JavaScript: Why is this causing an infinite loop?

Question

I'm trying to find the largest number by finding the point at which the JavaScript number line wraps around. E.g., if it could only hold numbers 0, 1, 127 then I'd find 127 by using the fact that "127 + 1 = 0". So I made a function

function getLargestNumber ( ) 
{
   var somethingBig = 12939123, last = (somethingBig - 1);
   while ( ++somethingBig > ++last );
   return last;
}

but that's causing an infinite loop (or crashing the browser for some other reason).

Is there anything wrong with the logic of my function?

score 2 · Answer 1 · edited May 23 '17 at 12:03

2

JavaScript numbers are always stored as floating point numbers, not integers like in C. Floating point numbers don't "wrap around" the same way that a C integer will.

This might help you What is JavaScript's highest integer value that a Number can go to without losing precision?

edited May 23 '17 at 12:03

Community

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answered Nov 23 '15 at 22:34

Ben Rubin

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It should eventually stop when the numbers get so large that the precision doesn't include the lowest digit. – Barmar Nov 23 '15 at 22:40
@Barmar if it eventually stops, then should I be looking for the point at which `somethingBig + 1 = somethingBig`? – Subpar Web Dev Nov 23 '15 at 22:43
True, but that's not the largest value that a JavaScript number can be. It's just the largest number before integer precision starts getting lost. – Ben Rubin Nov 23 '15 at 22:44
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@SubparWebDev Correct. At 1 million iterations/sec, that will take about 285 years. – Barmar Nov 23 '15 at 22:44
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@SubparWebDev What are you trying to accomplish? – Ben Rubin Nov 23 '15 at 22:44

score 1 · Answer 2 · answered Nov 23 '15 at 22:34

1

It looks like the while loop will run forever because you are always incrementing both somethingBig and last, therefore somethingBig will always be larger:

while ( ++somethingBig > ++last );

I'm not sure exactly what you are trying to accomplish but this would be the cause of the infinite loop.

Hope this helps!

answered Nov 23 '15 at 22:34

Matthew Spencer

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He's expecting `somethingBig` to overflow and wrap around to 0 before `last` does. – Barmar Nov 23 '15 at 22:37
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@Barmar Not necessarily to zero ... maybe to the smallest negative number. – Subpar Web Dev Nov 23 '15 at 22:40
Thanks @Barmar, unfortunately it's taking a little longer than expected :) – Matthew Spencer Nov 23 '15 at 22:40
@SubparWebDev Right, I was thinking of unsigned numbers. – Barmar Nov 23 '15 at 22:41

JavaScript: Why is this causing an infinite loop?

2 Answers2