Javascript gets a random number of four digits or letters_javascript skills

This chapter introduces how to implement a simple four-digit random number function through code examples.
A relatively simple way to achieve this is to randomly select four non-repeating characters from numbers and letters.

The code example is as follows:

function only(ele,arr){ 
 if(arr.length==0){ 
  return 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 meets our requirements. Let’s introduce the implementation process of the above code.

1. Code comments:
1.function only(ele,arr){}, this function can determine whether the specified index has been used, and place random numbers to avoid duplication.
2.if(arr.length==0){}, if the array is empty, it means there is no possibility of duplication and returns true.
3.for(var j=0;j 5.(function(){})(arr), a self-executing function and passes a parameter.
6.var randNum=null, declare a variable and assign the initial value to null, used to store randomly generated array index.
7.var old=[], creates an empty array to store array index values ​​that have already appeared.
8.var str="", creates an empty string to store the generated random number.
9.function done(){}, this function can be used to obtain a random number.
10.randNum=Math.floor(Math.random()*14), get the index value of the array.
11.if(only(randNum,old)){
str=str arr[randNum];
old.push(randNum);
}, determine whether it has been used, if not, obtain the array element and append it to the str string, and finally append this index value to the old array.
12.else{ done();
}, if it has been used, then get it again, using recursion here.
13.for(var index=0;index<4;index ){
done();
}, use a for loop to obtain 4 random numbers.

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