The JavaScript `Math`

object doesn’t have any built-in method to calculate the percentage of a number, so if you want to find out the percentage of a specific number, you need to create your own custom function.

For example, the code below will find what 20% of 250 is using the `calcPercent()`

function:

```
const percentVal = calcPercent(250, 20);
console.log(percentVal); // 50
```

To write the `calcPercent()`

code, we need to first understand the formula to calculate the percentage number.

To find 20% of 250, you need to divide `20`

by 100 and then multiply it by `250`

:

```
const num = 250;
const percentage = 20;
const percentVal = 250 * (20 / 100);
console.log(percentVal); // 50
```

Now that you know the formula for calculating percentages, you can extract the code as a helper function:

```
function calcPercent(num, percentage){
return num * (percentage / 100);
}
```

Now you can call the helper function `calcPercent()`

anytime you need to calculate a number’s percentage value:

```
function calcPercent(num, percentage){
return num * (percentage / 100);
}
let percentVal = calcPercent(250, 20);
console.log(percentVal); // 50
percentVal = calcPercent(210, 10);
console.log(percentVal); // 21
```

## Rounding the result to 2 decimals

Sometimes, a percentage calculation may return a floating number as follows:

```
const percentVal = calcPercent(20, 3.33);
console.log(percentVal); // 0.666
```

If you need to round the number to 2 decimals, then you can use a combination of `toFixed()`

and `parseFloat()`

methods to return the number in 2-decimal format.

See also: Rounding numbers with toFixed() method

Here’s the full function code:

```
function calcPercent(num, percentage){
const result = num * (percentage / 100);
return parseFloat(result.toFixed(2));
}
const percentVal = calcPercent(20, 3.33);
console.log(percentVal); // 0.67
```

Instead of returning `0.666`

, the function will return `0.67`

because it rounds the decimal numbers to a maximum length of two.