How to do BLE characteristic.Writevalue () asynchronously ?

[BangleBeginner]

My set up is Banglejs as BLE host and HM-10 as peripheral.
The Banglejs watch has a main task to perform, and occasionally
I would like the host to write a few bytes to HM-10 via peripheral 's service/characteristics.
I follow an example in the espruino reference but struggle to understand
the concept ...... After the BLE connection is established , how to use the
"characteristic" to transmit a few bytes ?

I have included my test javascript for your comments. Thank you
for any help/pointers in advance.
score2.txt

[gfwilliams]

What you're doing looks kind of ok, it's just that you're not waiting for the connection. characteristic.writeValue is asynchronous already - it returns a promise, which completes when the write is complete. You just need to pass that promise along and only execute the next code in a .then block. So like below (see my comments for what I added)...

var c;
g.setFont("Vector", 20);

function Init() {
// return added here
return NRF.connect("68:5E:1C:31:A7:95 public", { minInterval:7.5, maxInterval:40}).then(function(ga) {
    gatt = ga;
    //g.clear();
    g.drawString("1 Connected...", 10, 35);
    g.drawString("2 Get service...", 10, 50);
    gatt.device.on('gattserverdisconnected', function(reason) {
        g.drawString(reason, 10, 160);
        E.showAlert(reason).then(function() {
            Bangle.showLauncher();
        });
    });
  return gatt.getPrimaryService("0000ffe0-0000-1000-8000-00805f9b34fb");
}).then(function(service) {
  //g.clear();
  g.drawString("3 Got service...", 10, 65);
  return service.getCharacteristic("0000ffe1-0000-1000-8000-00805f9b34fb");
}).then(function(characteristic){
  g.drawString("4 Got chara", 10, 80);
  c=characteristic;
  
  // return characteristic.writeValue("+1"); // if doing it in this promise, use return to pass the writeValue promise on...
  // return characteristic.writeValue("+2");
}).catch(function(){console.log("Something wrong");});
}

function main() {
  g.clear();
  Init().then(function() { // .then added here
    if(NRF.getSecurityStatus().connected==true){
     g.drawString("5 Write", 10, 95);
      c.writeValue("+1").then(function() { // .then added here
        c.writeValue("+2");
      });
    }
  });
}
main();

[BangleBeginner]

Thank you for fixing my code, it really helps to move my project forward.
In the process, this is what I learn:

tst1(){

Init().then(function() {
charac.writeValue("+2\n").then(function() {
charac.writeValue("+2\n").then(function() {
charac.writeValue("+1\n").then(function() {
charac.writeValue("+1\n").then(function() {
charac.writeValue("+1\n");
});
});
});

Only two out of five writeValue are received !!
But when I entered individual charac.writeValue (empty promise return)
from the console, every single one is received.

---

Getting service invoked/involved before writeValue, then every write is received.
Like this:

tst2(){
Init().then(function() {
return getservice("+2\n").then(function(){
return getservice("+1\n").then(function(){
return getservice("+1\n").then(function(){
getservice("+2\n");
});
});
});
});
}

function getservice(pj)
{
return service.getCharacteristic(c_uuid).then(function(characteristic){
return characteristic.writeValue(pj);
}).catch(function() {
console.log("Bad");
});
}

Currently, when my main task needs to communicate (infrequently) with the BLE periperal,
I do this:

.
.
.
getservice(c);
.
.
.

Is this kosher ? I am thankful for any comments.

[gfwilliams]

It's possible that Init is returning before something is properly set up. It should be fine, but it's possible it is the peripheral you are trying to write to that has the problem. You could add:

.then(new Promise(resolve => setTimeout(resolve,500));

to the end of the Init function to add a delay. Or in fact you could manually add a delay after charac.writeValue in the same way.

If your getservice function works though, I don't see a huge problem in using that. It'd be slightly slower but if as you say you're only occasionally writing then it'd be ok