JavaScript Round .5 Down

When it comes to rounding numbers in JavaScript, the native Math.round() function works well for most cases. However, it has a quirk when it comes to numbers exactly halfway between two whole numbers, for example 1.5 or -2.5.

By default, Math.round() rounds these numbers up to the nearest whole number. But what if you want to round them down instead?

There are several ways to accomplish this, which we will explore in this article.

Option 1: Round Down with Math.floor()

One simple option is to use Math.floor(), which always rounds down to the nearest whole number.

const num = 1.5;
const roundedNum = Math.floor(num); // 1

This approach works well for positive numbers, but what about negative numbers?

const num = -2.5;
const roundedNum = Math.floor(num); // -3

In this case, Math.floor() rounds down to the next whole number less than -2.5, which is -3. This may not be the behavior you want.

Option 2: Round Down with Math.trunc()

Another option is to use Math.trunc(), which simply removes the decimal portion of a number without rounding. This works well for positive numbers:

const num = 1.5;
const roundedNum = Math.trunc(num); // 1

However, for negative numbers, Math.trunc() rounds toward zero instead of rounding down:

const num = -2.5;
const roundedNum = Math.trunc(num); // -2

This behavior may not be what you want either.

Option 3: Round to Even with Math.round()

If you're willing to accept a slight variation in your rounding, you can use the Math.round() function in a different way. Instead of rounding up for numbers exactly halfway between two whole numbers, you can round to the nearest even whole number.

const num = 1.5;
const roundedNum = Math.round(num - 0.5); // 1

const num2 = -2.5;
const roundedNum2 = Math.round(num2 + 0.5); // -2

By subtracting 0.5 from the positive number or adding 0.5 to the negative number before rounding, Math.round() will always round to the nearest even number. This is sometimes known as "banker's rounding".

Conclusion

Depending on your specific use case, any of the above rounding methods may be appropriate. If you need to always round down, Math.floor() is the way to go. If you want to round to the nearest even number, Math.round() can be used with a slight modification. And if you just need to remove the decimal portion of a number, Math.trunc() is a good option.