Is there a way to truncate scientific notation numbers in Javascript?

Question

As you all know since it is one of the most asked topic on SO, I am having problems with rounding errors (it isn't actually errors, I am well aware). Instead of explaining my point, I'll give an example of what possible numbers I have and which input I want to be able to obtain:

Let's say

var a = 15 * 1e-9;
alert(a)

outputs

1.5000000000000002e-8

I want to be able to obtain 1.5e-8 instead, but I cannot just multiply by 10e8, round and divide by 10e8 because I don't know if it will be e-8 or e-45 or anything else.

So basically I want to be able to obtain the 1.5000002 part, apply toFixed(3) and put back the exponent part.

I could convert into a string and parse but it just doesn't seem right...

Any idea ?

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(I apologize in advance if you feel this is one of many duplicates, but I could not find a similar question, only related ones)

Gael

Distract

Answer 1

You can use the toPrecision method:

var a = 15 * 1e-9;
a.toPrecision(2); // "1.5e-8"

Plaided

This, as documented, gives you a (typeof a ==) "string"; great for output. If you want to get a "number" back you can do var b = 1 * a.toPrecision(2); Beware of rounding in the middle of a computation. – Incitement

@Stephen, yes, or var b = +a.toPrecision(2); or var b = Number(a.toPrecision(2)); – Phip

Wasn't expecting an easy solution ! I shall look deeper into documentation next time ! – Distract

Answer 2

If you're doing scientific work and need to round with significant figures in mind: Rounding to an arbitrary number of significant digits

Coliseum

Answer 3

var a = 15 * 1e-9;

console.log(Number.parseFloat(a).toExponential(2));

//the above formula will display the result in the console as: "1.50e-8"

Genia

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