我认为您的问题位于此处:

I think that your problem is located here:

getRisks(): Observable<RiskListSummary[]> {
  return this.http.get(this.serviceUrl)
     .map(this.extractData()) <== passing result of function
     .catch(this.handleError()); <== passing result of function
}

你可以只使用传递 function 引用:

You could use just passing function reference:

getRisks(): Observable<RiskListSummary[]> {
  return this.http.get(this.serviceUrl)
     .map(this.extractData)
     .catch(this.handleError);
}

但是这样你会丢失this.

或者你可以使用 bind 方法保留this:

Or you could use bind method to retain this:

getRisks(): Observable<RiskListSummary[]> {
  return this.http.get(this.serviceUrl)
    .map(this.extractData.bind(this))
    .catch(this.handleError.bind(this));
}

但是你会失去类型检查.

我会利用 arrow 函数来能够使用词法this:

I would leverage arrow functions to be able to use lexical this:

getRisks(): Observable<RiskListSummary[]> {
  return this.http.get(this.serviceUrl)
    .map(res => this.extractData(res))
    .catch(err => this.handleError(err));
}

没有它，this 变量将指向进行调用的函数，而不是包含 getRisks() 的实例.

Without it, the this variable will point to the function where the call is made, instead of the instance that contains getRisks().