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.

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

Syntax:

``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.

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.

Example:

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

Output:

``09``

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.

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.

Example:

``````let num  = -9

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

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

// add the (-) minus sign back using + operator
}

Output:

``-09``

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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