- the Math.round() built-in function only rounds to integers
- commonly used rounding methods for decimal numbers produce incorrect results
Rounding Method Comparison
Test results of rounding 1,040,000 random numbers that end with a '5':
|Method||Errors||Fraction of total||Avg Err Ratio|
|exponent +/-||549||0.6602 %||NaN|
// using CommonJS const roundToFixed = require("round-to-fixed");
<!-- from an HTML file --> <script src="round-tofixed.min.js"></script>
Round a number to a fixed number of decimal places.
roundToFixed(num [, digits])
(number) - The number to round off
(positive integer) [optional]- the number of digits after decimal point
Defaults to zero.
(number) - The value of
num rounded to
digits decimal places. Returns
NaN on invalid inputs.
round-tofixed avoids the problems of commonly used rounding methods, all of which produce incorrect results in certain cases.
The most commonly used method for rounding a decimal number is Number.toFixed:
However, when a number ends in 5, it sometimes rounds in the wrong direction.
Another popular method is multiply-then-divide:
Math.round( x * (10 ** digits) ) / (10 ** digits);
While more accurate than using Number.toFixed(), it still rounds numbers ending in 5 incorrectly.
A more accurate method is exponent add-and-subtract:
Math.round( x + 'e' + digits ) + 'e-' + digits;
Exponent add-and-subtract rounds numbers correctly. But it has a fatal flaw—for input values smaller than 1e-6, it returns
round-tofixed uses the exponent add-and-subtract method, but it avoids the
NaN result, rounding correctly for all values.
Copyright (c) 2020 Terry Morse
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