Question

I’m having trouble with a JavaScript array adding an extra undefined object after pushing some strings to the array.

$(function() {
    var formTagArr = [];
    $( "button", "#start-button" ).click(function() {
            $.getJSON('http://127.0.0.1:8000/some_url/', function(data) {
                formTagArr.push(buildForm(data));
                console.log(formTagArr);
                displayForm(formTagArr);
            });
    return false;
    });

    function buildForm(data) {
        for (var i = 0; i < data.length; i++) {
            var html = "";
            var questionsTags = "<fieldset><p>" + data[i].question + "</p>";
            var answersTags = "";
            for (j = 0; j < data[i].answers.length; j++) {
                answersTags += "<input type='radio' name='" + data[i].qid + 
                    "' value='" + data[i].answers[j] + "' /" + ">" + 
                     data[i].answers[j] + "\n";
            }
            html = questionsTags + answersTags + "</fieldset>";
            formTagArr.push(html);
        }
    }

    function displayForm(arr) {
        if (arr.length === 0) {
            return false;
        }
        var info = arr.pop();
        $("#question-form").append(info[0]);
    }
}); 

/some_url/ returns this JSON:

[{"qid": 4, "question": "How many legs does a spider have?", "answers": ["4", "6", "8", "10"]}, {"qid": 2, "question": "When did Nigeria become a republic?", "answers": ["1960", "1961", "1962", "1963"]}, {"qid": 1, "question": "When did Nigeria gain independence?", "answers": ["1960", "1961", "1962", "1963"]}, {"qid": 3, "question": "How many days are in a leap year?", "answers": ["360", "362", "365", "366"]}]

and console.log(formTagArr); in the code above returns:

["<fieldset><p>How many l...e='10' />10\n</fieldset>", "<fieldset><p>When did N...963' />1963\n</fieldset>", "<fieldset><p>When did N...963' />1963\n</fieldset>", "<fieldset><p>How many d...'366' />366\n</fieldset>", undefined]

Because of this, displayForm() fails since info is undefined. Of course I could just use a conditional to skip the undefined object but I want to know exactly how the undefined object got there in the first place.
What did i do wrong?

Answer 1

formTagArr.push(buildForm(data));

Your buildForm function doesn’t return anything and the above code try to push the result of that function into the array. A function without a return statement would end up as undefined.

Seems like it should only be

buildForm(data)

As this function already pushed to the formTagArr array.