Error in code in ionic2/angular2

Question

Here, I am getting error in following code below indicated as I want to return promise but getting error in returning promise. How to returning promise?

import { Component } from '@angular/core';
import { NavController } from 'ionic-angular';
import { UsersPage } from '../users/users';
import { ShopPage } from '../shop/shop';

@Component({
  selector: 'page-home',
  templateUrl: 'home.html'
})
export class HomePage {
usrPage = UsersPage;
  constructor(public navCtrl: NavController) {}
  onGoToUsers(){
    this.navCtrl.push(this.usrPage)
    .catch((error)=> console.log('Access Denied, Argument was' + error));
  }

  ionViewCanLeave(): boolean | Promise<void> {
    const promise = new Promise((resolve, reject) => {
      setTimeout(()=>{
        resolve()
      }, 1000);
    });
    return promise;   // the line is producing error in returning promise.
  }

}

@Lazar, as I mentioned above, it is producing error in returning promise . — Manzer Hashmi
– Manzer Hashmi, Commented Sep 24, 2017 at 18:40

Mustansir Zia · Accepted Answer · 2017-09-24 19:03:29Z

2

Change ionViewCanLeave(): boolean | Promise<void> {

to this.

ionViewCanLeave(): boolean | Promise<any> {

Essentially, Promise<void> is not equal to Promise in your case. (which you return). Thus change the <T> of the promise to use any as the generic value.

The reason void is not equal to any is that void essentially means an absence of a type whereas any means any type. undefined and null can be assigned to a void type variable.

The line const promise = new Promise... declares the variable type implicitly as you do not give the type after the promise declaration, so it assumes it to be Promise<any>.
Thus, if you would write const promise: Promise<void> and it would also work.

Another way this could work would be to return the Promise directly like this: (Perhaps the tutorial you followed showed the function like this?)

return new Promise((resolve, reject) => {
   //
});

Since this is a return statement it would tell the typescript compiler to implicitly assume the correct return type that is Promise<void> instead of Promise<any> since that happens when you assign this Promise to a typeless const promise.

Hope I was able to shed some light. Read more about typescript types from here.

edited Sep 24, 2017 at 19:03

answered Sep 24, 2017 at 18:41

Mustansir Zia

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2 Comments

Manzer Hashmi Over a year ago

Thanks, but the same piece of code that i mentioned above run without any error on a tutorial, can't figure why it produced error in my case.

Mustansir Zia Over a year ago

Editing my answer to shed more light.

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