACE":               return " "; //32
		case "VK_COLON":               return ":"; //58
		case "VK_SEMICOLON":           return ";"; //59
		case "VK_LESS_THAN":           return "<"; //60
		case "VK_EQUALS":              return "="; //61
		case "VK_GREATER_THAN":        return ">"; //62
		case "VK_QUESTION_MARK":       return "?"; //63
		case "VK_AT":                  return "@"; //64
		case "VK_MULTIPLY":            return "*"; //106
		case "VK_ADD":                 return "+"; //107
		case "VK_SUBTRACT":            return "-"; //109
		case "VK_DECIMAL":             return "."; //110
		case "VK_DIVIDE":              return "/"; //111
		case "VK_CIRCUMFLEX":          return "^"; //160
		case "VK_EXCLAMATION":         return "!"; //161
		case "VK_DOUBLE_QUOTE":        return '"'; //162
		case "VK_HASH":                return "#"; //163
		case "VK_DOLLAR":              return "$"; //164
		case "VK_PERCENT":             return "%"; //165
		case "VK_AMPERSAND":           return "&"; //166
		case "VK_UNDERSCORE":          return "_"; //167
		case "VK_OPEN_PAREN":          return "("; //168
		case "VK_CLOSE_PAREN":         return ")"; //169
		case "VK_ASTERISK":            return "*"; //170
		case "VK_PLUS":                return "+"; //171
		case "VK_PIPE":                return "|"; //172
		case "VK_HYPHEN_MINUS":        return "-"; //173
		case "VK_OPEN_CURLY_BRACKET":  return "{"; //174
		case "VK_CLOSE_CURLY_BRACKET": return "}"; //175
		case "VK_TILDE":               return "~"; //176
		case "VK_COMMA":               return ","; //188
		case "VK_PERIOD":              return "."; //190
		case "VK_SLASH":               return "/"; //191
		case "VK_BACK_QUOTE":          return "`"; //192
		case "VK_OPEN_BRACKET":        return "["; //219
		case "VK_BACK_SLASH":          return "\\"; //220
		case "VK_CLOSE_BRACKET":       return "]"; //221
		case "VK_QUOTE":               return "'"; //222
	}
	return undefined;
}
var getVKCharMap = {
	VK_SPACE:               " ", //32
	VK_COLON:               ":", //58
	VK_SEMICOLON:           ";", //59
	VK_LESS_THAN:           "<", //60
	VK_EQUALS:              "=", //61
	VK_GREATER_THAN:        ">", //62
	VK_QUESTION_MARK:       "?", //63
	VK_AT:                  "@", //64
	VK_MULTIPLY:            "*", //106
	VK_ADD:                 "+", //107
	VK_SUBTRACT:            "-", //109
	VK_DECIMAL:             ".", //110
	VK_DIVIDE:              "/", //111
	VK_CIRCUMFLEX:          "^", //160
	VK_EXCLAMATION:         "!", //161
	VK_DOUBLE_QUOTE:        '"', //162
	VK_HASH:                "#", //163
	VK_DOLLAR:              "$", //164
	VK_PERCENT:             "%", //165
	VK_AMPERSAND:           "&", //166
	VK_UNDERSCORE:          "_", //167
	VK_OPEN_PAREN:          "(", //168
	VK_CLOSE_PAREN:         ")", //169
	VK_ASTERISK:            "*", //170
	VK_PLUS:                "+", //171
	VK_PIPE:                "|", //172
	VK_HYPHEN_MINUS:        "-", //173
	VK_OPEN_CURLY_BRACKET:  "{", //174
	VK_CLOSE_CURLY_BRACKET: "}", //175
	VK_TILDE:               "~", //176
	VK_COMMA:               ",", //188
	VK_PERIOD:              ".", //190
	VK_SLASH:               "/", //191
	VK_BACK_QUOTE:          "`", //192
	VK_OPEN_BRACKET:        "[", //219
	VK_BACK_SLASH:          "\\", //220
	VK_CLOSE_BRACKET:       "]", //221
	VK_QUOTE:               "'", //222
	__proto__: null
};
function getVKChar2(vk) {
	// Firefox doesn't have string representation for some codes...
	// https://developer.mozilla.org/en-US/docs/DOM/KeyboardEvent#Virtual_key_codes
	var chr = getVKCharMap[vk] || null;
	if(chr)
		return chr;
	if(/^VK_(?:NUMPAD)?([\d+A-Z])$/.test(vk))
		return RegExp.$1;
	return undefined;
}
function getVKChar3(vk) {
	// Firefox doesn't have string representation for some codes...
	// https://developer.mozilla.org/en-US/docs/DOM/KeyboardEvent#Virtual_key_codes
	var map = getVKCharMap;
	if(vk in map)
		return map[vk];
	if(/^VK_(?:NUMPAD)?([\d+A-Z])$/.test(vk))
		return RegExp.$1;
	return undefined;
}


var c = 100e3;
var codes = ["VK_SPACE", "VK_PLUS", "VK_F", "VK_NUMPAD8", "VK_QUOTE"];

var i = c + 1, t = performance.now();
while(--i)
	codes.forEach(getVKChar);
var dtOld = performance.now() - t;

var i = c + 1, t = performance.now();
while(--i)
	codes.forEach(getVKChar2);
var dtNew1 = performance.now() - t;

var i = c + 1, t = performance.now();
while(--i)
	codes.forEach(getVKChar3);
var dtNew2 = performance.now() - t;

alert(
	"Old: " + dtOld.toFixed(2)
	+ "\nNew 1: " + dtNew1.toFixed(2) + " x" + (dtOld/dtNew1).toFixed(1)
	+ "\nNew 2: " + dtNew2.toFixed(2) + " x" + (dtOld/dtNew2).toFixed(1)
);

For var c = 300e3; in Firefox 45.0.2:

Old: 182.09
New 1: 124.86 x1.5
New 2: 198.63 x0.9