Confusion arises from the error message 'No overload matches this call'

Currently, I am working on a weather application using Angular based on the concepts from the book Angular for Enterprise-Ready Web Applications - Second Edition. I am in the process of adding a search component to the app, which will allow users to display weather information based on the city they search for. However, I am encountering the error mentioned above in the 'city-search' component.ts file. The complete error message is as follows:

No overload matches this call. Overload 1 of 2, '(observerOrNext?: Partial<Observer<string | null>> | ((value: string | null) => void) | undefined): Subscription', gave the following error. Argument of type '(searchValue: string) => void' is not assignable to parameter of type 'Partial<Observer<string | null>> | ((value: string | null) => void) | undefined'. Type '(searchValue: string) => void' is not assignable to type '(value: string | null) => void'. Types of parameters 'searchValue' and 'value' are incompatible. Type 'string | null' is not assignable to type 'string'. Type 'null' is not assignable to type 'string'. Overload 2 of 2, '(next?: ((value: string | null) => void) | null | undefined, error?: ((error: any) => void) | null | undefined, complete?: (() => void) | null | undefined): Subscription', gave the following error.

city-search.component.ts:

import { Component, EventEmitter, OnInit, Output } from '@angular/core';
import { FormControl, Validators } from '@angular/forms';
import { debounceTime } from 'rxjs/operators';

import { WeatherService } from '../weather/weather.service';

@Component({
  selector: 'app-city-search',
  templateUrl: './city-search.component.html',
  styleUrls: ['./city-search.component.css']
})
export class CitySearchComponent implements OnInit {
  @Output() searchEvent = new EventEmitter<string>()
  search = new FormControl('', [Validators.minLength(2)])

  constructor(private weatherService: WeatherService){}
  ngOnInit(): void {
    this.search.valueChanges.pipe(debounceTime(1000)).subscribe((searchValue: string) => {
      if (!this.search.invalid) {
        this.searchEvent.emit(searchValue)
        const userInput = searchValue.split(',').map(s => s.trim())
      this.weatherService.getCurrentWeather(
        userInput[0],
        userInput.length > 1 ? userInput[1] : undefined
      ).subscribe(data => (console.log(data)))
    }
  })
}
}

I have shared the GitHub link where you can access the project: https://github.com/soorajKADAM/local-weather-app.git

I am currently struggling to comprehend the meaning of the error message and how to resolve it.

Answer №1

If you encounter an error indicating a type mismatch, it means that the valueChanges Observable is set to emit values of type string | null. While reactive form controls are typically nullable by default, this may not align with your specific expectations for a searchValue of type string.

To address this issue, there are two approaches you can take:

Option 1: Include null in the type definition

You can modify the type definition of searchValue to accommodate null values. By adding a simple check within the existing logic, you can handle the null scenario effectively. Here is an updated snippet of code:

this.search.valueChanges
  .pipe(debounceTime(1000))
  .subscribe((searchValue: string | null) => {
    if (searchValue !== null && !this.search.invalid) {
      // further code...
    }
  });

Option 2: Specify the form control as non-nullable

If it is necessary for the form control to not accept null values, you can explicitly define it as such:

search = new FormControl('', {
  nonNullable: true,
  validators: [Validators.minLength(2)]
});

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