I developed this in a rush for a client. We work with numbers as
large as a couple of millions and that have a maximum of two
decimals. There are a severals improvements that I would like to
do (patches are welcome!) but by the moment it’s working well for
my case so I decided to jump from alpha to stable.

Install

npm install number-to-spanish-words

Usage

const numberToSpanishWords = require('number-to-spanish-words');
numberToSpanishWords(10359); // "diez mil trecientos cincuenta y nueve"

How it works

I realised that in Spanish every number is built from cent to
cent, for example:

• Ciento cuarenta y uno (one hundred and forty one)
• Quince mil ciento cuarenta y uno (fifteen thousands and
  one hundred and forty one)
• Ciento cuarenta y un mil
• Ciento cuarenta y un mil docientos tres
• Cuento cuarenta y un millones

So first we split the number between the integer part and the
fractional part and then we process both parts the same way:

1. Divide the number each tree digits
2. Transform each digit into a word,
3. Concatenate each tree digits
4. Eliminate anomalies (i. e. transform diez y cuatro into
   catorce)
5. Join everything using mil, millones, and/or mil millones

This last part is pretty ugly and I would like to improve it.
After that I would like to process bigger numbers (up to a
billón).

I plan to replace all JavaScript array methods with Ramda because
I have been surprised by some native methods that mutate the array
instead to return a new one.