JavaScript inheritance Detailed (ii) _JS object-oriented

This

This represents the current object that refers to the current Page Object window if it is used globally, and if this is used in a function, this refers to what is invoked on the object based on this function at run time. We can also use apply and call two global methods to change the specific point of this in the function.

Let's look at an example of using this in a global scope:

  <script type= "Text/javascript" >
   console.log (this = = window);//True
   console.log (Window.alert = = This.alert); True
   Console.log (This.parseint ("021"));
  </script>
  

The This in the function is determined at run time, not when the function is defined, as follows:

  Defines a global function functions
  foo () {
   console.log (this.fruit);
  }
  Defines a global variable, equivalent to Window.fruit = "Apple";
  var fruit = "Apple";
  This point in the function Foo points to the Window object
  //This invocation and Window.foo (); is exactly equivalent
  foo ();///"Apple"/

  custom object, And point the object's property foo to the global function foo
  var pack = {
   fruit: "Orange",
   Foo:foo
  };
  This point in function foo points to the Window.pack object
  pack.foo ();//"Orange"
  

Global functions apply and call can be used to change the direction of this in a function, as follows:

  Defines a global function functions
  foo () {
   console.log (this.fruit);
  }
  
  Define a global variable
  var fruit = "Apple";
  Customize an object
  var pack = {
   fruit: "Orange"
  };
  
  Equivalent to Window.foo ();
  foo.apply (window); "Apple"/
  this time in Foo's this = = Pack
  foo.apply (Pack);//"Orange"
  

Note: Apply and call two functions have the same function, the only difference is that the parameters of the two functions are defined differently.

Because functions are also objects in JavaScript, we can see the following interesting examples:

  Defines a global function functions
  foo () {
   if (this = = window) {
    Console.log ("This is window.");
   }
  
  function Foo is also an object, so you can define the property boo for Foo as a function
  Foo.boo = functions () {
   if (this = = foo) {
    Console.log (' This is foo. ');
   else if (this = = window) {
    Console.log ("This is window.");
   }
  ;
  Equivalent to Window.foo ();
  Foo (); This is window.
  
  You can see the object Foo.boo () of this in the function pointing to the calling function
  ;//is foo.
  
  Use apply to change this in the function to point to
  foo.boo.apply (window);//this is window.
  

Prototype

We have used prototype to simulate class and inheritance implementations in chapter one. Prototype is essentially a JavaScript object. And each function has a default prototype property.
If this function is used in a scene that creates a custom object, we call this function a constructor. For example, here's a simple scenario:

  constructor function person
  (name) {
   this.name = name;
  }
  Defines the prototype of person, in which attributes can be referenced by custom objects
  Person.prototype = {
   getname:function () {return
    this.name;
   }
  }
  var Zhang = new person ("Zhangsan");
  Console.log (Zhang.getname ()); "Zhangsan"
  

As an analogy, consider the data types in JavaScript-strings, numbers, arrays (array), objects (object), dates (date), and so on. We have reason to believe that these types are implemented as constructors within JavaScript, such as:

  Defines the constructor of an array as a predefined type
  function array () {
   //...
  }
  
  of JavaScript. Initializes an instance of an array
  var arr1 = new Array (1,%,);
  However, we prefer the following syntax definition:
  var arr2 = [1, 56, 34, 12];
  

Many of the methods (such as Concat, join, push) of an array operation should also be defined in the prototype attribute.
In fact, all of the intrinsic data types of JavaScript have read-only prototype attributes (this is understandable: because if these types of prototype properties are modified, which predefined methods disappear), we can add our own extension methods to them.

  Extend a method to get the minimum value to the JavaScript intrinsic type Array
  Array.prototype.min = function () {
   var min = this[0];
   for (var i = 1; i < this.length i++) {
    if (This[i] < min) {
     min = this[i];
    }
   }
   return min;
  };
  
  Call the Min method on an instance of any array
  ([1, Console.log, 12].min ());/1
  

Note: There is a trap to add an extension method to the array's prototype, and the extension method is also recycled when the for-in loop array is used.
The following code illustrates this (assuming that the Min method has been extended to the array's prototype):

  var arr = [1,%, n];
  var total = 0;
  for (var i in arr) {Total
   = parseint (Arr[i];
  }
  Console.log (total); NaN
  
  

The solution is also simple:

  var arr = [1,%, n];
  var total = 0;
  for (var i in arr) {
   if (Arr.hasownproperty (i)) {total
    + = parseint (Arr[i], ten)
   ;
  }
  Console.log (total); 103
  

Constructor

Constructor always points to the constructor that creates the current object. For example, the following example:

  Equivalent to var foo = new Array (1,%,);
  var arr = [1,%, n];
  Console.log (Arr.constructor = = Array); True
  //equivalent to var foo = new Function ();
  var Foo = function () {};
  Console.log (Foo.constructor = = Function); True
  //The constructor instantiates an obj object
  var obj = new Foo ();
  Console.log (Obj.constructor = = = Foo); True
  
  //combine the above two pieces of code to get the following conclusion
  console.log (Obj.constructor.constructor = = Function);//True
  

But when constructor met prototype, something interesting happened.
We know that each function has a default attribute prototype, and this prototype constructor the default point to this function. As shown in the following example:

  function person (name) {
   this.name = name;
  };
  Person.prototype.getName = function () {return
   this.name;
  };
  var p = new Person ("Zhangsan");
  
  Console.log (P.constructor = = person); True
  console.log (Person.prototype.constructor = = person);//True
  //merge The last two lines of code to get the following result
  Console.log ( P.constructor.prototype.constructor = = person); True
  

When we redefined the prototype of a function (note: The difference between this and the previous example is not a modification but a overwrite), constructor's behavior is a bit strange, as follows:

  function person (name) {
   this.name = name;
  };
  Person.prototype = {
   getname:function () {return
    this.name;
   }
  };
  var p = new Person ("Zhangsan");
  Console.log (P.constructor = = person); False
  console.log (Person.prototype.constructor = = person);//False
  Console.log ( P.constructor.prototype.constructor = = person); False
  

Why, then?
It turns out that overwriting person.prototype is equivalent to doing the following code:

  Person.prototype = new Object ({
   getname:function () {return
    this.name;
   }
  });
  

and constructor always point to the constructor that created itself, so Person.prototype.constructor = = = Object At this point, that is:

  function person (name) {
   this.name = name;
  };
  Person.prototype = {
   getname:function () {return
    this.name;
   }
  };
  var p = new Person ("Zhangsan");
  Console.log (P.constructor = = = Object); True
  console.log (Person.prototype.constructor = = = Object);//True
  Console.log ( P.constructor.prototype.constructor = = = Object); True
  

How to fix this problem? The method is also very simple, the Person.prototype.constructor can be overwritten again:

  function person (name) {
   this.name = name;
  };
  Person.prototype = new Object ({
   getname:function () {return
    this.name;
   }
  });
  Person.prototype.constructor = person;
  var p = new Person ("Zhangsan");
  Console.log (P.constructor = = person); True
  console.log (Person.prototype.constructor = = person);//True
  Console.log ( P.constructor.prototype.constructor = = person); True
  

In the next chapter, we will improve the implementation of the Person-employee class and inheritance mentioned in the first chapter.