Mat-autocomplete 在 ngOnInit() 中不起作用，Map() 不起作用

Question

HTML- I'm trying to use Mat-autocomplete but its not working on very first time when component gets loaded ie in ngOnInit(), Its saying cannot read properties of null (reading 'map')

<mat-form-field appearance="outline">
                            <input type="text" placeholder="Pick one" aria-label="Number" matInput [formControl]="myControl" [(ngModel)]="element.NEW_AMENITY_NAME" [matAutocomplete]="auto">
                            <mat-autocomplete #auto="matAutocomplete" class="assignedSeat-select" (optionSelected)='onSelect($event.option, i)'>
                                <mat-option *ngFor="let item of filteredOptions | async" [value]="item">
                                    {{item}}
                                </mat-option>
                            </mat-autocomplete>
                        </mat-form-field>

TS file -

ngOnInit(): void {
  this.infoService.AmenityList.subscribe((res)=>{
      this.AmenityList = res; 
      this.assignForm.push(this.createForm(1))
      this.dataSource = [...this.assignForm.controls];
      console.log(this.dataSource.END_TIME , "data");
    })
    this.displayName = this.AmenityList.map(a=>a.DISPLAY_NAME);
    this.filteredOptions = this.myControl.valueChanges.pipe(
      startWith(''),
      map(value => this._filter(value || '')),
    );
   }

its not working on very first time when we are laoding the component, attached image of error-

Answer 1

Sync/Async issue.

I suggest you learn what is asynchronicity in JS before going further.

But quick solution.

private _data$ = this.infoService.AmenityList.pipe(shareReplay(1));
public names$ = this._data$.pipe(
  map(list => list.map(item => item.name))
);

ngOnInit(): void {
  this._data$.pipe(first()).subscribe((res)=>{
      this.assignForm.push(this.createForm(1))
      this.dataSource = [...this.assignForm.controls];
      console.log(this.dataSource.END_TIME , "data");
    })
    this.filteredOptions = this.myControl.valueChanges.pipe(
      startWith(''),
      map(value => this._filter(value || '')),
    );
   }

Answer 2

this.AmenityList is not set yet so its null, understand response only comes inside subscribe, anything outside runs before the result comes.

      ngOnInit(): void {
          this.infoService.AmenityList.subscribe((res)=>{ 
                // runs after response comes
              this.AmenityList = res; 
              this.assignForm.push(this.createForm(1))
              this.dataSource = [...this.assignForm.controls];
              console.log(this.dataSource.END_TIME , "data");
          //  }) you ended it here so the below code runs before the response comes
           
            this.displayName = this.AmenityList.map(a=>a.DISPLAY_NAME);
        }); // move it here, any code that depends on the response should be called after response 
          data.filteredOptions = this.myControl.valueChanges.pipe(
              startWith(''),
              map(value => this._filter(value || '')),
            );
           }