How to truncate up to n digits after the dot?

[PaulRBerg]

I looked through all issues and the docs, but I couldn't find an answer to my question.

const config = {
  number: "BigNumber",
  precision: 79,
};
const math = create({ expDependencies }, config);
const mbn = this.math.bignumber!;

function exp(x: string): string {
  const result = mbn(x).exp() as BigNumber;
  console.log(result.toString());
}

exp("-1");

The code above logs the following:

0.3678794411714423215955237701614608674458111310317678345078368016974614957448998"

Now, I would like to truncate the result to 18 digits after the dot. How can I do that? I looked at the format function but it seems like the maximum numbers of digits permitted is 16.

[PaulRBerg]

I found the answer myself. math.js is built on top of decimal.js, which has a toFixed method. It is similar to the vanilla toFixed available in JavaScript, but it can be used only with decimal.js instances.

Thus I would rewrite the example like this:

function exp(x: string): string {
  const result = mbn(x).exp() as BigNumber;
  console.log(result.toFixed(18));
}

And get this in the console:

0.367879441171442322

[josdejong]

There is a powerful function math.format that allows you to format numeric values and any type of value into a string

https://mathjs.org/docs/reference/functions/format.html

Your example would look like:

function exp(x: string): string {
  const result = mbn(x).exp() as BigNumber;
  console.log(math.format(result, { notation: 'fixed', precision: 18 }));
}

[PaulRBerg]

Thanks for the tip! I think that the docs should be updated .. it says that the maximum precision that can be set is 16.

[josdejong]

Ah, you're right. Updated via e6f3123