Parse Grove GPS values with Calliope Mini PXT block editor

calliope-grove-gps.js

/*
Snippet reads the first 3 digits of Latitude and Longitude values of Grove GPS connected to a Calliope Mini 
Links:
  - PXT editor for Calliope Mini: http://pxt.calliope.cc/index.html) 
  - Sensor description and Python example code for Grove GPS: http://wiki.seeed.cc/Grove-GPS/
  - GPS sensor spec: http://www.vis-plus.ee/pdf/SIM28@SIM68R@SIM68V_NMEA_Messages_Specification_V1.01.pdf
  
Licensed under the Apache Software License 2.0 (ASL 2.0)  
*/
let gpgga_value = ""
let indexplus = 0
let item = ""
let length = 0
let counter = 0
let value = ""
input.onButtonPressed(Button.A, () => {
    basic.showLeds(`
        # . . . .
        . . . . .
        . . . . .
        . . . . .
        . . . . .
        `)
    serial.redirect(
        SerialPin.C17,
        SerialPin.C16,
        BaudRate.BaudRate9600
    )
    serial.writeString("$PMTK104*37<CR><LF>")
})
serial.onDataReceived(serial.delimiters(Delimiters.Dollar), () => {
    value = serial.readString()
    music.playTone(262, music.beat(BeatFraction.Whole))
    length = value.length
    counter = 0
    for (let index = 0; index <= length; index++) {
        item = value.charAt(index)
        if (item == ",") {
            counter += 1
            // Latitude
            if (counter == 1) {
                indexplus = index
                indexplus += 1
                gpgga_value = value.substr(indexplus, 3)
                basic.showString(gpgga_value)
            }
            // Longitude
            if (counter == 3) {
                indexplus = index
                indexplus += 1
                gpgga_value = value.substr(indexplus, 3)
                basic.showString(gpgga_value)
            }
            music.playTone(147, music.beat(BeatFraction.Whole))
        }
    }
})