'undefined' appearing in alert
<p>I am using Javascript to validate some code, and it works fine, but whenever I call alert to show the errors, at the beginning of the alert message I get 'undefined'.  So when I should expect the alert to show 'Please enter a Low Target', instead I get 'undefinedPlease enter a Low Target'.  Can somebody tell me what is wrong with my code?</p>

<pre><code>//validation
        var lowTarget;
        var highTarget;
        var errorList;
        var isValid = true;

        lowTarget = $('input[name="txtLowTarget"]').val();
        highTarget = $('input[name="txtHighTarget"]').val();

        if (lowTarget == "") {
            errorList += "Please enter a Low Target\n";
            isValid = false;
        }
        else {
            if (isNumeric(lowTarget) == false) {
                errorList += "Low Target must be numeric\n";
                isValid = false;
            }
        }

        if (highTarget == "") {
            errorList += "Please enter a High Target\n";
            isValid = false;
        }
        else {
            if (isNumeric(highTarget) == false) {
                errorList += "High Target must be numeric\n";
                isValid = false;
            }
        }

        if (isValid == true) {
            if (!(parseFloat(highTarget) &gt; parseFloat(lowTarget))) {
                errorList += "High Target must be higher than Low Target\n";
                isValid = false;
            }
        }

        if (isValid == false) {
            alert(errorList);
        }
</code></pre>
