Add leading zero to a number in JavaScript (Code Example)

In this article, we will see how to add a leading zero to a number in Javascript.

Let’s say we have a number 9 which we want to print out in two digits eg 09, then we have to add a zero in front of it.

To achieve that we can use the in-built padStart() method in Javascript.

Add leading zero using padStart() in Javascript

The padStart() method is used to pad a given string with another string until it reaches the given length.


string.padStart(targetLength, padstring)

The targetLength is the length of the final string after padding.

The padstring is the string that we want to pad to the current string.

So to add a leading zero to a number:

  1. First, convert the number to a string using toString().
  2. Call the padStart() method on that string and pass on 2 as the length and ‘0’ as the pad string.

Let’s check the example below to understand it.


let num  = 9
// convert to string
let num_str = num.toString()

let lead_zero = num_str.padStart(2,'0')




Here, we have passed 2 as the length of the string we want and 0 is the string we want to add as the leading value.

Now, it is important to note that the output i.e 09 is a String and not a Number.

And if we try to convert the string value back to number then it will drop the leading 0 from it. It is because 09 is equal to 9 (09 === 9) in Javascript.

Add a leading zero to a negative number in Javascript

If you want to add a leading zero in front of a negative number for example -9 to -09, then we have to use the slice() method, to separate the minus sign and then add it after we have added the zero to the given number.


let num  = -9

function addLeadingZero(num){

  // convert number to string
  let num_str = num.toString()

  // remove the minus from the string
  remove_minus = num_str.slice(1)

  let lead_zero = remove_minus.padStart(2,'0')

  // add the (-) minus sign back using + operator
  return '-' + lead_zero




Here, we have a function addLeadingZero(num) that takes in the number as a parameter.

First, we converted the number to string using toString() method. And then using slice(1) we removed the - sign from the number.

Next using padStart() method we have added the leading zero to the given number.

In the end, we used the + operator to add the - (minus) sign back to the number and returned the value.

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