为什么要使用Math.random（）及其作用

Question

Here I have a piece of code:

if (Math.random() < 0.80) {
    var img = $('#img');
}

$(document).mousemove(function(event) {
    var mouse_x = event.pageX;
    var mouse_y = event.pageY;
    $(img).css({
        'top': mouse_y+'px', 
        'left': mouse_x+'px',
        'display' : 'block',
        'position' : 'absolute'
    }); 
});

In this script, I don't understand what the if (Math.random() < 0.80) line is doing. And how does Math.random() gets its value, from where?

Answer 1

From developer.mozilla.org

The Math.random() function returns a floating-point, pseudo-random number in the range [0, 1) that is, from 0 (inclusive) up to but not including 1 (exclusive), which you can then scale to your desired range. The implementation selects the initial seed to the random number generation algorithm; it cannot be chosen or reset by the user.

In your code Math.random() generates a psudeo random number < 1 and then if that number is less than 0.80 the code inside the if block executes.