Error: “Encountered undefined provider! Usually th

Question

Here's a somewhat useless error I'm getting in my Angular / TypeScript application. Until someone makes the error message better, what can we do about this? What are the most likely situations that cause this to happen?

Uncaught Error: Encountered undefined provider! Usually this means you have a circular dependencies (might be caused by using 'barrel' index.ts files.
    at Object.syntaxError 
    at eval     at Array.forEach (native) [<root>]
    at CompileMetadataResolver._getProvidersMetadata 
    at CompileMetadataResolver.getNgModuleMetadata 
    at CompileMetadataResolver.getNgModuleSummary 
    at eval 
...

Answer 1

For me just helped to restart the NG SERVE

Answer 2

@Component({
  selector: "app-dispatching-history",
  templateUrl: "./dispatching-history.component.html",
  styleUrls: ["./dispatching-history.component.css"],
  providers: [RecommendationService, Location, { provide: HashLocationStrategy, useClass: HashLocationStrategy }]
})

I just added Location, { provide: HashLocationStrategy, useClass: HashLocationStrategy } as a provider in one of my component and didn't do any other necessary changes in files like app.module.ts. I don't need it anymore, so I simply remove it. And the command ng build -c deploy --build-optimizer --aot --prod --sourceMap works again.