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.

• If the absolute value of this is greater or equal to 10^21 then this methods returns an exponential representation computed by this.toStringAsExponential().

Examples:

1000000000000000000000.toStringAsExponential(3); // 1.000e+21
• Otherwise the result is the closest string representation with exactly fractionDigits digits after the decimal point. If fractionDigits equals 0 then the decimal point is omitted.

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