If you just want your code to work, simply remove the extra i++ and change the += line slightly, like this:

function products() {
    var items = _rsq.executor.sentWave.order_properties.order_items;
    var product = '';
    for (var i = 0; i < items.length; i++) {
        product +=
            _rsq.executor.sentWave.order_properties.order_items[i].name + ',';
    }
    console.log(product);
}
products();

However, I would suggest using one of these three alternatives to for loops. If you use methods like these, you will altogether avoid the issue of accidentally writing i++ twice, and your code will be more readable in general.

const items = [
    {
        id: 0,
        name: 'Sleep',
        price: 24.99,
    },
    {
        id: 1,
        name: 'Revive',
        price: 24.99,
    },
];

const productsUsingReduce = () => {
    return items.slice(1).reduce((result, item) => {
        return `${result}, ${item.name}`;
    }, items[0].name);
};

const productsUsingFor = () => {
    let result = items[0].name;
    items.slice(1).forEach((item) => {
        result += `, ${item.name}`;
    });
    return result;
};

const productsUsingJoin = () => {
    return items
        .map((item) => {
            return item.name;
        })
        .join(', ');
};

console.log(productsUsingReduce());
console.log(productsUsingFor());
console.log(productsUsingJoin());

The only* reason I have found where you actually need a for loop is if you are using await to run multiple asynchronous calls in a specific order.

*If you are forced to use old version(s) of javascript/node for some strange reason, then you may end up having to use for loops.

When you do x += 2, it is the same as if you did x = x + 2, so the second product in product += product has to be removed in order to get the correct result.

You can shorten the _rsq.executor.sentWave.order_properties.order_items to just items, because you declared it before the for-loop as items and you never changed it.

Also, you added extra i++, which is not necessary, because you already increment i in the for-loop, specifically at the end of this line: for (var i=0; i < items.length; i++).

This should work:

function products() {
    var items = _rsq.executor.sentWave.order_properties.order_items;
    var product = "";

    for (var i=0; i < items.length; i++) {
        product += items[i].name + ",";
    }

    console.log(product);
}

products();