JavaScript function encapsulates random color verification code (complete code)

​
function verify(a = 6,b = "num"){
  //Define three random verification codes verification code library var num = "0123456789"
  var str = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNIPQRSTUVWXYZ"
  var mixin = num + str;
  
  //Define an empty string to store the verification code var verify=""
  if(a == undefined || b == undefined){
    //Verify whether the input is legal. If it fails, throw an exception. throw new Error("Parameter exception");
  }else{
      if(a == "" || b==""){
        //Judge whether the user has not input throw new Error("Illegal parameter.");
      }else{
        //Detect the input type to determine whether to enter var typea = typeof(a);
        var typeb = typeof(b);
        if(typea == "number" && typeb == "string"){
            if(b == "num"){
                  
                //Define a loop to receive the verification code pure digital verification code for(var i=0;i<a;i++){
                     //Define a variable to store the random value of color var r1 = Math.random()*255;
                     var g1 = Math.random()*255;
                     var b1 = Math.random()*255;
 
                     //Determine the random index var index = Math.floor(Math.random()*(num.length-1))
                     //Determine the random verification code var char = num[index];
                     //Add color to the random verification code verify += `<span style ='color:rgb(${r1},${g1},${b1})'>${char}</span>`
                 }
                 //Return to the array itself return verify;
            }else if(b == "str"){
                 for(var i=0;i<a;i++){
                   //Verification code of pure letters var r1 = Math.random()*255;
                    var g1 = Math.random()*255;
                    var b1 = Math.random()*255;
 
                    var index = Math.floor(Math.random()*(str.length-1));
                    var char = str[index];
 
                    verify += `<span style ='color:rgb(${r1},${g1},${b1})'>${char}</span>`
                 }
                 return verify;   
            }else if(b == "mixin"){
                 // Mixed verification code for(var i=0;i<a;i++){
                    var r1 = Math.random()*255;
                    var g1 = Math.random()*255;
                    var b1 = Math.random()*255;
 
                    var index = Math.floor(Math.random()*(mixin.length-1));
                    var char = mixin[index];
 
                    verify += `<span style ='color:rgb(${r1},${g1},${b1})'>${char}</span>`
                }
                return verify;
            }else{
                //If the verification fails, an exception is thrown. throw new Error("Input type is illegal.")
            }
        
        }else{
            //If the verification fails, an exception is thrown. throw new Error("Input type is illegal.")
        }
      }
  }
}
 
​

  //The first value is the length entered by the user, and the second is the type! 
 var arr = verify(8,"mixin");
     document.write(arr)