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.

**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.

So to add a leading zero to a number:

- First, convert the number to a string using
`toString()`

. - 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()
let lead_zero = num_str.padStart(2,'0')
console.log(lead_zero)
```

**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.

## 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.

**Example:**

```
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
}
console.log(addLeadingZero(num))
```

**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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