javascript

Chapter 6: Object-Oriented Programming
be determined if the in operator returns true but hasOwnProperty() returns false . 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 person = new Person();
alert(hasPrototypeProperty(person, “name”)); //true
person.name = “Greg”;
alert(hasPrototypeProperty(person, “name”)); //false
In this code, the name property first exists on the prototype, so hasPrototypeProperty() returns true .
Once the name property is overwritten, it exists on the instance, so hasPrototypeProperty() returns
false .
When using a for - in loop, all properties that are accessible by the object and can be enumerated will be
returned, which includes properties on both the instance and on the prototype. Instance properties that
shadow a nonenumerable prototype property (a property that has [[DontEnum]] set) will be returned
in the for - in loop since all developer - defined properties are enumerable by rule, except in Internet
Explorer (IE).
The IE implementation of JScript has a bug where properties that shadow nonenumerable properties will
not show up in a for - in loop. Here ’ s an example:
var o = {
toString : function(){
return “My Object”;
}
};
for (var prop in o){
if (prop == “toString”){
alert(“Found toString”);
}
}
//won’t display in Internet Explorer
When this code is run, a single alert should be displayed indicating that the toString() method was
found. The object o has an instance property called toString() that shadows the prototype ’ s
toString() method (which is not enumerable). In IE, this alert is never displayed because it skips over
the property, honoring the [[DontEnum]] flag that was set on the prototype ’ s toString() method. This
same bug affects all properties and methods that aren ’ t enumerable by default: hasOwnProperty() ,
propertyIsEnumerable() , toLocaleString() , toString() , and valueOf() . Some browsers set
[[DontEnum]] on the constructor and prototype properties, but this is inconsistent across
implementations.
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