Already specified in function comments that sendAmount input must be a number

is chronik's output for utxo.value a string? if so, worth a comment

could use some comments here

the min fee is 1 sat/byte. To use this, you need to make sure you have accurately determined the bytecount of your tx.

...really, for an example like this, we should accurately calculate the bytecount of the tx and use 1.0 -- which we should also extend to Cashtab

For an example, we should use "the" dust amount - 546 sats

Cashtab's use of 550 is arbitrary + an additional safeguard against confusion with SLP utxos

Not clear why we are using Number() so much

If the input type is a Number, just use that.

We only really need Number if we are converting a string.

For a general constant like this, use style of

const ECASH_DECIMALS = 2

Add unit tests here with large numbers approaching total XEC supply to verify that Number is ok

add

• one that goes over total circulating supply to prove that usage of Number is acceptable for xec txs
• one that does the same but also has 2 decimal places, e.g. totalCirculatingSuply.42

This doesn't seem relevant for the example

Nit: capitalize everywhere it's needed

That's not really what you want to check, but rather than the balance is sufficient

2 remarks;

• You can avoid this whole explanation by providing a simple isToken() method in some of the libs.
• I don't understand how you can get a mix of eToken and XEC, does chronik returns tokens when calling for utxos ?

There should be a function in the API to do this. You can't really expect every user to implement a coin selection algorithm, which is not necessarily an easy task, before being able to create some transaction.

This is the kind of things that we don't want a user to have to deal with, and we should provide a better API