Multiplying float with BigInt in JavaScript? [duplicate]

Question

I can calculate for example

x = 0.28
y = 1e18
x * y = 280,000,000,000,000,000

How can I do this in javascript while x could be any number, including ints, and y could be any other big number like 1e12 or whatever.

BigInt cannot be mixed with other types. What is the result you expect? — Coloured Panda
– Coloured Panda, Commented Aug 22, 2023 at 14:16

Barmar · Accepted Answer · 2023-08-22 15:22:23Z

2

You can't mix BigInt with other types in a calculation, so you need to use BigInt for all the operands.

So instead of multiplying the float by 0.28, multiply by 28n and divide by 100n.

console.log((BigInt(1e18) * 28n) / 100n)

answered Aug 22, 2023 at 15:22

Barmar

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3 Comments

user2640145 Over a year ago

As I said, the numbers can be unknown at runtime

Barmar Over a year ago

You need to require the input to be supplied in this form. If 0.28 represents a percentage, ask for the percentage rather than a fraction, so they enter 28 and you do the division by 100 in your code.

Barmar Over a year ago

Or count the number of digits after the decimal point, then scale up. If they enter .285, use BigInt(Math.round(input*1000)) to convert it, then divide by 1000n.

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