I get undefined at start of array in js

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, …]

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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.

>Solution :

As Musa advised in an earlier comment, you do not initialize X and Y, so basically what you are trying to do is the following:

 y += div.offsetTop+" ";
 x += div.offsetLeft + " ";

That translates to

undefined += div.offsetTop+" ";
undefined += div.offsetLeft + " ";

At the top you should define that X and Y are strings, for example:
let x = "";
let y = "";

Also, I would suggest using let and const instead of var, its 2022, so no need for var anymore.

Hope that helps