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:

My Personal Notes

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JavaScript | Pad a number with leading zeros