That one is just integers. If you need decimal fractions too, you'll have to find something else.
If you just want to check that a 20-digit string looks like a number, and you don't need to know the value, you can do that with a regular expression.
You can keep 20 digits (or 200 or 2000) intact as a string, or as an array of digits, but to do any math on it you need a big integer object or library.
Normally you use Number() or parseInt("", [radix]) to parse a string into a real number.
I am guessing you are asking about what happens when the string you parse is above the int - threshold. In this case it greatly depends on what you are trying to accomplish.
In short: working with VERY large number or exact decimals is a challenge in every programming language and often requires very specific mathematical libraries (which are less convenient and often a lot slower than when you work in "normal" (int/long) areas) - which obviously is not an issue when you REALLY want those big numbers.
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