See the docs for num.toStringAsFixed().
String toStringAsFixed(int fractionDigits)
Returns a decimal-point string-representation of this.
Converts this to a double before computing the string representation.
Examples:
1000000000000000000000.toStringAsExponential(3); // 1.000e+21
The parameter fractionDigits must be an integer satisfying: 0 <= fractionDigits <= 20.
Examples:
1.toStringAsFixed(3); // 1.000
(4321.12345678).toStringAsFixed(3); // 4321.123
(4321.12345678).toStringAsFixed(5); // 4321.12346
123456789012345678901.toStringAsFixed(3); // 123456789012345683968.000
1000000000000000000000.toStringAsFixed(3); // 1e+21
5.25.toStringAsFixed(0); // 5