Getting the result from a fetch function in javascript

I think that you're getting confused with promises. The fetch function returns a promise and not the data.

I recommend that you read this modern tutorial: https://javascript.info/fetch

This is a very trivial function that gets the url and returns the response data in json:

async function callApi(url) {
  const response = await fetch(url);
  const data = await response.json();

  return data;
}

let data = await callApi('https://api.website.net/?');
// do whatever you want with the data

When dealing with primises there are 2 syntaxes, the old .then() and the new async/await. Using async/await is better, in general, so I recommend using the async/await syntax. With .then() it would look like this:

// Not recommended alternative

fetch('https://api.website.net/?')
  .then(response => response.json())
  .then(data => {
    // do whatever you want with the data
  });

Try this:

fetch(....)
    .then(response => response.json())
    .then(data => /* use data */);

Try this approach:

Calling a function in your code.

document.addEventListener("click", async function(e) {
        if (e.target) {
            if (Boolean(e.target.closest('span')?.classList.contains('addProduct'))) {
                e.preventDefault();
                if ('disabled' in e.target) e.target.disabled = true;
                productId = parseInt(e.target.closest('tr')?.getAttribute('id').replace('product-', '')) || 0;

                let form = new FormData();
                form.append('productId', productId);
                let result = await callApi('url-here', form);
                console.log(result);

                if ('disabled' in e.target) e.target.disabled = false;
            }
        }
    });

Wrapper function (needed for processing Promise)

async function callApi(url, form, method = 'POST', type = 'json') { // text, json, blob, formData, arrayBuffer
        try {
            return await callApiSafe(url, form, method, type);
        } catch (error) {
            console.error('Error in callApiSafe:', error);
            return null;
        }
    }

Data acquisition function

async function callApiSafe(url, form, method, type) {
        try {
            const response = await fetch(url, {
                method: method,
                cache: "no-store",
                headers: { "X-Requested-With": "XMLHttpRequest" },
                body: form
            });

            if (!response.ok) {
                if (response.status === 404) throw new Error('404, Not found');
                if (response.status === 500) throw new Error('500, Internal server error');
                throw new Error(`HTTP error: ${response.status}`);
            }

            switch (type) {
                case 'text':
                    return await response.text();
                case 'json':
                    return await response.json();
                case 'blob':
                    return await response.blob();
                case 'formData':
                    return await response.formData();
                case 'arrayBuffer':
                    return await response.arrayBuffer();
                default:
                    throw new Error('Unsupported response type');
            }
        } catch (error) {
            console.error('Request failed:', error);
            throw error;
        }
    }