function subStrAfterChars(str, char, pos)  
    {  
      if(pos=='b')  
       return str.substring(str.indexOf(char) + 1);  
      else if(pos=='a')   
       return str.substring(0, str.indexOf(char));  
      else  
      return str;    
    }  
      
    console.log(subStrAfterChars('w3resource: JavaScript Exercises', ':','a'));  
      
    console.log(subStrAfterChars('w3resource: JavaScript Exercises', 'E','b'));