我创建了一个网站，可以像 Cat and Mash 一样随机挑选一只猫，但出现了一个我无法理解的错误。

我有一个 JSON 对象，其中有包含图像的 URL。我需要随机显示图像，但不是两次显示相同的图像。

控制台：

为什么 length 未定义？

import { Component, OnInit } from '@angular/core';
import { CatService } from '../services/cat.service';
import { CatList, Cat } from '../model/cat';

@Component({
  selector: 'app-cat',
  templateUrl: './cat.component.html',
  styleUrls: ['./cat.component.css']
})
export class CatComponent implements OnInit {
    twoCatsArray: Cat[] = [];
    allcats: Cat[];
    constructor(private catService: CatService) {}
    ngOnInit() {
        this.showMeTwoCats();
    }
    showMeTwoCats() {
        this.catService.getCats().subscribe((cats: CatList) = > {
            this.allcats = cats.images;
            this.twoCatsArray = this.chooseTwoRandomCats(this.allcats);
        });
    }
    chooseTwoRandomCats(cats: Cat[]): Cat[] {
        const firstCatIndex = this.getRandomIndex(cats.length);
        const secondCatIndex = this.getRandomIndex(cats.length, firstCatIndex);
        return [cats[firstCatIndex], cats[secondCatIndex]];
    }
    getRandomIndex(maxValue: number, differentThanValue ? : number): number {
        let index: number;
        do {
            index = this.getRandomInt(maxValue);
        } while (index === differentThanValue);
        return index;
    }
    getRandomInt(max): number {
        return Math.floor(Math.random() * Math.floor(max));
    }
    voteForThisCat(id: string) {
        const likedCatindex = this.allcats.findIndex((cat: Cat) = > cat.id === id);
        const newRating = this.getIncrementedCatRatingValue(this.catService.catVote[likedCatindex].rating);
        this.catService.catVote[likedCatindex].rating = newRating;
        this.twoCatsArray = this.chooseTwoRandomCats(this.allcats);
    }
    getIncrementedCatRatingValue(rating: number | undefined): number {
        return rating ? ++rating : 1;
    }
}