Use a parameter in Promise javascirpt

Question

I have a function to handle the scroll in puppeteer.

async function autoScroll(page, custom_scroll = null) {
        await page.evaluate(async () => {
            await new Promise((resolve, reject) => {
                var totalHeight = 0;
                var distance = 100;
                var timer = setInterval(() => {
                    var scrollHeight = document.body.scrollHeight-1200;
                    window.scrollBy(0, distance);
                    totalHeight += distance;

                    if (totalHeight >= scrollHeight) {
                        clearInterval(timer);
                        resolve();
                    }
                }, 100);
            });
        });
    }

I want to pass custom_scroll and use it in document.body.scrollHeight-1200 But if I use document.body.scrollHeight-custom_scroll I get an error