Object undefined/HTMLElement being output using jQuery each

Question

I'm using jQuery's each/getJSON to loop through a my data.json file, grab and format the data, and output it onto my page inside the #output div.

It works as expected, except for the [object HTMLElement] that's strangely prepended to the rendered HTML.

$.getJSON( 'json/data.json', function ( data ) {
    var output;
    $.each( data, function ( index, entry ) {
        output += '<div class="entry" id="' + entry.date + '">';
        $.each( entry.exercises, function ( index, exercise ) {
            var i = 1;
            output += '<table class="exercise"><tr><th>' + exercise.name 
                   + '</th><th>Weight</th><th>Reps</th></tr>';
            $.each( exercise.stats, function ( index, stat ) {
                output += '<tr><td>SET ' + i++ + '</td><td><input type="number" placeholder="' 
                   + stat.weight + '"></td><td><input type="number" placeholder="' 
                   + stat.reps + '"></td></tr>';
            });
            output += '</table>';
        });
        output += '</div>';
    }); // each
    $( '#output' ).html( output );
}); // getJSON

data.json

[
    {
        "date" : 12252014,
        "exercises" : [
            {
                "name" : "Squats",
                "stats" : [
                    { "weight" : 135, "reps" : 5 },
                    { "weight" : 225, "reps" : 10 },
                    { "weight" : 315, "reps" : 15 }
                ]
            },
            {
                "name" : "Bench",
                "stats" : [
                    { "weight" : 435, "reps" : 20 },
                    { "weight" : 525, "reps" : 15 },
                    { "weight" : 615, "reps" : 30 }
                ]
            },
            {
                "name" : "Rows",
                "stats" : [
                    { "weight" : 735, "reps" : 35 },
                    { "weight" : 825, "reps" : 40 },
                    { "weight" : 915, "reps" : 45 }
                ]
            }
        ]

    }
]

HTML error:

<div id="output">
    "[object HTMLElement]"
    <div class="entry" id="12252014" style="display: none;">
        <table class="exercise">
            <tbody>
                <tr>
                    <th>Squats</th>
                    <th>Weight</th>
                    <th>Reps</th>
                </tr>
                <tr>
                    <td>SET 1</td>
                    <td><input type="number" placeholder="135"></td>
                    <td><input type="number" placeholder="5"></td>
                </tr>
                <tr>
                    <td>SET 2</td>
                    <td><input type="number" placeholder="225"></td>
                    <td><input type="number" placeholder="10"></td>
                </tr>
                <tr>
                    <td>SET 3</td>
                    <td><input type="number" placeholder="315"></td>
                    <td><input type="number" placeholder="15"></td>
                </tr>
            </tbody>
        </table>
        [...]
    </div>
</div>

Why's [object HTMLElement] being added to the output?

Answer 1

This for me looks like the variable output allready was in use before and filled with a html element.
In your case it's your wrapper div with the id output which is exposed as a global variable.

You should be able to overcome this by giving output en initial value:

var output = ''; 

Because using var does not cleanup an existing variable:

var output = 'Lorem ipsum';
var output;

alert(output)

Answer 2

Change

var output;

to

var output = '';

and this will fix your problem. There is probably an element in your HTML document with id="output", by default, elements with ids are exposed as global variables of the same name, so <a id="hello">text</a> will appear as window.hello.