In my opinion, there are two big things in Javascript's world - Closure and Prototype.
Question 1.
What does Prototype do in JavaScript?
In a single word, it's a feature that aimed at reducing code duplication.
We all know in computer world there is a famous philosophy - Don't Repeat Yourself.
In classic object-oriented languages such as c++/c#/java, we have inheritance to encapsulate some common methods into a super class to reduce code lines.
But in JavaScript, there is no concept of Class, we use Function and Prototype concept to imitate the similar behavior.
Question 2.
Who owns the Prototype?
Function owns the prototype property, not object itself.
In JavaScript, all things are object(include function), so function can have it's own property.
In fact, every function has a prototype property, which is an Object.
Object created from function has a hidden link to the function's prototype object.
Functions created in this propose are called constructor function, which should start with a capital letter.
For example:
function User(name) {
this.name = name;
}
User.prototype.getName = function() {
return this.name;
};
var user = new User('Zhang San');
alert(user.getName());In this example, User is a constructor function.
user is a instance of User which has privileges to access prototype method getName.
Questions 3.
Who create me?
Every object has a constructor property, which indicate the constructor function.
see this exmple:
function User(name) {
this.name = name;
}
var user = new User('Zhang San');
alert(user.constructor === User); // true
alert(user.constructor.prototype === User.prototype); // true
alert({}.constructor === Object); // true
alert([].constructor === Array); // true
alert(''.constructor === String); // trueBecause User.prototype is an object, it has constructor property:
alert(user.constructor.prototype.constructor); // User
we can redefine the constructor function's prototype like this:
function Person(sex) {
this.sex = sex;
}
function User(name) {
this.name = name;
}
User.prototype = new Person('man');
var user = new User('Zhang San');
alert(user.sex); // 'man'
alert(user.constructor); // 'Person'Note that user.constructor is Person now, not User.
We can fix it using a trick:
function Person(sex) {
this.sex = sex;
}
function User(name) {
this.name = name;
}
User.prototype = new Person('man');
User.prototype.constructor = User;var user = new User('Zhang San');
alert(user.sex); // 'man'
alert(user.constructor); // 'User'Question 4.
Object/Array are types?
You'd better think them as constructor functions, just like User has done.
Maybe there are definition in JavaScript core like this:
function Object() {
}
function Array() {
}The pre-defined constructor functions have read-only prototypes.
For examples:
Array.prototype.max = function() {
var maxValue = this[0];
for (var i = 1; i < this.length; i++) {
maxValue = this[i];
}
}
return maxValue;
};
alert([2,33,25].max()); // 33
Array.prototype = {
'max': function() {
var maxValue = this[0];
for (var i = 1; i < this.length; i++) {
maxValue = this[i];
}
}
return maxValue;
}
};
alert([2, 33, 25].max()); // [2, 33, 25].max is not a function