SOURCE

function unique(arr) {
    if (!Array.isArray(arr)) {
        console.log('type error!')
        return
    }
    var array = [];
    for (var i = 0;i < arr.length; i++) {
        if(!array.includes(arr[i])) {
            array.push(arr[i]);
        }
    }
    return array
}

var arr = [1,1,2,3,null,null]
console.log(unique(arr))