Javascript prototype inheritance issue
<p>I'm having an issue with prototype inheritance and I can't find out why it's not working properly.</p>

<p>The snippet is this:</p>

<pre><code>function Field(newKey, val) {
    this.key = newKey || "";
    this.value = val || "";
}

Field.prototype.isEmpty = function() {
    return this.value === undefined || this.value === null || this.value === "";
};

function DoubleField(newKey, val) {
    this.base = Field;
    this.base(newKey, val);
}
DoubleField.prototype = new Field();
DoubleField.prototype.constructor = DoubleField;

function IntegerField(newKey, val) {
    this.base = DoubleField;
    this.base(newKey, val);
}
IntegerField.prototype = new DoubleField();
IntegerField.prototype.constructor = IntegerField;

var f = new Field('keyfield', 'valField');
var d = new DoubleField('keydouble', 'valDouble');
var i = new IntegerField('keyinteger');

var res = f.isEmtpy();
</code></pre>

<p>The call to f.isEmpty is failing? Why? Calls to d.isEmpty or i.isEmpty work just fine as expected.</p>

<p>I cannot realize what I'm doing wrong. Any help would be much appreciated!</p>
