JavaScript原型继承问题

Question

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

The snippet is this:

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();

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

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

Answer 1

The error is in the last line of code:

var res = f.isEmtpy();//That's what you wrote

The correct is:

var res = f.isEmpty();

Answer 2

You haven't said how it's failing. As I noted in a comment, at the very least there's a typo: var res = f.isEmtpy(); should be var res = f.isEmpty();

But beyond that (which I assume is just a typo in the question , not the code), note that this line in your derived constructors:

this.base = DoubleField;

...will not work as you intend. Consider the case of IntegerField . You call new IntegerField and it sets this.base to DoubleField and calls it. But the DoubleField constructor then sets this.base to Field . So you have an IntegerField instance with a base property pointing to Field , not DoubleField .

You can't use instance properties to trace lineage. It works for a parent/child situation, but you run into the "grandchild" problem described above.

I recommend using one of the inheritance helper scripts out there. There are several, including my own Lineage . These scripts help you build inheritance hierarchies, handling the plumbing for you so you can focus on building whatever it is you're trying to build. We only need them temporarily; JavaScript is getting syntactic sugar to help with this in the next version (but of course, it'll be years before we see widespread support of it, once it's actually finalized).