TypeError: Found non-callable @@iterator - Migration of Angular 7 to 8 (upgrading Ionic: 4.6.0 > 4.7.1)

Troubleshoot migration problems

TypeError: Found non-callable @@iterator

TypeError: Found non-callable @@iterator

The non-descriptive error message Found non-callable @@iterator, for new Angular 8 user like me, has made the upgrading journey a difficult experience.

And my finding, the cause of the problem above is due to the way interceptor written in Angular 7.

Causes: HttpRequest's headers "set"

I noticed the use of request.headers.set() causing the issue above. My sample code (causes):

intercept(req: HttpRequest, next: HttpHandler): Observable> {
  let headers = req.headers;
  const sampleToken = 'SAMPLE_TOKEN';
  headers = req.headers.set('Authorization', sampleToken);

  return next.handle(req.clone({ headers }));
}

After multiple trial and error, the only way to resolve is avoid using the same way to add header with such solution.

Solution: Headers modification

- on one single header modification, do the following

const yourToken = 'YOUR_TOKEN';
const apiReq = req.clone({
  headers: new HttpHeaders({'Authorization', yourToken})
});
return next.handle(apiReq);

- on multiple headers modification, do the following

const headers = { Authorization: authToken };
const apiReq = req.clone({
  setHeaders: headers
});
return next.handle(apiReq);

P/S: header value must be in string value

Angular 7 + Ionic 4: No provider for NavController! (Clue: incorrect module name imported)

NullInjectorError: No provider for NavController!

After switching from AngularJs to Angular 7 and Ionic 4, I've came across a few new issues where led me to high dependence on StackOverflow.
For certain edge cases without a certified answer, as plus Ionic 4 is still in beta build (during my writing of this article). If SO don't has the answer, I'll have to spend more hours on the issue by diving into the error message and core library.

First thing came into my mind when I first look at the error message:

What is NavController has to do with my code while I've never imported it into any of my component. I didn't use NavController at all in my code as Ionic mentioned they are deprecated in Ionic4

After spent 2-3 days of clueless bug hunting, it's found how I imported IonicModule is the culprit!

Capital letter '@Ionic/angular' should be '@ionic/angular'

In short, the solution is to check your module name if they are written in correct case format (yes case sensitive, I've learned my painful lesson).

Quick Solution:
@Ionic/angular should be @ionic/angular

No provider for NavController

...
ERROR Error: StaticInjectorError(AppModule)[HrefDelegate -> NavController]: 
StaticInjectorError(Platform: core)[HrefDelegate -> NavController]: 
  NullInjectorError: No provider for NavController!
  at NullInjector.push../node_modules/@angular/core/fesm5/core.js.NullInjector.get (core.js:717)
  at resolveToken (core.js:954)
  at tryResolveToken (core.js:898)
  at StaticInjector.push../node_modules/@angular/core/fesm5/core.js.StaticInjector.get (core.js:795)
  at resolveToken (core.js:954)
  at tryResolveToken (core.js:898)
  at StaticInjector.push../node_modules/@angular/core/fesm5/core.js.StaticInjector.get (core.js:795)
  at resolveNgModuleDep (core.js:17656)
  at NgModuleRef_.push../node_modules/@angular/core/fesm5/core.js.NgModuleRef_.get (core.js:18345)
  at resolveDep (core.js:18716)
...

Like the error message above, Ionic4 didn't really provide very informative clue that helps debugger identify what is rootcause. So yeah, there are risks when we use beta library building our project...
I believe soon later, the error message would be clearer once it's near its official stable build release date.
There is also a slightly useful warning message obtained from browser console, posted as below

...
There are multiple modules with names that only differ in casing.
This can lead to unexpected behavior when compiling on a filesystem with other case-semantic.
Use equal casing. Compare these module identifiers:
// ... list of affected files ...
/node_modules/@ionic/angular/dist/providers/action-sheet-controller.js
/node_modules/@Ionic/angular/dist/util/overlay.js
/node_modules/@ionic/angular/dist/index.js
/node_modules/@Ionic/angular/dist/providers/action-sheet-controller.js
/node_modules/@Ionic/angular/dist/util/util.js
...

Well, the messages in debugger do shine some light about the ambiguous and conflicting module names, the list of affected files don't really tell what has gone wrong.
Hope this article could help some not-so-lucky guys out there who fell into traps set by ourselves.