How can I randomly generate a number between -0.5 and 0.5?

Quoting MDN on Math.random

.

The function Math.random()

returns a pseudo-random floating point number in the range [0, 1), which ranges from 0 (inclusive) to, but not including 1 (excluding)

So, create random numbers between 0 and 1 and subtract 0.5 from it.

Math.random() - 0.5

      

        
        
        
      

    

Note. Math.random()

does not expect any parameters. Thus, you cannot specify a range.

If you want to generate random numbers between a range use the snippet examples on MDN.

// Returns a random number between min (inclusive) and max (exclusive)
function getRandomArbitrary(min, max) {
  return Math.random() * (max - min) + min;
}

      

        
        
        
      

    

Demo with range

function getRandomArbitrary(min, max) {
  return Math.random() * (max - min) + min;
}


function appendToResult(value) {
  document.getElementById("result").innerHTML += value + "<br />";
}

document.getElementById("result").innerHTML = "";

for (var i = 0; i < 10; i += 1) {
  appendToResult(getRandomArbitrary(-0.35, 0.35));
}
      

        
        
        
      

    

<pre id="result" />
      

        
        
        
      

    

This works because the expression

Math.random() * (max - min) + min

      

        
        
        
      

    

will be evaluated as follows. Firstly,

Math.random()

      

        
        
        
      

    

will evaluate to get a random number between 0 and 1 and then

Math.random() * (max - min)

      

        
        
        
      

    

it will be multiplied by the difference between the value max

and min

. In your case, say max

- 0.35

, and min

- -0.35

. So it max - min

becomes 0.7

. So, Math.random() * (max - min)

gives a number in the range 0

and 0.7

(because even if it Math.random()

does 1

, when you multiply it by 0.7

, the result will be 0.7

). Then you do

Math.random() * (max - min) + min

      

        
        
        
      

    

So, for your case, what basically becomes,

Math.random() * (0.35 - (-0.35)) + (-0.35)
(Math.random() * 0.7) - 0.35

      

        
        
        
      

    

So, if it (Math.random() * 0.7)

generates any value greater than 0.35

since you subtract from it 0.35

, it becomes some number between 0

and 0.35

. If it (Math.random() * 0.7)

generates a value less 0.35

as you subtract from it 0.35

, it becomes a number between -0.35

and 0

.