fractionDigits

Optional. Number of digits after the decimal point. Must be in the range 0 – 20, inclusive.

The toExponential method returns a string representation of a number in exponential notation. The string contains one digit before the significand's decimal point, and may contain fractionDigits digits after it.

If fractionDigits is not supplied, the toExponential method returns as many digits necessary to uniquely specify the number.

Version 5.5

Reference