JavaScript | Pad a number with leading zeros

Last Updated : 20 May, 2019

The String prototype property is used to pad a number with leading zeros. This property allows for adding new properties and methods to existing object types.

Syntax:

object.prototype.name = value

Return value: It returns a reference to the String.prototype object.

Example 1: This example adds leading zeros to a number by creating a function and inside this function, if the number is less than the width provided then it adds the leading zeros.

<!DOCTYPE html>  
<html>  
    <head>  
        <title>  
            JavaScript | Pad a number with leading zeros 
        </title> 
    </head>  
      
    <body style = "text-align:center;" id = "body">  
      
        <h1 style = "color:green;" >  
            GeeksForGeeks  
        </h1>  
      
        <p id = "GFG_UP" style = "font-size: 16px;"></p> 
      
        <button onclick = "gfg_Run()">  
            click here 
        </button> 
          
        <p id = "GFG_DOWN" style = "color:green; 
                    font-size: 20px; font-weight: bold;"> 
        </p> 
          
        <script> 
            var el_up = document.getElementById("GFG_UP"); 
            var el_down = document.getElementById("GFG_DOWN"); 
            var num = 2213; 
            el_up.innerHTML = 'Number = ' +num; 
            function pad(n, width) { 
                n = n + ''; 
                return n.length >= width ? n :  
                    new Array(width - n.length + 1).join('0') + n; 
            } 
            function gfg_Run() {                  
                el_down.innerHTML = pad(num, 7);  
            }  
        </script>  
    </body>  
</html>                     

Output:

Before clicking on the button:
After clicking on the button:

Example 2: This example adds leading zeros to a number by creating a prototype pad. In this example we can pass a string to the object, whatever we want to pad with the number.

<!DOCTYPE html>  
<html>  
    <head>  
        <title>  
            JavaScript | Pad a number with leading zeros 
        </title> 
    </head>  
  
    <body style = "text-align:center;" id = "body">  
          
        <h1 style = "color:green;" >  
            GeeksForGeeks  
        </h1>  
          
        <p id = "GFG_UP" style = "font-size: 16px;"></p> 
          
        <button onclick = "gfg_Run()">  
            click here 
        </button> 
          
        <p id = "GFG_DOWN" style = "color:green;  
                font-size: 20px; font-weight: bold;"> 
        </p> 
          
        <script> 
            var el_up = document.getElementById("GFG_UP"); 
            var el_down = document.getElementById("GFG_DOWN"); 
            var num = '29213'; 
            el_up.innerHTML = 'Number = ' + num; 
              
            String.prototype.pad = function(String, len) { 
                var str = this; 
                while (str.length < len) 
                    str = String + str; 
                return str; 
            } 
            function gfg_Run() {                  
                el_down.innerHTML = num.pad("0", 9);  
            }  
        </script>  
    </body>  
</html>                     

Output:

Before clicking on the button:
After clicking on the button:

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