Math.random() in javascript is able to return 1, right? Which means if I would be to use it to get a random index on my array the following code could fail:

var arr = [ 1, 2, 3 ],
    index = Math.floor(Math.random() * arr.length);

// index could be 3?
alert(arr[index]);

Could someone shed some light on this?

Math.random() in javascript is able to return 1, right? Which means if I would be to use it to get a random index on my array the following code could fail:

var arr = [ 1, 2, 3 ],
    index = Math.floor(Math.random() * arr.length);

// index could be 3?
alert(arr[index]);

Could someone shed some light on this?

• javascript
• arrays
• random

• 5 Did you even read the documentation you linked to? The answer is right there on the first sentence. – R. Martinho Fernandes Commented Apr 7, 2011 at 11:54
• I did, but I wasn't sure what exclusive meant. – Kevin Commented Apr 7, 2011 at 11:56
• 1 @Kevin: Then this question is off-topic and would have been better served on english.stackoverflow., posed purely as a language question. – Lightness Races in Orbit Commented Apr 7, 2011 at 11:58
• wanna clear it in the separate topic? :) No, it can't be. – Emmerman Commented Apr 7, 2011 at 11:58
• @Emmerman: Stack Overflow is not a forum; it has questions and answers, not "topics". – Lightness Races in Orbit Commented Apr 7, 2011 at 11:58

4 Answers 4

The link you posted takes me to a site that says:

Returns a pseudo-random number in the range [0,1) — that is, between 0 (inclusive) and 1 (exclusive). The random number generator is seeded from the current time, as in Java.

"inclusive" means the value is part of the range, whereas "exclusive" means that the value is not part of the range.

So Math.random() returns a value from 0 to just-less-than 1.

No, it returns from 0 inclusive to 1 exclusive

See https://developer.mozilla/en/JavaScript/Reference/Global_Objects/Math/random

Note however the caveat in that page:

Note that as numbers in JavaScript are IEEE 754 floating point numbers with round-to-nearest-even behavior, these ranges, excluding the one for Math.random() itself, aren't exact, and depending on the bounds it's possible in extremely rare cases (on the order of 1 in 2⁶²) to calculate the usually-excluded upper bound.

For these purposes, though, you should be fine.

I am pretty sure the number returned by

Math.random()

is smaller than 1 but equal or greater than zero.

between 0 (inclusive) and 1 (exclusive) - cannot be 1

Your code is all right