JS function calls and issues related to implicit parameters arguments and this

##1. Several different ways of calling functions:

#1. Calling as a function

#2. Calling as a method, Call on the object, support object-oriented programming

#3. Call as a constructor to create a new object#4. Call through the apply() or call() method, This method is relatively the most complicated


##2. Function calls will pass two implicit parameters and exist in the scope of the function: arguments and this

Function parameters How to deal with inconsistencies with formal parameters:
#1. Actual parameters>Formal parameters--》The excess actual parameters will not be allocated

#2. Actual parameters

##3.

#1. Function context: depends on the calling method of the function (four calling methods).

#2. arguments: a collection of all parameters, with a length attribute, which can be obtained using arguments[2], etc.; but arguments are not arrays, and those methods without arrays

#3. this: References an object that is implicitly associated with the function call, called the function context

##Four:

#1. "As a function" call: Refers to the first method of the above four methods, which is different from the other three methods
For example: function test(a,b){}/var test = function(a,b){}
The function context at this time It is window, that is, **this==window**

#2. "As a method" call: When a function is assigned to an object and the attribute that references the function is used
I. The object to which a method belongs can be called with this in the method body

var o = {};
o.test = function(){};
o.test();

The function context at this time is the object o, that is, **this==o**

#3. "As constructor" call: Same as function declaration, except how to call the function, use the new keyword before the function call
function test(){return this};

new test();#########Special features of the constructor:#######I. Create a new empty object#######II. Pass to the constructor The object is this object, thus becoming the context of the constructor#######III. If there is no explicit return value, the newly created object is returned as the return value of the constructor######

function Ninja(){
this.skulk = function(){return this};
//无显式返回，故返回this===o
}===function Ninja(){
var o = {};
o.skulk = function(){return this;};
return o;
}
var ninja1 = new Ninja();
var ninja2 = new Ninja();
ninja1.skulk === ninja1;
ninja2.skulk === ninja2;

# ######apply() and call() methods: Explicitly specify any object as a function context ###apply() requires two parameters: 1. The object as the function context 2. Composed of function parameters The array###call() also requires two parameters: the difference is that the second parameter is a parameter list########You can allow any element to be used as the context of apply() and call()###

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