How to find percentage change of two BigNumbers of ether.js?

Question

So I have this for calculating percentage change between two BigNumbers:

const percentageChange = (a: BigNumber, b: BigNumber) => b.sub(a).div(a).mul(BigNumber.from('100'))

Now the issue is when I call toString() or toNumber() on it I always get 0.

The a and b are numbers in wei and are very close together, so percentage should be something like 0.000021 or some such.

How do I calculate the percentage and get JS-native type without rounding it to zero?

Answer 1

BigNumbers don't accept decimal points, it means that when doing

b.sub(a).div(a) 
if b.sub(a) < a will always return 0

I would suggest you multiply for a factor for instance 1e6 a and b before doing the operations and once you have your percentage*1e6 as number, you can divided by 1e6 and multiply by 100

Hope it helps