Explain Math.floor(Math.random())

Question

I have seen many places using Math.floor() and Math.random()

like below

$('a.random-color').hover(function() { //mouseover
    var col = 'rgb(' + (Math.floor(Math.random() * 256)) + ',' + (Math.floor(Math.random() * 256)) + ',' + (Math.floor(Math.random() * 256)) + ')';
    $(this).animate({
      'color': col,
      'paddingLeft': '20px'
    },1000);
  },function() { //mouseout
    $(this).animate({
      'color': original,
      'paddingLeft': '0'
    },500);
  });
});

Why used Math.floor() and Math.random()?

Answer 1

Math.random will give you a floating point number between 0 (inclusive) and 1 (exclusive).

Multiplying that by 256 will give you a number in the range 0 (inclusive) through 256 (exclusive), but still floating point.

Taking the floor of that number will give you an integer between 0 and 255 (both inclusive).

It's the integer from 0 to 255 that you need to construct RGB values like rgb(72,25,183).

Answer 2

It seems a random color is desired - one with each component random between 0 and 255.

Math.random() returns a random number on [0,1) (ie it may be exactly zero or up to but not including one).

Multiplying that random value by 256 gives a random number on the range [0,256) (ie it may be 255.99, but never 256). Almost there, but not quite.

Math.floor() rounds the number downwards to the nearest whole integer, making the result an integer on [0,255] as desired.