Using exponential notation to prevent rounding errors.js

console.log(Number(Math.round(1.005 + "e2") + "e-2")) // 1.01

const roundAccurately = (number, decimalPlaces) => Number(Math.round(number + "e" + decimalPlaces) + "e-" + decimalPlaces)

console.log(roundAccurately(1.005, 2)) // 1.01

const roundAccurately = (number, decimalPlaces) => Number(Math.round(Number(number + "e" + decimalPlaces)) + "e" + decimalPlaces * -1);

console.log(roundAccurately(23456789.32452342345, -5)) // 23500000

This will not work if the number is already in exponential notation :(

@hamidgasmi Nope! JavaScript's built-in math isn't great if you actually need precision or are working with exponential notation. I'd recommend using a library. MathJS is a good place to start. https://mathjs.org/

Can anyone help explain what 'e' and 'e-' means in this scenario? I tried it out on code.org and I think it multiplies the value by 10 but I don't understand how the composition of the code can round. Thank You!

@DoctorDerek

The lower case e is exponential notation, so 10 to the power of positive 2 or minus 2.

Since the built-in rounding function only works on the first decimal place, you can move the decimal point (with the e notation), round, and move the decimal point back.

I'd recommend trying each piece of the code individually, reading the docs on Math.round, and just completely avoiding this use case if it doesn't make sense to you.

Again, the "right" solution here would be to use something more robust. Math.round(1.005) is 1, not 1.01 or 1.0 as you might expect. @y-a11y

@DoctorDerek Thank you so much! This makes a lot more sense now

@hamidgasmi Nope! JavaScript's built-in math isn't great if you actually need precision or are working with exponential notation. I'd recommend using a library. MathJS is a good place to start. https://mathjs.org/

great, thanks.

very helpful.