Several topics about javascript functions_javascript skills

1. Function objects and function pointers
A function is actually an executable object, and any way of accessing a function object is a function pointer.
No matter which method is used
var a=new Function(...);
function a(...){...}
var a=function(...) {...}
The obtained function name a is a pointer to the function. You can use var b=a; to create another pointer to it.
But they point to the same function. Because the function name is a pointer, it can be passed as a parameter. The
() operator acting on a function pointer will execute the function it is in, and of course will pass in the parameters.


2. Function direct quantity (function literal quantity)
Actually, the term function direct quantity is not accurate, but it can explain some usage in a popular way
125 "good" is all direct Quantities can be used in code without variable names. Functions have similar usage
function(){...} represents the value of a function pointer. It points to the function defined by function(){...}
Then (function(){...})() executes the function a=function(){...} and creates a copy of this pointer

3. Members of the function object
The function object inherits from the object object. It also has its own properties and methods
That’s all I found. Tips from vs2005
f.apply(thisValue, argArray); // Execute on thisValue object. The parameters are provided by the array argArray. Provide
f.call(thisValue); // Execute on thisValue object. It does not have no parameters. If you want to use parameters, just follow thisValue directly.
f.prototype; // Prototype is very common and has not been used before. Said.
f.length;//The length of the parameter list

f.hasOwnProperty(propertyName);//Method inherited from Object, checks whether the object has the specified property
f.propertyIsEnumerable(propertyName); //Method inherited from Object, check whether the specified property can be enumerated by for in (except for prototype under firefox, other properties cannot be enumerated)
f.toLocaleString();//Method inherited from Object, Convert to string
f.toString();//Method inherited from Object, converted to string Automatically called when alert document.write, etc.
f.valueOf();//Method inherited from Object , evaluation, automatically called when the object performs arithmetic operations, if the default is toString

3. The range that the javascript function can access
What resources can the javascript function access? It consists of three parts: the scope of the function, this pointer, and parameters. In addition, the function cannot access any resources.
If you want to understand the function more clearly, you should imagine that the function has three sets of parameters:
The first set is the scope, which is determined at the time of definition and is determined by the position of the function definition. It is temporarily called the outer scope and is a variable that can be accessed at the function definition. and functions can also be accessed within a function, no matter where the pointer to the function is passed
The second group is this which is determined when the function is called
If the function is called with () it defaults to pointing to the function to which the function belongs Object: If the function is a member of an object, then this points to the object, otherwise it points to the global object (most of the time, window)
If the function is called with apply or call, this points to the specified thisvalue
The third group It is a parameter that is determined when the function is called. Parameters in the parameter list can be accessed directly. Parameters that are no longer in the parameter list can be accessed using arguments

4. Generation and access of function scope
In addition to being able to access Outside a scope, each execution will generate a new scope (closure)
Code outside this scope cannot access the variables of this scope. Code within this scope can access this scope and this function. The scope at the time of definition.

Cross-scope access can only be through pointers. Object in js is a pointer type.
If you want to get a pointer to a certain scope, you must pass the pointer out from this scope: that is, assign the value of the pointer type variable in this scope to a variable in other scopes
As mentioned above The resources that can be accessed in the function include this outer scope parameters, so variables in other scopes can be accessed in this way.
A very bt example

[Ctrl A select all Note: If you need to introduce external Js, you need to refresh to execute

]<script> //想办法访问一个很深的嵌套中的函数对象 var o=new Object(); function a(obj){ var pointer; (function(){ var p=new function(){ function f(){ var x=function(){alert("Try to visit me!")};//x是一个指针 return x;//作为返回值传递出第一层; } this.x=f();//用this传递出第二层; } pointer=p.x;//用外作用域传递出第三层 })() obj.x=pointer;//用参数传递出第四层 } a(o); o.x(); </script>
This last example is too BT. In fact, although js function has such ability, we generally have a restriction when using it, that is, when accessing variables outside the closure in the closure, there should not be "side effects" " expression, for example, -- or assignment, that is to say, the content of the closure's access to the external domain is limited to "reading" and "expression calculation". You can modify the value of the object actually referenced by the external variable, but do not use it in the closure. Modify the value of the external variable itself within the package. So the following example
function c(a){
return function(x)
{
return x a ;
}
}
is usually legal but unreasonable, Because this is a closure with side effects. This can easily lead to confusion when the program logic is complex. Because it is completely uncertain when this closure is called. The existence of this closure makes the return value of each call of this function completely different, even if the same parameters are used. This obviously violates the basic principle of functional "formulation".

However, in fact, the above function has a special purpose, used to generate some special counters :)
So the limit is not absolute...