Round a Number to 2 Decimal Places | JavaScript

You can use:

Math.round((9.9876 + Number.EPSILON) * 100) / 100 //returns 9.99
Or
+9.9876.toFixed(2) //returns 9.99

But, it seems like Math.round() is a better solution, but it is not! In some cases it will NOT round correctly. Also, toFixed() will NOT round correctly in some cases.

Approximate Rounding

To address rounding issues associated with Math.round() and toFixed(), you can create a custom rounding function that incorporates a "nearly equal" test. This test helps decide whether a fractional value is close enough to a midpoint value to warrant midpoint rounding adjustments. By using this method, you can achieve more accurate rounding results.

The following function return the value of the given number rounded to the nearest integer accurately.

Number.prototype.roundTo = function(decimal) { return +(Math.round(this + "e+" + decimal) + "e-" + decimal); }
var num = 9.7654; console.log( num.roundTo(2));
Output:
9.77

When using a custom rounding function that incorporates a "nearly equal" test for halfway numbers (numbers that are equidistant between two possible rounded values), you might still occasionally encounter scenarios where the result is the lower rounded value due to the inherent limitations of floating-point arithmetic. This behavior is a result of how computers represent and manipulate numbers with finite precision. While the custom rounding function improves accuracy in many cases, it's essential to be aware that rounding can still produce unexpected outcomes for certain inputs.

Using Number.toLocaleString()

The toLocaleString() method in JavaScript is primarily designed for formatting numbers according to the user's locale, including adding thousands separators and using the appropriate decimal separator. However, it doesn't inherently provide accurate rounding functionality.

While you mentioned a custom rounding function using Number.toLocaleString() for accurate rounding to the nearest integer, it's important to clarify that toLocaleString() itself doesn't perform rounding operations in the way other rounding methods do. If you could provide the details or code of the custom function you're referring to, I'd be happy to help clarify its behavior and accuracy.

const roundTo = (num, decimals) => num.toLocaleString('en-US', { minimumFractionDigits: 2, maximumFractionDigits: 2, }); console.log(roundTo(5.015)); //returns 5.02 console.log(roundTo(1.555)); //returns 1.56

Rounding

Rounding involves approximating a number while maintaining its value close to the original. There isn't a one-size-fits-all solution applicable to everyone. Various rounding algorithms exist, and your implementation might differ based on your specific requirements.

Rounding Numbers in JavaScript

In many instances, JavaScript developers employ Math.round() or toFixed() to round numbers to the nearest integer. However, while Math.round() might appear superior, it isn't infallible. There are situations where it fails to round accurately. Similarly, toFixed() also exhibits shortcomings and doesn't consistently achieve correct rounding in certain scenarios.

Math.round()

var num = 5.015 console.log( Math.round((num+ Number.EPSILON)* 100)/100);
Output:
5.01 //wrong (should be 5.02)

In the above case the output is 5.01 instead of 5.02. If you execute the 5.015 with the above function roundTo() the output will be 5.02, that is the correct answer.

toFixed()

var num = 1.555 console.log( parseFloat(num).toFixed(2));
Output:
1.55 //wrong (should be 1.56)

In the above case the output is 1.55 instead of 1.56 . If you execute the 1.555 with the above function roundTo() the output will be 1.56, that is the correct answer.


Round to 2 decimal places in JavaScript

When the rounded number is larger than the original, the given number is considered rounded up. Conversely, if the rounded number is smaller, it is considered rounded down.

Conclusion

To round a number to two decimal places in JavaScript, you can use the toFixed() method on a number to directly obtain a string representation with the desired precision. Alternatively, you can use Math.round() in combination with multiplication and division to achieve the same result. Remember that rounding can introduce minor precision discrepancies due to the nature of floating-point arithmetic.






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