How to Accurately Round Numbers to One Decimal Place in JavaScript?

Rounding to One Decimal Place in JavaScript

Rounding numbers in JavaScript can be tricky, but it's crucial for many applications. One common task is to round a number to one decimal place.

The *10, Round, /10 Method

A common approach is to multiply the number by 10, round it, and then divide by 10. However, this method leaves two decimals at the end of the integer.

The Correct Approach

To correctly round to one decimal place, use the following formula:

Math.round(num * 10) / 10

Example

var number = 12.3456789;
var rounded = Math.round(number * 10) / 10;
// rounded is 12.3

Adding Trailing Zeros

If you need the rounded number to always have one decimal place, even if that would be a zero, use the following:

var fixed = rounded.toFixed(1);
// 'fixed' is always to one decimal point

Custom Rounding Function

For more flexibility, you can create a custom rounding function that takes precision as an argument:

function round(value, precision) {
    var multiplier = Math.pow(10, precision || 0);
    return Math.round(value * multiplier) / multiplier;
}

This function allows you to round to any precision, including rounding to the nearest whole number (precision 0).

Additional Notes:

• The .toFixed() method returns a string, so you'll need to use parseFloat() to convert it back to a number if necessary.
• To prevent trailing zeros from being lost in calculations, perform rounding as the last step before outputting the number.
• Negative numbers are handled correctly by this method.

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