我有一個問題，我正在嘗試將playlist[]數組從 component.ts傳遞給 header.ts。ERROR TypeError: Cannot read property 'length' of undefined下面的代碼：組件.tsimport { Component, OnInit } from '@angular/core';import { ApiService } from '../../../services/api.service';import { FormGroup, FormControl } from '@angular/forms';import { FormBuilder } from '@angular/forms';import { faSearch } from '@fortawesome/free-solid-svg-icons';import { faRedo } from '@fortawesome/free-solid-svg-icons';import { faHeadphones } from '@fortawesome/free-solid-svg-icons';import { faExternalLinkAlt } from '@fortawesome/free-solid-svg-icons';import { faPlus } from '@fortawesome/free-solid-svg-icons';@Component({  selector: 'app-content',  templateUrl: './content.component.html',  styleUrls: ['./content.component.scss']})export class ContentComponent {  public data = [];  public playlist = [];  public apiData: any;  public results = [];  public loading = false;  public noData: any;  p: number = 1;  faSearch = faSearch;  faRedo = faRedo;  faHeadphones = faHeadphones;  faExternalLinkAlt = faExternalLinkAlt;  faPlus = faPlus;  searchQuery: string = "";  clickMessage = '';  constructor(private service: ApiService) { }  getAll() {    this.service.getAll(this.searchQuery).subscribe((results) => {      this.loading = true;      console.log('Data is received - Result - ', results);      this.data = results.results;      this.loading = false;      if (this.data.length <= 0) {        this.noData = true;      } else if (this.data.length >= 1) {        this.noData = false;      } else {        this.noData = false;      }    })  }  closeAlert() {    this.noData = false;  }  addSongToPlaylist(itunes) {    this.playlist.push(itunes);    console.log('Playlist - ', this.playlist);}  refresh(): void {    window.location.reload();  }