var randomizer = function () {
    return Math.random() > 0.02;
};

var randomizer = {
    _ratio: 0.02,  // default ratio
    setRatio: function (falses, total) {
        this._ratio = falses / total;
    },
    getResult: function () {
        return Math.random() > this._ratio;
    }
};

randomizer.getResult();
randomizer.setRatio(60, 1000); // 60 out of 10000

function mostlyFalse() {
    return Math.random() <= 0.05;
}

//Get random number from 1 to 20, if equals 20 return false
if (Math.floor((Math.random()*20)+1) == 20)
{
  return false;
}
else{
  return true;
}

function suchRandom(chanceInPercent){
    return Math.random() > chanceInPercent/100;
}

console.log(suchRandom(2)); //because 1000/50 = 20 true / 1000 calls = 2%

function foo () { return Math.random() > 0.02; }

// 1 / 50 == 0.02, so if it's less than 0.02 return false

var num = Math.floor((Math.random()*50)+1);

function chance( c = 0.5 ){
    return Math.random() < c;
}

// Block has a 60% chance of being executed.
if( chance( .6 ) ){
    //Do stuff
}

// Or, to solve your problem specifically
var c = chance( 49 / 50 );

var c = 0;
function f()
{
    return ++c % 50 == 0 ? false : true;
}