JavaScript closures, variable promotion

One, add (2) (3) Summation function (closure)

1 function Add (x) {2     var a = x; 3     return function (b) {4         sum = a + b; 5         console.log (sum); 6     }7}8 Add (2) (3);        // 5

Second, the variable promotion

1 //first Case2 3 varA = 100;//Global Variables4 5 functionTest () {6alert (a);//Search up Global variables-->1007A = 10;//no var modifier, so A is a global variable, overwriting the previous global variable8alert (a);//-->109 }Ten  OneTest ();

1 // The second case 2 var a = +; 3 function Test () {4     alert (a);        // variable lift,  undefined5     var a = ten; 6     alert (a);        // Ten 7 }8 test ();

1 //The third case2 varA = 100;3 functionTest () {4alert (a);//variable promotion, undefined5A = 10;6alert (a);//Ten7     varA;8alert (a);//Ten9 }TenTest ();

Third, look at the code, to output the results

 1  for  ( var  i = 1; I <= 3; I++ 2  setTimeout (function   () { 3   Console.log (i);  4 },0);  5  };  6  //  4 4 4 

1  for (var i = 1; I <= 3; i++) {2     setTimeout (function(a) {3         Console.log (a); 4     }) (i), 0); 5 }; 6 // 1  2  3

Or

 1  for  ( var  i = 0; I < 3; I++ 2  setTimeout (function   () { 3   Console.log (i);  4 },0);  5   Console.log (i);  6  }  7  //  0 1 2 3 3 3  

 1  for  ( var  i = 0; I < 3; I++ 2  setTimeout ((function   (n) { 3   Console.log (n);  4 }) (i), 0 5   Console.log (i);  6  }  7  //  0 0 1 1 2 2  

1  for (var i = 0; i < 3; i++) {2     setTimeout (functionreturnfunction< /c8>() {3        console.log (n); 4     };}) (i), 0); 5     Console.log (i); 6 }7//  0  1  2  0  1  2

JavaScript closures, variable promotion