The usage example of the JS function This is an analysis of _javascript techniques

This example describes the use of the JS function this. Share to everyone for your reference. Specifically as follows:

When writing functions in JS, a lot of this is used. What this is, this is a keyword, is a pointer to the execution environment scope, also known as the context.
Let's say the function is a code block that is called repeatedly in the language.
In JS, the function is assigned to the object's properties, called the method
Such as:

var m={};
m.title= ' title ';
M.show=function () {
alert (this.title)
}
m.show ()

is to call the function as the object M method.
In this case, this point is the object of M.

Called function of direct function name:

var a=1212;
Function Show () {
alert (a)//1212
} show
()//1212

In the global context, global variables can be interpreted as window properties, and global functions are window methods
Look at the following example:

var m ={};
M.id= ' mmm ';
M.show=function () {
  alert (this.id);
}
var a={};
A.id= ' AAA ';
A.show=m.show;
A.show (); AAA

A.show=m.show;  Understand this sentence first, because the function is an object,
m.show=function () {
  alert (this.id)
}

This expression is equivalent to the a.show and m.show references pointing

function () {
  alert (this.id)
}

actually equals

A.show=function () {
  alert (this.id)
}

So when calling A.show (), this is pointing to a object,
And look at one of the chestnuts below.

var m ={};
M.id= ' mmm '
m.show=function () {
  alert (this.id)
}
var a={}
a.id= ' aaa '
a.show= function () {
  m.show ()
};
A.show (); Mmm

So calling A.show () is equivalent to calling the M.show () method, so this. is the M object.

Look at one of the following examples, at the beginning still not very understand

var color= ' red ';
var app = {};
App.color= "Green";
App.paint=function (node) {
node.style.color=this.color;
  alert (This.color);
}
function Findnode (callback) {
  var btn =document.queryselector ('. btn ');
  Callback (BTN);//Pass in,
}
Findnode (app.paint);
alert (This.color); Red, not green.

When a function argument is passed, it is passed by value, not by reference

So Findnode (App.paint), when it came in, was actually

function (node) {
  node.style.color=this.color;
  alert (This.color);
}

, and because Findnode is defined globally, this points to window OR UNDEFINED;

About arguments, passing the past by value

Function Show (a) {
  alert (a)
}

When the parameter is a basic data type, it's understandable.

var b=10;
Show (b)//alert (10);

As for the object

var c ={};
C.prop=true;
var showprop=function (obj) {
obj.prop=false
}
Showprop (c);//c.prop = False

Some people think the above example is by reference to the argument
In fact, above or by the value of the argument, Showprop (c) into the function, C is actually equivalent to a reference, the function of Obj.prop=false, the equivalent of the reference to the object of {Prop:false}

Let's look at one of the following examples

var c ={};
C.prop=true;
var showprop=function (obj) {
  obj = new Object ();
  Obj.prop=false return
  obj;
}
Showprop (c);
alert (C.prop); True

Here obviously the incoming obj modified, if the function according to the reference parameters, the changes in the function will be reflected to the external

I hope this article will help you with your JavaScript programming.