错误错误：未捕获（承诺）：TypeError：无法读取未定义的属性“ map”

Question

The below error happens only when it tries to return value from the catch . Hope I'm doing a fundamentally wrong thing here.Please give me a clue.Why can't I return an observable from catch ?

.ts

  getMyTopic() {
    return this.topicSer.getMyTopics().map((res: any) => res.json()).map((res: any) => res = res.categories)
      .catch((err: any) => {
        console.log('err', err);
        return []
      })
  }    

provider.ts

  getMyTopics(): Observable<any> {
       if (typeof this.userService.userDetails != 'undefined') {
          this.localCacheService.getItem(this.localCacheService.discoverTopicsKey).then((val) => {
        if (val != null && val.length == 0) return this.apiService.get(config.discover_topic_WITHAUTH);
         }).catch(() => {
       return this.apiService.get(config.discover_topic_WITHOUTAUTH);//here is the issue
          });
        }

Error

ERROR Error: Uncaught (in promise): TypeError: Cannot read property 'map' of undefined
TypeError: Cannot read property 'map' of undefined
    at Discover.webpackJsonp.402.Discover.getMyTopic (discover.ts:123)
    at Discover.webpackJsonp.402.Discover.getData (discover.ts:69)
    at discover.ts:51
    at t.invoke (polyfills.js:3)
    at Object.onInvoke (core.js:4626)
    at t.invoke (polyfills.js:3)
    at r.run (polyfills.js:3)
    at polyfills.js:3
    at t.invokeTask (polyfills.js:3)
    at Object.onInvokeTask (core.js:4617)
    at Discover.webpackJsonp.402.Discover.getMyTopic (discover.ts:123)
    at Discover.webpackJsonp.402.Discover.getData (discover.ts:69)
    at discover.ts:51
    at t.invoke (polyfills.js:3)
    at Object.onInvoke (core.js:4626)
    at t.invoke (polyfills.js:3)
    at r.run (polyfills.js:3)
    at polyfills.js:3
    at t.invokeTask (polyfills.js:3)
    at Object.onInvokeTask (core.js:4617)
    at c (polyfills.js:3)
    at polyfills.js:3
    at t.invokeTask (polyfills.js:3)
    at Object.onInvokeTask (core.js:4617)
    at t.invokeTask (polyfills.js:3)
    at r.runTask (polyfills.js:3)
    at o (polyfills.js:3)
    at e.invokeTask [as invoke] (polyfills.js:3)
    at p (polyfills.js:2)
    at IDBRequest.v (polyfills.js:2)
defaultErrorLogger @ core.js:1350
ErrorHandler.handleError @ core.js:1411
IonicErrorHandler.handleError @ ionic-error-handler.js:61
next @ core.js:5373
schedulerFn @ core.js:4220
SafeSubscriber.__tryOrUnsub @ Subscriber.js:238
SafeSubscriber.next @ Subscriber.js:185
Subscriber._next @ Subscriber.js:125
Subscriber.next @ Subscriber.js:89
Subject.next @ Subject.js:55
EventEmitter.emit @ core.js:4200
(anonymous) @ core.js:4648
t.invoke @ polyfills.js:3
r.run @ polyfills.js:3
NgZone.runOutsideAngular @ core.js:4574
onHandleError @ core.js:4648
t.handleError @ polyfills.js:3
r.runGuarded @ polyfills.js:3
(anonymous) @ polyfills.js:3
n.microtaskDrainDone @ polyfills.js:3
o @ polyfills.js:3
e.invokeTask @ polyfills.js:3
p @ polyfills.js:2
v @ polyfills.js:2
IndexedDB (async)
(anonymous) @ localforage.js:826
t.invoke @ polyfills.js:3
onInvoke @ core.js:4626
t.invoke @ polyfills.js:3
r.run @ polyfills.js:3
(anonymous) @ polyfills.js:3
t.invokeTask @ polyfills.js:3
onInvokeTask @ core.js:4617
t.invokeTask @ polyfills.js:3
r.runTask @ polyfills.js:3
o @ polyfills.js:3
e.invokeTask @ polyfills.js:3
p @ polyfills.js:2
v @ polyfills.js:2

Answer 1

The getMyTopics method doesn't return anything. Also the catch operator is returning an empty array when it should return an Observable.