JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" placeholder="1">
    <input type="text" placeholder="2">
    <input type="text" placeholder="3">
        <br/>
    <input type="radio">
        <input type="checkbox">
            <input type="submit" name="sign_in" value="Registrieren" class="success_button" onclick="alert(onbeforeunload());" />

JavaScript

window.onbeforeunload = function (e) {
        var input = document.getElementsByTagName("input");
        var returnArr = [];
        for (var i = 0; i < input.length; i++) {
             if((input[i].value == "" && input[i].type == 'text') || (input[i].checked == false && input[i].type != 'text') ){
            returnArr.push(false);
        }
            else{
                returnArr.push(true);
            }
        };
        returnArr = sort_unique(returnArr);
        if(returnArr[0] == false && returnArr[1] == true)
        return "You have to fill the whole form.";
    };

    function sort_unique(arr) {
        arr = arr.sort(function (a, b) { return a*1 - b*1; });
        var ret = [arr[0]];
        for (var i = 1; i < arr.length; i++) { // start loop at 1 as element 0 can never be a duplicate
        if (arr[i-1] !== arr[i]) {
            ret.push(arr[i]);
        }
        }
        return ret;
    }