Fully Understand javascript caller, callee, call, and apply concepts _ Javascript tutorial

Javascript: a complete understanding of javascript caller, callee, call, apply concepts, and Javascript tutorials  

Before talking about the above concepts, we should first talk about the implicit parameter of the function in javascript: arguments

Arguments

This object represents the function being executed and the parameters of the function that calls it.

[Function.] arguments [n]
Parameter function: option. Name of the Function object currently being executed. N: option. The index of the parameter value starting from 0 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 similar to an array but not an array. It is similar to an array because it has the same access nature and method as an array, you can use arguments [n] to access the values of a single parameter and have the length attribute of the array length. In addition, the arguments object stores the parameters actually passed to the function, not limited to the list of parameters defined in the function declaration, and cannot explicitly create the arguments object. The arguments object is available only when the function starts. The following example details these properties:

// Usage of the arguments object.
Function ArgTest (a, B ){
Var I, s = "The ArgTest function expected ";
Var numargs = arguments. length; // obtain the value of the passed parameter.
Var expargs = ArgTest. length; // obtain 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 + = "\ n"
For (I = 0; I <numargs; I ++) {// obtain the parameter content.
S + = "Arg" + I + "=" + arguments [I] + "\ n ";
}
Return (s); // return the parameter list.
}

A code indicating that arguments is not an Array (Array class) is added here:

Array. prototype. selfvalue = 1;
Alert (new Array (). selfvalue );
Function testAguments (){
Alert (arguments. selfvalue );
}

Run the code and you will find that the first alert shows 1, which indicates that the array object has the selfvalue attribute and the value is 1. When you call the testAguments function, "undefined" is displayed, indicating that it is not an attribute of arguments, that is, arguments is not an array object.

Caller
Returns a reference to the function that calls the current function.
FunctionName. caller
The functionName object is the name of the executed function.
Description
For a function, the caller attribute is defined only when the function is executed. If the function is called by the top layer, caller contains null. If the caller attribute is used in the string context, the result is the same as functionName. toString, that is, the decompilation Text of the function is displayed.
The following example illustrates the usage of caller attributes:

// Caller demo {
Function callerDemo (){
If (callerDemo. caller ){
Var a = callerDemo. caller. toString ();
Alert ();
} Else {
Alert ("this is a top function ");
}
}
Function handleCaller (){
CallerDemo ();
}

Callee

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

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

Description

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

The callee attribute is a member of the arguments object. It indicates a reference to the function object itself, which facilitates anonymity.
Recursion of a function or encapsulation of a function. For example, the following example recursively calculates the sum of natural numbers from 1 to n. This attribute
It is available only when the related function is being executed. Note that callee has the length attribute, which is sometimes
It is better for verification. Arguments. length is the length of the real parameter, and arguments. callee. length is
The length of the parameter to determine whether the length of the parameter is consistent with that of the actual parameter.

Example

// 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 length of the form parameter and real parameter is correct! ");
Return;
} Else {
Alert ("real parameter length:" + arguments. length );
Alert ("parameter length:" + arguments. callee. length );
}
}
// Recursive Calculation
Var sum = function (n ){
If (n <= 0)
Return 1;
Else
Return n + arguments. callee (n-1)
}
Typical recursive functions:

Var sum = function (n ){
If (1 = n) return 1;
Else return n + sum (n-1 );

Call time: alert (sum (100 ));

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