javascript动态原型

Question

I want extend a new JS object while creation with other object passing a parameter. This code does not work, because I only can extend object without dynamic parameter.

otherObject = function(id1){
    this.id = id1;
};

otherObject.prototype.test =function(){
    alert(this.id);
};

testObject = function(id2) {
    this.id=id2;
};

testObject.prototype = new otherObject("id2");/* id2 should be testObject this.id */


var a = new testObject("variable");
a.test();

Any suggestion?

Answer 1

Apart from the obvious syntax error, the correct JavaScript way of inheritance is this:

// constructors are named uppercase by convention
function OtherObject(id1) {
    this.id = id1;
};
OtherObject.prototype.test = function() {
    alert(this.id);
};

function TestObject(id2) {
    // call "super" constructor on this object:
    OtherObject.call(this, id2);
};
// create a prototype object inheriting from the other one
TestObject.prototype = Object.create(OtherObject.prototype);
// if you want them to be equal (share all methods), you can simply use
TestObject.prototype = OtherObject.prototype;


var a = new TestObject("variable");
a.test(); // alerts "variable"

You will find lots of tutorials about this on the web.

Answer 2

I do not understand exactly what you desire, but

otherObject.prototype.test = function () { 
    alert(this.id); 
}; 

would be correct.

And this

testObject.prototype = new otherObject(id2);

will not work unless id2 is set before.

Try the following

   var OtherObject = function () {
   }
   OtherObject.prototype.test = function () {
       alert (this.id);
   }

   var TestObject = function (id) {
       this.id = id;
   }
   TestObject.prototype = new OtherObject ();

   var a = new TestObject("variable");
   a.test ();

Answer 3

Fixed your code

otherObject = function(id1){
    this.id = id1;
};

otherObject.prototype.test =function(){
    alert(this.id);
};

testObject = function(id2) {
    this.id=id2;
};

testObject.prototype = new otherObject("id2");/* id2 should be testObject this.id */


var a = new testObject("variable");
a.test();

Answer 4

testObject = function(id2) {
    otherObject.call(this, id2); // run the parent object constructor with id2 parameter
    this.id=id2;
};

testObject.prototype = new otherObject(); // no id2 parameter here, it doesn't make sense

Note that while creating an instance of testObject , the constructor of otherObject is called twice - once to create the prototype and once to initialise the object.

To prevent duplicate initialisation, we can halt the constructor immediately when we are only using it to create the prototype.

otherObject = function(id1){
    if (typeof id1 == 'undefined') {
        /* as there is no parameter, this must be the call used to create
         * the prototype. No initialisation needed here, we'll just return.
         */
        return;
    }
    this.id = id1;
};

PS Please use capital camelcase with objects.