SOURCE

var arr = [1,null,'3',3,'3',null,[1,2],[1,2]]
function fun(arr){
    let map = new Map(),res=[],count=0
    for(let i=0;i<arr.length;i++){
        let temp = arr[i]
        if(map.get(temp)){
            map.set(temp,map.get(temp)+1)
        }else{
            map.set(temp,1)
        }
        count=count>map.get(temp)?count:map.get(temp)
    }
    console.log(map)
    for(let item of map){
        console.log(item,'iii',count)
        if(item[1]===count){
            res.push(item[0])
        }

    }
    return res
}
console.log(fun(arr),'res')