How to use the implicit parameters (arguments, callee, caller) of js_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] Parameters
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. Description 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 explicitly created. The arguments object is only available at the beginning of the function.

The following example details these properties

Copy the code The code is as follows:

//Usage of arguments object
function ArgTest(a, b){
var i, s = "The ArgTest function expected ";
var numargs = arguments.length; // Get the passed parameters numerical value.
var expargs = ArgTest.length; // Get the value of the expected parameter.
if (expargs s = expargs " argument. ";
else
s = expargs " arguments. ";
if (numargs s = numargs " was passed.";
else
s = numargs " were passed.";
s = " "
for (i =0; i s = " Arg " i " = " arguments " ";
}
return(s); // Return the parameter 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 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: alert(arguments instanceof Array);
alert(arguments instanceof Object);

caller

Returns a reference to the function that called the current function.
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, 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,
Note: Function.toString() can realize the decompilation of Function Function. If the recursive function is added, the function will be more powerful
The following example illustrates the usage of the caller attribute:

Copy code The code is as follows:

// caller demo {
function callerDemo() {
if (callerDemo.caller) {
var a= callerDemo.caller.toString( );
                                                                      ; ();
}


callee

Returns the Function object being executed, which is the body of the specified Function object. [function.]arguments.callee optional function parameter is the name of the Function object currently being executed. Note that 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. This is beneficial to the recursion of anonymous functions or to ensure the encapsulation of functions. For example, the following example recursively calculates the sum of natural numbers from 1 to n. This property is only available when the relevant function is executing. It should also be noted that callee has a length attribute, which is sometimes better 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);
}
//Used to verify parameters
function calleeLengthDemo(arg1, arg2) {
if (arguments.length= =arguments.callee.length) {
        window.alert("Verify that the formal and actual parameter lengths are correct!");
        return; arguments.length);
alert("Formal parameter length: " arguments.callee.length);
}
}
//Recursive calculation
var sum = function(n){
if (n if (1==n) return 1;
else return n sum (n-1);
}


When calling: 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. In this case, use callee would be a better way.


apply and call

Both of them bind a function to another object to run. They only differ in the way they define parameters:
apply(thisArg,argArray); call(thisArg[,arg1,arg2…] ]);

That is, the this pointer inside all functions will be assigned the value of thisArg, which can realize the purpose of running the function as a method of another object. Apply's description If argArray is not a valid array or is not an arguments object, it will cause 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, then the Global object is used as thisArg. Related tips: There is another trick in applying call and apply. After using call and apply to apply another function (class), the current function (class) It has the methods or properties of another function (class), which can also be called "inheritance".

Look at the example below:

Copy the 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, you can implement the class creation pattern example in prototype:

Copy the 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); >