JavaScript Advanced (6)---function declaration

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• The function in JS is also an object
• The return value of the function is determined by return, and no return undefined

1. Function declaration method

1. 1.  function   add (A, b) {A  = a + A;　　　　　b  = b + b;}  

      2, function expression method

   2.  var  add = function  (A, b) {...}; 

   3.  (function  () {...}
         ());

   4.  return  function  () {...} 

   5.  var  add =function  foo (A, b) { ......}; //
         named function expression, there is a compatibility problem and is not commonly used  

• 1. • Why can I call this function ahead of time in front of a function declaration?
       
       • Because function declarations are pre-processed at load time, in function expressions, only variables declared in the function are pre-processed.

       • Add (5,4); var add =function foo (A, b) {alert (a+b)};

       • And the function expression itself does not. Therefore, calling the function before the function expression will result in an error.
       • (I didn't think of chestnuts at the moment)

     3. Function constructor

     • var New Function (' A ', ' B ', ' Console.log (a+b); ' ) func (4,5);

　　　　　4. Arrow function

• • • A new function has been added under the ES 6 standard: arrow function
    • For example:

      x = x*// equivalent to function  (x) {    return x*x;  }

    • Arrows are the equivalent of omitting the key words "function", "{}", "Return"
    • Special changes are required in the following special circumstances
      1. The argument is not a case

         // two parameters ()= $;       // No Parameters // variable-parameter conditions (x, Y, ... rest) = {    return x+y;  }

      2. The returned content is the case of the object

         x = {foo:x}     // error notation  = ({foo:x})   // correct wording

      • The arrow function completely fixes the point of the This keyword, in the arrow function, this always points to the lexical scope, that is, the outer caller

         var  obj_arrow = {birth:  1990, Getage:  function   () { var  b = this  .birth;  var  fn = () =>new  Date ().    getFullYear ()-this  .birth;   return   FN ();  }}console.log (Obj_arrow.getage ());  // 26 here directly points to obj  

        The arrow function uses apply with call, because its internal this is fixed to this method scope, then call and apply The first parameter passed in is ignored 　　　　

JavaScript Advanced (6)---function declaration