JavaScript gets a random number of four digits or letters _javascript tips

This section describes how to implement a simple four-bit random number function by using a code example.
A simpler way to do this is to randomly extract four characters from numbers and letters.

The code example is as follows:

function only (Ele,arr) { 
 if (arr.length==0) { 
  true; 
 } 
 for (Var j=0;j<arr.length;j++) { 
  if (Ele==arr[j]) {return 
   false; 
  } else{return 
   true; 
  } 
 } 
 
var arr=[0,1,2,3,4,5,6, "A", "B", "C", "D", "E", "F", "G"]; 
 
(function () { 
 var randnum=null; 
 var old=[]; 
 var str= ""; 
 function done () { 
  Randnum=math.floor (math.random () *14); 
  if (only (Randnum,old)) { 
   str=str+arr[randnum]; 
   Old.push (Randnum); 
  } 
  else{done 
   (); 
  } 
 for (Var index=0;index<4;index++) {done 
  (); 
 } 
 Console.log (str); 
}) (arr)

The above code to achieve our requirements, the following describes the implementation of the code above.

A. Code comments:
1.function only (Ele,arr) {}, which enables you to determine whether the specified index has been used, and to place random numbers in duplicate.
2.if (arr.length==0) {}, returns true if the array is empty and the case cannot be duplicated.
3.for (Var j=0;j<arr.length;j++) {}, the array is not empty, it traverses the elements in the array, makes the comparison, returns true without repetition, otherwise returns false. 4.var arr=[0,1,2,3,4,5,6, "a", "B", "C", "D", "E", "F", "G"], get an array of random numbers, and of course can be expanded.
5. (function () {}) (arr), a self execution function, and passing a parameter.
6.var Randnum=null, declares a variable and assigns the initial value to NULL to store the randomly generated array index.
7.var old=[], creates an empty array to store the array index values that have already occurred.
8.var str= "" To create an empty string that holds the generated random number.
9.function done () {}, this function can be used to get a random number.
10.randnum=math.floor (Math.random () *14), gets the index value of the array.
11.if (only (Randnum,old)) {
Str=str+arr[randnum];
Old.push (Randnum);
To determine if it has been used, if not, get the element of the array, append to the STR string, and finally append the index value to the old array.
12.else{done ();
If you've already used it, get it again, here's how to use recursion.
13.for (var index=0;index<4;index++) {
Done ();
, use a For loop to get 4 random numbers.

From: http://www.softwhy.com/forum.php?mod=viewthread&tid=16493