It’s not possible to natively deal with a 21-digit precision number in JavaScript.
JavaScript only has one kind of number: “number”, which is a IEEE-754 Double Precision (“double”) value. As such, parseFloat in JavaScript is the equivalent of a “parse double” in other languages.
However, a number/”double” only provides 16 significant digits (decimal) of precision and so reading in a number with 21-digits will lose the 5 least significant digits1.
For more precision (or accuracy) a “big number” library must be used;
- What is the standard solution in JavaScript for handling big numbers (BigNum)?
1 Information can be lost when encoding as an IEEE “double”, which cannot encode all decimal values exactly, but that’s another question..