Why does !search equate to falsey but null/undefined don't?

Hi all, I was wondering if someone could maybe put the concept of falsey into simpler terms for me. In the video Guil was able to set the first conditional statement to falsey by typing "!search", which makes sense because I understand that putting "!" in front of a variable means "not". What I don't understand is why when I try and change the conditional to something like "search === undefined" or "search === null" all that pops up is the html, but no values have been plugged in. This tells me the program doesn't recognize these conditionals as falsey (unless I'm mistaken) even though I know they're falsey values.

Thanks for any feedback! This question comes from the "Search for a Value in an Array" video.

const inStock = ['pizza', 'cookies', 'eggs', 'apples', 'milk', 'cheese', 'bread', 'lettuce', 'carrots', 'broccoli', 'potatoes', 'crackers', 'onions', 'tofu', 'limes', 'cucumbers'];
const search = prompt('Search for a product.');

let message;

if(!search){
 message = `<strong>In stock:</strong> ${inStock.join(', ')}`; 

}else if (inStock.includes(search.toLowerCase())) {
    message = `Yes we have <strong>${search}!</strong>It's #${inStrock.indexOf(search.toLowerCase())+ 1} on the list!`;
    }else{
     message = `Sorry we do not have <strong>${search}!</strong>`;
    }


document.querySelector("main").innerHTML = `<p>${message}</p>`;