How Do I Round Numbers to One Decimal Place in JavaScript?

How to Round Numbers to One Decimal Place in JavaScript

Rounding a number to a specific precision can be a common task in many programming scenarios. In JavaScript, one may encounter the need to round a number to exactly one decimal place. Here's how it can be achieved:

To properly round a number to one decimal place in JavaScript, the following technique can be utilized:

Math.round(num * 10) / 10

For example, consider the number 12.3456789:

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

This method ensures proper rounding, as it multiplies the number by 10, rounds it to the nearest integer, and then divides it back by 10.

If the requirement is to have exactly one decimal place, even if the value would be a 0, the toFixed method can be used:

var fixed = rounded.toFixed(1)
// 'fixed' is always to one decimal point
// NOTE: .toFixed() returns a string!

To convert the string back to a number format, the parseFloat method can be employed:

parseFloat(number.toFixed(2))
// 12.34
// but that will not retain any trailing zeros

It's important to note that the toFixed method always returns a string, so ensure it is the last step before output, and utilize a number format during calculations.

For convenience, a custom round function can be defined to handle rounding with various precisions:

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

Usage example:

round(12345.6789, 2) // 12345.68
round(12345.6789, 1) // 12345.7

The defaults to round to the nearest whole number (precision 0). It can also be used to round to the nearest 10, 100, etc., and handles negative numbers корректно.

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