Adding array to json and post ajax cart

I’ve basically got an ajax call where I’m adding multiple products to cart via one event.

The products are added to an array like this

let crossSellSelected = false
    const crossSellElements = productFormContainer?.querySelectorAll('.crossSell__Item.selected');
    let crossSell
    let crossSellArray = new Array();

    if (crossSellElements) {
      crossSellSelected = true
      crossSellElements.forEach(crossSellElement => {
          const selectedCrossSell = crossSellElement.querySelector('.crossSell__Swatch.selected .crossSell__Radio').value
              crossSell = {
                  id: selectedCrossSell,
                  quantity: 1
                  
           }
           crossSellArray.push(crossSell)
      })
  }

They are then converted to json and added in an ajax call like this

const response = await fetch(
      `${window.themeVariables.routes.cartAddUrl}.js?sections=mini-cart`,
      {
        body: JSON.stringify({
          items: crossSellSelected
            ? [crossSellArray, product]
            : [product],
          attributes: {
            _ttp: getCookie("_ttp"),
            ttclid: getCookie("ttclid"),
          },
        }),
        method: "POST",
        headers: {
          "X-Requested-With": "XMLHttpRequest",
          "Content-Type": "application/json",
        },
      },
    );

But the post method fails in this case. However if I change it to

const response = await fetch(
          `${window.themeVariables.routes.cartAddUrl}.js?sections=mini-cart`,
          {
            body: JSON.stringify({
              items: crossSellSelected
                ? [crossSellArray[0], product]
                : [product],
              attributes: {
                _ttp: getCookie("_ttp"),
                ttclid: getCookie("ttclid"),
              },
            }),
            method: "POST",
            headers: {
              "X-Requested-With": "XMLHttpRequest",
              "Content-Type": "application/json",
            },
          },
        );

it works. So it looks to me the issue is that it is an array but I’m not really sure why that is an issue and how to fix it. Any help would be greatly appreciated

>Solution :

I think using spread operator will do the job:

change this:

[crossSellArray, product]

To this:

[...crossSellArray, product]