I made this code :

function get_coordinates(container) {
    var x;
    var y;
    var divs = container.getElementsByTagName('div');

    Array.from(divs).forEach(div => {
        y += div.offsetTop+" ";
        x += div.offsetLeft + " ";
    });
    const a = x + ";" + y;
    console.log(divs);
    return x+";"+y;
}

this is the console log :

HTMLCollection(10) [div#selector_1.selector, div#selector_2.selector, div#selector_3.selector, div#selector_4.selector, div#selector_5.selector, div#selector_6.selector, div#selector_7.selector, div#selector_8.selector, div#selector_9.selector, div#selector_10.selector, selector_1: div#selector_1.selector, selector_2: div#selector_2.selector, selector_3: div#selector_3.selector, selector_4: div#selector_4.selector, selector_5: div#selector_5.selector, …]

and this the return :

undefined0 0 0 0 0 0 0 0 0 0 ;undefined0 0 0 0 0 0 0 0 0 0

I dont understand why i get undefined at the start of both.