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

Question

Could anyone help with the error

"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)"

I am trying to get the values of an inner array from a json file to a variable that I have created. My json file is in the following format.

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

I have created couple of models to store the values

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

export interface A {

    type?;

    id?;

    title?;

    b?: B[];

}

B.model.ts
export interface B {

    id?;

    title?;

}

My Service that returns the json has the following

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; });
  }

In the component.ts I have written the following code.

<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;
  }
}

Can anyone help me telling what mistake I am doing? Or is there a way to get the 'B' array values.

Thanks in advance.

Answer 1

The issue is that this.someService.get is asynchronous and you are trying to access the result of it as if it was synchronous:

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

The fix is to use this.aArray in the callback of the asynchronous method:

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
});