Examples and understanding of arguments, caller, callee, call, apply under javascript_javascript skills

Before mentioning the above concepts, I first want to talk about the implicit parameters of functions in JavaScript: arguments
Arguments
This object represents the parameters of the function being executed and the function that calls it.

[function.]arguments[n]
Arguments function: options. The name of the Function object currently executing. n: option. The 0-based index of the parameter value to be passed to the Function object.
Explanation

Arguments is a hidden object created in addition to the specified parameters when calling a function. Arguments is an object that is similar to an array but not an array. It is said to be similar to an array because it has the same access properties and methods as an array. The value of the corresponding single parameter can be accessed through arguments[n], and it has the array length attribute length. Also, the arguments object stores the parameters actually passed to the function, not limited to the parameter list defined by the function declaration, and the arguments object cannot be created explicitly. The arguments object is only available at the beginning of the function. The following example details these properties:

Copy code The code is as follows:

//arguments object usage.
function ArgTest(a, b){
var i, s = "The ArgTest function expected ";
var numargs = arguments.length; // Get the value of the passed parameter.
var expargs = ArgTest.length; // Get the value of the expected parameter.
if (expargs < 2)
s = expargs " argument. ";
else
s = expargs " arguments. ";
if (numargs < 2)
s = numargs " was passed.";
else
s = numargs " were passed.";
s = "nn"
for (i =0 ; i < numargs; i ){ / / Get parameter content.
s = " Arg " i " = " arguments[i] "n";
}
return(s); // Return the argument list.
}

Added a code here to explain that arguments is not an array (Array class):

Copy code The code is as follows:

Array.prototype.selfvalue = 1;
alert(new Array().selfvalue);
function testAguments(){
alert(arguments.selfvalue);
}

Run the code and you will find that the first alert displays 1, which means that the array object has a selfvalue attribute with a value of 1, and when you call When calling the function testAguments, you will find that "undefined" is displayed, indicating that it is not a property of arguments, that is, arguments is not an array object.
Attached here is a simple method recommended by everyone:

Copy the code The code is as follows:

alert(arguments instanceof Array);
alert(arguments instanceof Object);

caller
Returns a reference to a function that The current function is called.
functionName.caller
functionName object is the name of the function being executed.
Explanation
For functions, the caller attribute is only defined when the function is executed. If the function is called from the top level, then caller contains null . If the caller attribute is used in a string context, the result is the same as functionName.toString, that is, the decompiled text of the function is displayed.
The following example illustrates the usage of the caller attribute:

Copy the code The code is as follows:

// caller demo {
function callerDemo() {
if (callerDemo.caller) {
var a= callerDemo.caller.toString();
alert(a);
} else {
alert("this is a top function");
}
}
function handleCaller() {
callerDemo();
}

callee

Returns the Function object being executed, which is the body of the specified Function object.

[function.]arguments.callee
The optional function parameter is the name of the Function object currently being executed.

Explanation

The initial value of the callee attribute is the Function object being executed. The

callee attribute is a member of the arguments object, which represents a reference to the function object itself, which is beneficial to the recursion of the anonymous
function or to ensure the encapsulation of the function, such as the recursive calculation of 1 to n in the example below The sum of natural numbers. And this attribute
is only available when the related function is executing. It should also be noted that callee has a length attribute, which is sometimes
used for verification. arguments.length is the actual parameter length, and arguments.callee.length is the
formal parameter length. From this, you can determine whether the formal parameter length is consistent with the actual parameter length when calling.

Example

Copy code The code is as follows:

// callee can print itself
function calleeDemo() {
alert(arguments.callee);
}
//For verifying parameters
function calleeLengthDemo(arg1, arg2) {
if (arguments.length==arguments.callee.length) {
window.alert("Verify that the formal and actual parameter lengths are correct!");
return;
} else {
alert( "Actual parameter length: " arguments.length);
alert("Formal parameter length: " arguments.callee.length);
}
}
//Recursive calculation
var sum = function(n){
if (n <= 0)
return 1;
else
return n ＋arguments.callee(n - 1)
}

Relatively general recursive function:

Copy code The code is as follows:

var sum = function (n){
if (1==n) return 1;
else return n sum (n-1);

When called: alert(sum(100));
The function contains a reference to sum itself. The function name is just a variable name. Calling sum inside the function is equivalent to calling
a global variable. It cannot well reflect that it is calling itself. At this time Using callee would be a better method.

apply and call

Both of them bind a function to another object to run. The only difference between the two is the way to define parameters:

apply(thisArg,argArray);

call(thisArg[,arg1,arg2...] ]);

That is, the this pointer inside all functions will be assigned thisArg, this It can realize the purpose of running a function as a method of another object

Instructions for apply

If argArray is not a valid array or is not an arguments object, it will result in a TypeError.
If neither argArray nor thisArg is provided, the Global object will be used as thisArg,
and no parameters can be passed.

Description of call

The call method changes the object context of a function from the initial context to the new object specified by thisArg.
If the thisArg parameter is not provided, the Global object is used as thisArg

Related tips:

There is another trick in applying call and apply, which is to use call and apply to apply another After a function (class), the current
function (class) has the methods or attributes of another function (class), which can also be called "inheritance". Look at the following example:

Copy code The code is as follows:

// Demonstration of inheritance
function base() {
this.member = " dnnsun_Member";
this.method = function() {
window.alert(this.member);
}
}
function extend() {
base.call(this);
window.alert(member);
window.alert(this.method);
}

As can be seen from the above example, extend can inherit the methods and properties of base after calling.

By the way, apply is used in the JavaScript framework prototype to create a pattern that defines a class.

The implementation code is as follows:

Copy code The code is as follows:

var Class = {
create: function() {
return function() {
this .initialize.apply(this, arguments);
}
}
}

Analysis: From the code point of view, this object only contains one method: Create, which returns a function, that is, a class. But this is also the
constructor of the class, which calls initialize, and this method is the initialization function defined when the class is created. In this way,
can implement the class creation mode in prototype

Example:

Copy code The code is as follows:

var vehicle=Class.create();
vehicle.prototype={
initialize:function(type){
this.type=type;
}
showSelf:function(){
alert("this vehicle is " this.type);
}
}

var moto=new vehicle("Moto") ;
moto.showSelf();

For more detailed information about the prototype, please visit its official website.

There are more exciting understandings of this in the comments, so I won’t add them here. You can take a look to deepen your understanding.
After reading the above code, that’s it. The following code is for those who want to see the effect immediately. Test more.

Copy code The code is as follows: