Cannot read .length property of undefined (sometimes)

I’m trying to get checkbox values as an array, so later I can use them as part of a GET request.

Checkboxes are like this:

<input type="checkbox" name="diet" value="gluten"/>
<input type="checkbox" name="diet" value="vegetarian"/>
<input type="checkbox" name="diet" value="vegan"/>

I’ve (almost) catch them with this:

function getCheckboxValues(form){
    var values= [];
    var diet = form.diet;

    for (let i = 0; i <diet.length; i++) {
        if (diet[i].checked) {
            values.push(diet[i].value);
        }
    }
    return values;
}

Then I want to get them into:

var reqDiet = []; //global

So I can merge it into my get params

document.getElementById("submit-btn").addEventListener("click", function (){
    reqParams = "&diet="+reqDiet+",
    getRecipe(ASK_RECIPE, reqParams)
})

async function getRecipe(reqRecipe, reqParameters){
    let response = await fetch(reqRecipe+API_KEY+reqParameters);
    recipe = await response.json();

    let recipeStored = JSON.stringify(recipe);
    sessionStorage.setItem("recipe",recipeStored);
};

The problem is, if i run the getCheckboxValues("form_id") in console, it returns the correct array with items, but if i do something like

reqDiet = getCheckboxValues("form_id");

Console says :

Uncaught (in promise) TypeError: Cannot read properties of undefined (reading ‘length’)
at getCheckboxValues

What I can understund from this is that my program is not recognizing “diet.length” as an array when I do the last one, but do recognize it when i run the function by itself.
I prefer to do this in vanilla JS since I’m new here and trying to do this small projects.

P.S. any feedback in the post is apreciated since is my first one and i think is pretty large :/ sorry about that