more textarea insert value in only one

[not finished yet]

by slawe

HTML

<input type="checkbox" name="custom7" value="1" id="custom7" checked="checked">
<input type="text" id="textfield_id_0" value="1234567890">
<textarea id="textfield_id_1"></textarea>
<textarea id="textfield_id_2"></textarea>
<textarea id="textfield_id_3"></textarea>
<input type="hidden" id="textfield_id_4" value="1234567890">
<input type="hidden" id="hiddenfield_id" value="">

JavaScript

function buildhidden() {    
    var joinedvalues = 'text::' + document.getElementById('textfield_id_1').value + ','+ 
        'description::' + document.getElementById('textfield_id_2').value + ',' + 
        'name::' + document.getElementById('textfield_id_3').value + ',' +
        'id::' + document.getElementById('textfield_id_4').value + ',' +
        'active::' + document.getElementById('custom7').value;
    
    document.getElementById("hiddenfield_id").value = joinedvalues;
}

var x = document.getElementById("custom7");

x.addEventListener('change', function() {
    this.value ^= 1;
    document.getElementById("hiddenfield_id").value += this.value;
});

var first = document.getElementById('textfield_id_0');

if (first.value != '') {
    buildhidden();
} else {
    alert('fuck');
}

addEventListener('keyup', buildhidden);

//<?php
//$fruits = '0101010101010101010101010';
//echo substr($fruits, -1); 
//?>