indexaCapitalWidget.js

// Read parameter from widget
let parameter = args.widgetParameter
var parameters = "";
if (parameter) parameters = parameter.split(";")
else
{
  // If no parameters were found, ask the user to put them
  let w = new ListWidget()
  error = w.addText("Introduce tu cuenta y token de IndexaCapital separados por un ; en la configuración del Widget")
  error.font = new Font("Avenir-Book", 11)
  Script.setWidget(w)
  Script.complete()
  return
}

let portfolio = await loadItems("portfolio", parameters)
let performance = await loadItems("performance", parameters)

let widget = createWidget(portfolio, performance)

Script.setWidget(widget)
Script.complete()

// Get the data from the API in the given endpoint
async function loadItems(endpoint, parameters) 
{
  const url = "https://api.indexacapital.com/accounts/" + parameters[0] + "/" + endpoint
  const request = new Request(url)
  request.headers = { 'X-Auth-Token':parameters[1] }
  const response = await  request.loadJSON()
  return response
}

// Create the widget with the portfolio and performance info
function createWidget(recportfolio, recperformance) 
{  
  let totalfont = new Font("Avenir-Heavy",20)
  let retfont = new Font("Avenir-Book",17)
  
  let w = new ListWidget()
  w.backgroundColor = new Color("#123456")
  
  let takes = 0
  let cgs = 0
  let total = 0
  
  total_amount = (recportfolio["portfolio"]["total_amount"]).toFixed(2).toString()+"€"
  time_return = (recperformance["return"]["time_return"] * 100).toFixed(2).toString()+"%"
  
  amount = w.addText(total_amount)
  ret = w.addText(time_return)
  
  amount.font = totalfont
  ret.font = retfont
  
  amount.textColor = Color.white()
  ret.textColor = Color.white()
  ret.textOpacity = 0.7
  
  w.backgroundColor = new Color("#217ec1") 
  
  return w
}  

Great idea! I think you can simplify it a bit by getting the total amount from the portfolio, if I'm not mistaken:

#!/bin/bash

ACCOUNT_ID="$1"
TOKEN="$2"
ENDPOINT=performance
METHOD=GET

RESPONSE=`curl https://api.indexacapital.com/accounts/$ACCOUNT_ID/$ENDPOINT \
    -H "X-Auth-Token: $TOKEN" \
    -X $METHOD`

TOTAL=`echo $RESPONSE | jq '.return.total_amount' | cut -d'.' -f1`
INVESTED=`echo $RESPONSE | jq '.return.investment'`
TIME_RETURN=`echo $RESPONSE | jq '.return.time_return'`

echo "$INVESTED - $TOTAL ($TIME_RETURN)"

Sorry for the bash, always end up using that when I'm testing something...

Edit: I actually did that, plus adding the widget a small title, check it out if you're interested - https://gist.github.com/deigote/faf6fc75db9289893c538639f96e66c8 . Again, great idea, I also didn't know about scriptable which looks like fun 😄 .