javascript

Chapter 6: Object-Oriented Programming
The startsWith() method in this example returns true if some given text occurs at the beginning of a
string. The method is assigned to String.prototype , making it available to all strings in the
environment. Since msg is a string, the String primitive wrapper is created behind the scenes, making
startsWith() accessible.
Although possible, it is not recommended to modify native object prototypes in a
production environment. This can often cause confusion and create possible name
collisions if a method that didn ’ t exist natively in one browser is implemented
natively in another. It ’ s also possible to overwrite native methods accidentally.
Problems with Prototypes
The prototype pattern isn ’ t without its faults. For one, it negates the ability to pass initialization
arguments into the constructor, meaning that all instances get the same property values by default.
Although this is an inconvenience, it isn ’ t the biggest problem with prototypes. The main problem comes
with their shared nature.
All properties on the prototype are shared among instances, which is ideal for functions. Properties that
contain primitive values also tend to work well, as shown in the previous example, where it ’ s possible to
hide the prototype property by assigning a property of the same name to the instance. The real
problem occurs when a property contains a reference value. Consider the following example:
function Person(){
}
Person.prototype = {
constructor: Person,
name : “Nicholas”,
age : 29,
job : “Software Engineer”,
friends : [“Shelby”, “Court”],
sayName : function () {
alert(this.name);
}
};
var person1 = new Person();
var person2 = new Person();
person1.friends.push(“Van”);
alert(person1.friends); //”Shelby,Court,Van”
alert(person2.friends); //”Shelby,Court,Van”
alert(person1.friends === person2.friends); //true
Here, the Person.prototype object has a property called friends that contains an array of strings.
Two instances of Person are then created. The person1.friends array is altered by adding another
string. Because the friends array exists on Person.prototype , not on person1 , the changes made are
also reflected on person2.friends (which points to the same array). If the intention is to have an array
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