probability_AB.js

//The sum of 'weights' elements should be equal 1.
let weights = [0.1, 0.9]; // probabilities

//The possible results
let results = [0, 1]; // values to return

function getRandom () {
    let num = Math.random(),
        s = 0,
        lastIndex = weights.length - 1;

    for (let i = 0; i < lastIndex; ++i) {
        s += weights[i];
        if (num < s) {
            return results[i];
        }
    }

    return results[lastIndex];
};

let number = getRandom()
let element = document.getElementById('box')

number === 1 ? element.setAttribute("class", "square-box blue") : 
  element.setAttribute("class", "square-box red")

console.log(number)

let timer = window.setTimeout(() => {
  location.reload(true)
}, 1000)