“错误：未捕获（承诺）：类型错误：无法读取未定义的属性‘长度’”

Question

任何人都可以帮助解决错误

"Error: Uncaught (in promise): TypeError: Cannot read property 'length' of undefined

TypeError: Cannot read property 'length' of undefined
    at SomeComponent.webpackJsonp.805.SomeComponent.getBList (http://localhost:4200/1.chunk.js:23613:34)
    at SomeComponent.webpackJsonp.805.SomeComponent.ngOnInit (http://localhost:4200/1.chunk.js:23610:14)
    at checkAndUpdateDirectiveInline (http://localhost:4200/vendor.bundle.js:11957:19)
    at checkAndUpdateNodeInline (http://localhost:4200/vendor.bundle.js:13336:17)
    at checkAndUpdateNode (http://localhost:4200/vendor.bundle.js:13304:16)
    at debugCheckAndUpdateNode (http://localhost:4200/vendor.bundle.js:13933:59)
    at debugCheckDirectivesFn (http://localhost:4200/vendor.bundle.js:13874:13)
    at Object.eval [as updateDirectives] (ng:///SeasonPageModule/SeasonPageComponent_Host.ngfactory.js:16:5)
    at Object.debugUpdateDirectives [as updateDirectives] (http://localhost:4200/vendor.bundle.js:13859:21)
    at checkAndUpdateView (http://localhost:4200/vendor.bundle.js:13271:14)
    at callWithDebugContext (http://localhost:4200/vendor.bundle.js:14259:42)
    at Object.debugCheckAndUpdateView [as checkAndUpdateView] (http://localhost:4200/vendor.bundle.js:13799:12)
    at ViewRef_.detectChanges (http://localhost:4200/vendor.bundle.js:11368:63)
    at RouterOutlet.activateWith (http://localhost:4200/vendor.bundle.js:32580:42)
    at ActivateRoutes.placeComponentIntoOutlet (http://localhost:4200/vendor.bundle.js:31760:16)
    at ZoneAwareError (http://localhost:4200/vendor.bundle.js:95038:33)
    at resolvePromise (http://localhost:4200/vendor.bundle.js:94728:31)
    at resolvePromise (http://localhost:4200/vendor.bundle.js:94713:17)
    at http://localhost:4200/vendor.bundle.js:94762:17
    at ZoneDelegate.invokeTask (http://localhost:4200/vendor.bundle.js:94502:35)
    at Object.onInvokeTask (http://localhost:4200/vendor.bundle.js:5362:37)
    at ZoneDelegate.invokeTask (http://localhost:4200/vendor.bundle.js:94501:40)
    at Zone.runTask (http://localhost:4200/vendor.bundle.js:94378:47)
    at drainMicroTaskQueue (http://localhost:4200/vendor.bundle.js:94660:35)"

我正在尝试将内部数组的值从 json 文件获取到我创建的变量。我的 json 文件采用以下格式。

JSON filename: file.json
    {
      "A":[
        {
          "id": "123",
          "title": "title of A",
          "type": "asf",
          "B": [
            {
              "id": "1",
              "title": "title1"
            },
            {
              "id": "2",
              "title": "title2"
            }
          ]
      ]
    }

我创建了几个模型来存储值

A.model.ts
import {B} from './b.model';

export interface A {

    type?;

    id?;

    title?;

    b?: B[];

}

B.model.ts
export interface B {

    id?;

    title?;

}

我返回 json 的服务具有以下内容

someService.service.ts

import {A} from '../model/a.model';
constructor(private _http: Http, private _authService: AuthService) {}

  get() {
    return this._http.get('app/file.json')
                    .toPromise()
                    .then(res => <A[]> res.json().showList)
                    .then(A => { return A; });
  }

在 component.ts 中，我编写了以下代码。

<somename>.component.ts

import { Component, OnInit } from '@angular/core';
import { Routes, Router } from '@angular/router';
import {Observable} from 'rxjs';

import {A} from '../../shared/model/a.model';
import {SomeService} from '../../shared/service/someService.service';
import {B} from '../../shared/model/b.model';
import {Location} from '@angular/common';

class AList implements A {
  constructor(public title?, public id?, public type?, public bList?: B[]) {}
}
class BList implements B {
  constructor(public title?, public id?) {}
}

@Component({
  selector: 'ms-season-page',
  styleUrls: [],
  template: require('./season-page.component.html')
})
export class SomeComponent implements OnInit {

  b: B = new BList();

  bArray: B[] = [];

  aArray: A[];

  constructor(private router: Router, private someService: SomeService) {
  }

  ngOnInit() {
    this.someService.get().then(a => this.aArray = a);
    this.bArray = this.getBList(this.aArray);
  }

  getBList(aArray: AList[]): B[] {
    let bList: B[] = [];
    for(let j=0; j<aArray.length; j++) {
        for (let i=0; i<aArray[j].seasonList.length; i++) {
          let b = new BList(aArray[j].B[i].title, aArray[j].B[i].id);
          bList.push(b);
        }
    }
    return bList;
  }
}

谁能帮我说说我在做什么错误？或者有没有办法获得'B'数组值。

提前致谢。

Answer 1

问题是它this.someService.get是异步的，并且您试图像同步一样访问它的结果：

this.someService.get().then(a => this.aArray = a); // This is asynchronous
this.bArray = this.getBList(this.aArray); // This will be called before `this.aArray` is set to anything. Now `this.aArray` will be undefined

this.aArray修复方法是在异步方法的回调中使用：

this.someService.get().then(a => {
  this.aArray = a;
  this.bArray = this.getBList(this.aArray); // Now `this.aArray` will have some value provided `someService.get()` returns something
});