Javascript原型方法“无法设置属性”

Question

I'm always getting Cannot set property 'saySomething' of undefined but why? Am I making a mistake somewhere?

var Person = new Object();

Person.prototype.saySomething = function ()
{ 
  console.log("hello"); 
};

Person.saySomething();

Answer 1

Debugging tip: You get this ..of undefined errors when you try to access some property of undefined.

When you do new Object() , it creates a new empty object which doesn't have a prototype property.

I am not sure what exactly are we trying to achieve here but you can access prototype of function and use it.

 var Person = function() {}; Person.prototype.saySomething = function() { console.log("hello"); }; var aperson = new Person(); aperson.saySomething();

Answer 2

The prototype property exists on functions, not on instantiated objects.

 var Person = new Object(); console.log(Person.prototype); // undefined var Person2 = function () {} console.log(Person2.prototype); // {}

This is useful because things put on the prototype of a function will be shared by all object instances created with that function (by using new ).

 var Person = function() {}; Person.prototype.saySomething = function() { console.log("hello"); }; console.log( new Person().saySomething === Person.prototype.saySomething // true. they are the same function );

If all you want is to add a method to the person object, there's no need for a prototype:

 var Person = {}; Person.saySomething = function() { console.log("hello"); }; Person.saySomething();

You can even use object literal syntax:

 var Person = { saySomething: function() { console.log("hello"); } }; Person.saySomething();

Answer 3

i was trying out some code thought of posting it, might help others.

<script>

        var MODULE = {};
        MODULE = (function (my) {
            my.anotherMethod = function () {
                console.log("hello ");
            };

            my.newMethod = function(){
                console.log("hi new method ");

            }
            return my;
        }(MODULE));


        MODULE.anotherMethod(); 
        MODULE.newMethod(); 


    </script>

And please not var MODULE ={}, if this is not initialized with {} then it give cannot set property.

Answer 4

You can check here .

var Person = function(name) {
  this.canTalk = true;
  this.name = name;
};
Person.prototype.greet = function() {
  if (this.canTalk) {
    console.log('Hi, I am ' + this.name);
  }
};

bob = new Person('bob');
bob.greet();