Attempted to assign to readonly property even though same command works elsewhere?

Question

I have a Javascript function (in an Angular 2 NativeScript based mobile app) which is triggered on a button press, it should hide the button and show an activity indicator in its place, carry out a Bluetooth scan, which when completed switches off the activity indicator and shows the original button.

bluetoothAdd() {
    this.isScanning = true;
    var plusIcon = this.page.getViewById("add");
    plusIcon.style.opacity = 0;
    bluetooth.hasCoarseLocationPermission().then(
        function (granted) {
            if (!granted) {
                bluetooth.requestCoarseLocationPermission();
            } else {
                bluetooth.startScanning({
                    serviceUUIDs: ["133d"],
                    seconds: 4,
                    onDiscovered: function (peripheral) {
                        console.log("Periperhal found with UUID: " + peripheral.UUID);
                    }
                }).then(function () {
                    console.log("scanning complete");
                    this.isScanning = false;
                    plusIcon.style.opacity = 1;
                }, function (err) {
                    console.log("error while scanning: " + err);
                });
                this.isScanning = false;
            }
        });
}

Unfortunately, the this.isScanning = false; line throws all of these errors. What have I done wrong?

CONSOLE ERROR file:///app/tns_modules/angular2/src/platform/server/parse5_adapter.js:55:75: EXCEPTION: Error: Uncaught (in promise): TypeError: Attempted to assign to readonly property.
CONSOLE ERROR file:///app/tns_modules/angular2/src/platform/server/parse5_adapter.js:53:75: STACKTRACE:
CONSOLE ERROR file:///app/tns_modules/angular2/src/platform/server/parse5_adapter.js:53:75: resolvePromise@file:///app/tns_modules/zone.js/dist/zone-node.js:496:41
file:///app/tns_modules/zone.js/dist/zone-node.js:532:32
invokeTask@file:///app/tns_modules/zone.js/dist/zone-node.js:314:43
onInvokeTask@file:///app/tns_modules/angular2/src/core/zone/ng_zone_impl.js:35:51
invokeTask@file:///app/tns_modules/zone.js/dist/zone-node.js:313:55
runTask@file:///app/tns_modules/zone.js/dist/zone-node.js:214:58
drainMicroTaskQueue@file:///app/tns_modules/zone.js/dist/zone-node.js:432:43
promiseReactionJob@[native code]
UIApplicationMain@[native code]
start@file:///app/tns_modules/application/application.js:233:26
file:///app/tns_modules/nativescript-angular/application.js:65:26
ZoneAwarePromise@file:///app/tns_modules/zone.js/dist/zone-node.js:542:38
nativeScriptBootstrap@file:///app/tns_modules/nativescript-angular/application.js:64:23
anonymous@file:///app/main.js:5:36
evaluate@[native code]
moduleEvaluation@[native code]
[native code]
promiseReactionJob@[native code]
CONSOLE ERROR file:///app/tns_modules/zone.js/dist/zone-node.js:419:27: Unhandled Promise rejection: Attempted to assign to readonly property. ; Zone: angular ; Task: Promise.then ; Value: TypeError: Attempted to assign to readonly property.
CONSOLE ERROR file:///app/tns_modules/zone.js/dist/zone-node.js:421:23: Error: Uncaught (in promise): TypeError: Attempted to assign to readonly property.
CONSOLE LOG file:///app/Pages/Home/home.component.js:99:32: scanning complete

Answer 1

The issue is that once you enter the Promise your are in a different context; the "this" no longer points to the "this" you thought it did, so you need to save the "this" into a another variable; some people use "that", "self" or even "_this"...

So the solution to this issue is;

bluetoothAdd() {
    this.isScanning = true;
    var plusIcon = this.page.getViewById("add");
    plusIcon.style.opacity = 0;

    var self = this; // THIS LINE ADDED

    bluetooth.hasCoarseLocationPermission().then(
        function (granted) {
            if (!granted) {
                bluetooth.requestCoarseLocationPermission();
            } else {
                bluetooth.startScanning({
                    serviceUUIDs: ["133d"],
                    seconds: 4,
                    onDiscovered: function (peripheral) {
                        console.log("Periperhal found with UUID: " + peripheral.UUID);
                    }
                }).then(function () {
                    console.log("scanning complete");
                    self.isScanning = false; // CHANGED!
                    plusIcon.style.opacity = 1;
                }, function (err) {
                    console.log("error while scanning: " + err);
                });
                self.isScanning = false; // CHANGED!
            }
        });
}

---

Update for ES6 method -- You can also use ES6 arrow functions => For example you can change the first line to this:

   bluetooth.hasCoarseLocationPermission().then(
            (granted) => {

In this case because you used an ES6 arrow function the this will automatically be from the parent scope; and so then you don't need to use the self, _this, or that tricks.

As of NativeScript 2.4 ES6 is supported in both iOS and Android.