javascript - What makes this pseudo-GUID generator better than math.random()? - Stack Overflow

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I found the question here:

Create GUID / UUID in JavaScript?

The answer provides the following JS:

function S4() {
   return (((1+Math.random())*0x10000)|0).toString(16).substring(1);
}

function guid() {
   return (S4()+S4()+"-"+S4()+"-"+S4()+"-"+S4()+"-"+S4()+S4()+S4());
}

Now, Some of this seems silly to me. Why so much repetition? I planned on using this to name file being uploaded to my server so that they didn't override each other. This doesn't look like it will always generate a unique number.

What is the above codes benefit over just naming the file math.random(). It doesn't even change the seed.

Sorry, I've never worked with GUID / UUID ever and some of the code doesn't really make any sense to me...

CLARIFICATION

A lot of people aren't answering the question like I asked it. A lot of people are explaining that GUID isn't always unique, blah blah blah. That isn't what I'm asking. I'm asking, what was the point of using it over just math.random().

Joe seems to have given the best answer for me in the ments.

I found the question here:

Create GUID / UUID in JavaScript?

The answer provides the following JS:

function S4() {
   return (((1+Math.random())*0x10000)|0).toString(16).substring(1);
}

function guid() {
   return (S4()+S4()+"-"+S4()+"-"+S4()+"-"+S4()+"-"+S4()+S4()+S4());
}

Now, Some of this seems silly to me. Why so much repetition? I planned on using this to name file being uploaded to my server so that they didn't override each other. This doesn't look like it will always generate a unique number.

What is the above codes benefit over just naming the file math.random(). It doesn't even change the seed.

Sorry, I've never worked with GUID / UUID ever and some of the code doesn't really make any sense to me...

CLARIFICATION

A lot of people aren't answering the question like I asked it. A lot of people are explaining that GUID isn't always unique, blah blah blah. That isn't what I'm asking. I'm asking, what was the point of using it over just math.random().

Joe seems to have given the best answer for me in the ments.

• javascript
• guid
• uuid

Share Improve this question Follow edited May 23, 2017 at 12:19 CommunityBot 111 silver badge asked Oct 29, 2011 at 17:47 FreesnöwFreesnöw 32.2k3131 gold badges9494 silver badges140140 bronze badges 13

• 4 Even that answer says: "do you want actual GUIDs, or just random numbers that look like GUIDs?" Because those aren't real GUIDs. Also from the original thread: "There's no way to generate real GUIDs in Javascript, because they depend on properties of the local puter that browsers do not expose." so my question is: do you need a GUID? Or just a random filename? There's nothing magical about a GUID as a consumer, it is not how it looks, it's how it's generated. For a random filename, using clock ticks + a random number would be (at least as) effective... – Joe Commented Oct 29, 2011 at 17:51
• 7 Someone wanted a thing that looked like a GUID, but was easier to get. Why do people buy fake Rolexes? – ObscureRobot Commented Oct 29, 2011 at 17:56
• 3 There is no guarantee that a GUID is unique. A GUID is semi-unique. Even considering not-well-formatted GUID, there are only 2^128 possible GUID. Thanks to the Birthday Paradox, if you generate 2^64 of them you have like 50% of generating a duplicate. – xanatos Commented Oct 29, 2011 at 18:31
• 1 @xanatos: that is true, though 2^64 is eighteen quintillion and change... – Joe Commented Oct 29, 2011 at 19:16
• 1 @Joe Still it isn't "unique". You couldn't "sync" all the world on a single "unchecked" GUID. In the end a GUID isn't a Globally Unique ID. – xanatos Commented Oct 29, 2011 at 19:20

 |  Show 8 more ments

2 Answers 2

Sorted by: Reset to default 4

Even that answer says: "do you want actual GUIDs, or just random numbers that look like GUIDs?" Because those aren't real GUIDs. Also from the original thread: "There's no way to generate real GUIDs in Javascript, because they depend on properties of the local puter that browsers do not expose." so my question is: do you need a GUID? Or just a random filename? There's nothing magical about a GUID as a consumer, it is not how it looks, it's how it's generated. For a random filename, using clock ticks + a random number would be (at least as) effective...

In your case, no reason. But if you have JS code that is talking to something that is expecting a GUID or something in that form, you would need to pass something of a similar format since you can't generate the real thing in pure JS.

To make an example, even .NET GUID aren't "tied" to a machine. .NET uses v4 UUID for Guid.NewGuid() ( Simple proof that GUID is not unique ) so 122 bits of randomness plus 6 fixed bits. By reading here Are GUID collisions possible? it seems SQL Server uses a full random number (but I don't have an SQL Server to check... checked... It seems to use the same format of v4. There is a "fixed" 4 and the next "block" always begins with 8, 9, A, B. So 122 bits of randomness).

So what problem do you have with this random-generator that gives 128 bits of randomness in a "know" format? Would you prefer to see a non-fixed-format 128 bit number? Wow! Very practical... Here... Take one 43438471087229589138546501885363994076 (it's a GUID converted to its numericvalue, base 10). Now, how would you like to save it do your favourite DB? VARCHAR or NUMERIC? Isn't it easier to pack it in a GUID?

As a sidenote... What is the advantage over math.random()? Well... Over a single use of math.random() it's quite clear :-) I don't know how many bits of randomness a single call of math.random has, but at max it's 63 bits (a number in JS is 64 bits, but 1 bit is for the sign). And probably many many less.

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