If subclassing a "class" in JavaScript is done like so:
var ParentClass = function() {
// something
};
var ChildClass = function() {
// something
};
ChildClass.prototype = new ParentClass();
... what should I do when the parent class has required parameters?
var ParentClass = function(requiredParameter) {
if (typeof requiredParameter === 'undefined') {
throw new TypeError("'requiredParameter' is required!");
}
};
var ChildClass = function() {
// something
};
ChildClass.prototype = new ParentClass();
// ^ Throws TypeError
Thanks.
This is how its done:
function Parent( a ) {
this.a = a;
}
function Child( a, b ) {
Parent.call( this, a ); // this is crucial
this.b = b;
}
Child.prototype = Object.create( Parent.prototype );
Child.prototype.constructor = Child;
Live demo: http://jsfiddle.net/ECCgt/ (analyze the instances in the console)
The way you're doing it
ChildClass.prototype = new ParentClass();
is a dirty hack which is broken and should be avoided. Use Object.create
to set up the inheritance relationship between the two prototype objects.
The second line
Child.prototype.constructor = Child;
is somewhat optional. We are correcting the constructor
property because we had to overwrite Child.prototype
in order to set up the inheritance. If you don't care about the constructor
property, just leave out that line.
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