Generates a globally unique identifier (GUID) in JavaScript.

```
/**
* RFC4122: The version 4 UUID is meant for generating UUIDs from truly-random
* or pseudo-random numbers.
*
* The algorithm is as follows:
*
* Set the two most significant bits (bits 6 and 7) of the
* clock_seq_hi_and_reserved to zero and one, respectively.
*
* Set the four most significant bits (bits 12 through 15) of the
* time_hi_and_version field to the 4-bit version number from Section
* 4.1.3. Version4
*
* Set all the other bits to randomly (or pseudo-randomly) chosen values.
*
* UUID = time-low "-" time-mid "-"time-high-and-version "-"clock-seq-reserved and low(2hexOctet)"-" node
* time-low = 4hexOctet
* time-mid = 2hexOctet
* time-high-and-version = 2hexOctet
* clock-seq-and-reserved = hexOctet:
* clock-seq-low = hexOctet
* node = 6hexOctet
*
* Format: xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx
*
* y could be 1000, 1001, 1010, 1011 since most significant two bits needs to be 10
* y values are 8, 9, A, B
*
* Source:
* https://github.com/AzureAD/azure-activedirectory-library-for-js/blob/dev/lib/adal.js
*
* @return {String} A new guid.
*/
function newGuid() {
var guidHolder = "xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx";
var hex = "0123456789abcdef";
var r = 0;
var guidResponse = "";
for (var i = 0; i < 36; i++) {
if (guidHolder[i] !== "-" && guidHolder[i] !== "4") {
// each x and y needs to be random
r = (Math.random() * 16) | 0;
}
if (guidHolder[i] === "x") {
guidResponse += hex[r];
} else if (guidHolder[i] === "y") {
// clock-seq-and-reserved first hex is filtered and remaining hex values are random
r &= 0x3; // bit and with 0011 to set pos 2 to zero ?0??
r |= 0x8; // set pos 3 to 1 as 1???
guidResponse += hex[r];
} else {
guidResponse += guidHolder[i];
}
}
return guidResponse;
}
```