digits specifies how many digits should appear in total. The value must between 0 and some arbitrary number that's hopefully at least larger than 20 (for Node it's 21. Why? Who knows).
Returns a string representing the given value in fixed-point or scientific notation
The output will be rounded or padded with zeroes if necessary.
toPrecision differs from toFixed in that the former will count all digits against the precision, while the latter will count only the digits after the decimal point. toPrecision will also use scientific notation if the specified precision is less than the number for digits before the decimal point.
raisesRangeError
if digits is not in the range accepted by this function (what do you mean "vague"?)