javascript

Chapter 6: Object-Oriented Programming
Prototypes and the in Operator
There are two ways to use the in operator: on its own or as a for - in loop. When used on its own, the in
operator returns true when a property of the given name is accessible by the object, which is to say that
the property may exist on the instance or on the prototype. Consider the following example:
function Person(){
}
Person.prototype.name = “Nicholas”;
Person.prototype.age = 29;
Person.prototype.job = “Software Engineer”;
Person.prototype.sayName = function(){
alert(this.name);
};
var person1 = new Person();
var person2 = new Person();
alert(person1.hasOwnProperty(“name”)); //false
alert(“name” in person1); //true
person1.name = “Greg”;
alert(person1.name); //”Greg” - from instance
alert(person1.hasOwnProperty(“name”)); //true
alert(“name” in person1); //true
alert(person2.name); //”Nicholas” - from prototype
alert(person2.hasOwnProperty(“name”)); //false
alert(“name” in person2); //true
delete person1.name;
alert(person1.name); //”Nicholas” - from the prototype
alert(person1.hasOwnProperty(“name”)); //false
alert(“name” in person1); //true
Throughout the execution of this code, the property name is available on each object either directly or
from the prototype. Therefore, calling ” name ” in person1 always returns true , regardless of whether
the property exists on the instance. It ’ s possible to determine if the property of an object exists on the
prototype by combining a call to hasOwnProperty() with the in operator like this:
function hasPrototypeProperty(object, name){
return !object.hasOwnProperty(name) & & (name in object);
}
Since the in operator always returns true so long as the property is accessible by the object and
hasOwnProperty() returns true only if the property exists on the instance, a prototype property can
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