Compound Promises in Typescript / Javascript? -

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i writing angular2 based mobile app, using typescript nativescript runtime, facing issues promises. have homecomponent, able call various bluetooth functions. these require human interaction (like selecting device) , wait periods bluetooth scan, take several seconds complete.

i want abstract methods promises, have done this:

homecomponent.ts:

bluetoothadd() {     this.isscanning = true;     this._ble.scan().then(         // log fulfillment value         function (val) {             this.isscanning = false; //hide activity indicator             this.connect(this.ble._peripheral);         })         .catch(         function (reason) {             this.isscanning = false; //hide activity indicator             console.log('handle rejected promise (' + reason + ') here.');         }); }

bluetoothutils.ts (this imported above this._ble):

var devices: array<string>; var _peripheral: any;  export function scan() {     return new promise((resolve, reject) => {         bluetooth.hascoarselocationpermission().then(             (granted) => {                 if (!granted) {                     bluetooth.requestcoarselocationpermission();                 } else {                     bluetooth.startscanning({                         serviceuuids: ["133d"],                         seconds: 4,                         ondiscovered: (peripheral) => {                             console.log("periperhal found uuid: " + peripheral.uuid);                         }                     }).then(() => {                         console.log("scanning complete");                      }, (err) => {                         console.log("error while scanning: " + err);                     });                 }             });     }); }

having stepped through app debugger, when bluetoothadd() called in homecomponent, scan() function in bluetoothutils works expected , in 1 scenario, executes console.log("error while scanning: " + err); line, saying:

error while scanning: bluetooth not enabled

however, neither then or catch parts of homecomponent this._ble.scan() promise executed? why this? can compound promises (i.e. promise in promise) trying do? or ideas how ot debug further?

there's few problems.

1/ you're using anti pattern. there's no need create new promise in scan since you're dealing promises.

2/a) if want use anti pattern, need call resolve , reject functions, i.e. in current situation new promise never resolved nor rejected, , therefore "main" promise in scan never resolved or rejected either.

2/b) better solution: return promises created. if don't return become separate promise branches , won't influence "main" promise branch in scan, i.e.:

export function scan() {     return bluetooth.hascoarselocationpermission().then(         (granted) => {             if (!granted) {                 return bluetooth.requestcoarselocationpermission();             } else {                 return bluetooth.startscanning({                     serviceuuids: ["133d"],                     seconds: 4,                     ondiscovered: (peripheral) => {                         console.log("periperhal found uuid: " + peripheral.uuid);                     }                 }).then(() => {                     console.log("scanning complete");                  }, (err) => {                     console.log("error while scanning: " + err);                 });             }         }); }

note 3 added returns: 1 @ top of scan , before bluetooth.requestcoarselocationpermission , bluetooth.startscanning

if want know other promise anti patterns exist, here's great resource


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