can not read get property of undefined angular 2 error - api

Question

hi I am trying to get city name from google api but getting that error below is my code
appcomponent class
import {Component, OnInit} from 'angular2/core';
import {marketComponent} from './market.component';
import {RouteConfig, ROUTER_DIRECTIVES} from 'angular2/router';
import {introComponent} from './intro.component';
import {geoService} from './service.geo';
import {JSONP_PROVIDERS} from 'angular2/http';
declare var google: any;
#Component({
selector: 'my-app',
templateUrl: 'app/app.component.html',
directives: [ROUTER_DIRECTIVES],
providers: [JSONP_PROVIDERS, geoService]
})
#RouteConfig([
{ path: '/intro', name: 'Intro', component: introComponent, useAsDefault: true },
{ path: '/market', name: 'Market', component: marketComponent },
])
export class AppComponent {
constructor(private _http: geoService) { }
public maps;
public cat_error: Boolean = false;
public xml_Latitude :string;
public xml_Lang: string;
ngOnInit() {
if (navigator.geolocation) {
navigator.geolocation.getCurrentPosition(this.showPosition);
} else {
alert("Geolocation is not supported by this browser.");
}
var input: any = document.getElementById('google_places_ac');
var autocomplete = new google.maps.places.Autocomplete(input, {});
google.maps.event.addListener(autocomplete, 'place_changed', function () {
var place = autocomplete.getPlace();
console.log(place)
});
}
showPosition(position) {
this.xml_Latitude = position.coords.latitude;
this.xml_Lang = position.coords.longitude;
this._http.getPlaces(this.xml_Latitude, this.xml_Lang).subscribe(
data => { this.maps = data },
err => { this.cat_error = true }
);
var result = this.maps.results;
var city = result[0].address_components[4].long_name + "," + result[0].address_components[6].long_name;
alert(city);
}
}
and geoservice file
import {Injectable} from 'angular2/core';
import { Response, Jsonp} from 'angular2/http';
import 'rxjs/add/operator/map';
#Injectable()
export class geoService {
constructor(private http: Jsonp) { }
public xml_Latitude: string;
public xml_Lang: string;
public getPlaces(xml_Latitude, xml_Lang) {
return this.http.get(`http://maps.googleapis.com/maps/api/geocode/json?latlng=
'${this.xml_Latitude}','${this.xml_Lang}'&sensor=true`)
.map((res: Response) => res.json())
.catch(this.handleError);
}
private handleError(error: Response) {
console.error(error);
return error.json().error || 'Server error';
}
}
error also says getplaces is not a function, I think I am missing something but don't know what....

Answer 1

In addition to the callback ordering problem identified by Thierry, you have a lost this context on this line:
navigator.geolocation.getCurrentPosition(this.showPosition);
The Problem
You have the classic JavaScript problem known as the incorrect this context.
The this keyword in JavaScript behaves differently than in does in other languages like C# and Java.
How this works
The this keyword, in a function, is determined as follows:
* If the function was created through a call to .bind, the this value is the argument provided to bind
* If the function was invoked through a method call, e.g. expr.func(args), then this is expr
* Otherwise
* If the code is in strict mode, this is undefined
* Otherwise, this is window (in a browser)
Let's look at how this works in practice:
class Foo {
value = 10;
doSomething() {
// Prints 'undefined', not '10'
console.log(this.value);
}
}
let f = new Foo();
window.setTimeout(f.doSomething, 100);
This code will print undefined (or, in strict mode, throw an exception).
This is because we ended up in the last branch of the decision tree above.
The doSomething function was invoked, the function wasn't a result of a bind call, and it wasn't invoked in a method syntax position.
We can't see the code for setTimeout to see what its invocation looks like, but we don't need to.
Something to realize is that all doSomething methods point to the same function object.
In other words:
let f1 = new Foo();
let f2 = new Foo();
// 'true'
console.log(f1.doSomething === f2.doSomething);
We know that setTimeout can only see the function we passed it, so when it invokes that function,
there's no way for it to know which this to provide.
The this context has been lost due to our referencing the method without invoking it.
The Red Flag
Once you know about this problems, they're easy to spot:
class Foo {
value = 10;
method1() {
doSomething(this.method2); // DANGER, method reference without invocation
}
method2() {
console.log(this.value);
}
}
The Solution
You have a few options here, each with its own trade-offs.
The best option depends on how often the method in question is invoked from differing call sites.
Arrow Function in Class Definition
Instead of using the normal method syntax, use an arrow function to initialize a per-instance member.
class DemonstrateScopingProblems {
private status = "blah";
public run = () => {
// OK
console.log(this.status);
}
}
let d = new DemonstrateScopingProblems();
window.setTimeout(d.run); // OK
Good/bad: This creates an additional closure per method per instance of your class. If this method is usually only used in regular method calls, this is overkill. However, if it's used a lot in callback positions, it's more efficient for the class instance to capture the this context instead of each call site creating a new closure upon invoke.
Good: Impossible for external callers to forget to handle this context
Good: Typesafe in TypeScript
Good: No extra work if the function has parameters
Bad: Derived classes can't call base class methods written this way using super.
Bad: The exact semantics of which methods are "pre-bound" and which aren't create an additional non-typesafe contract between your class and its consumers.
Function Expression at Reference Site
Shown here with some dummy parameters for explanatory reasons:
class DemonstrateScopingProblems {
private status = "blah";
public something() {
console.log(this.status);
}
public run(x: any, y: any) {
// OK
console.log(this.status + ': ' + x + ',' + y);
}
}
let d = new DemonstrateScopingProblems();
// With parameters
someCallback((n, m) => d.run(n, m));
// Without parameters
window.setTimeout(() => d.something(), 100);
Good/bad: Opposite memory/performance trade-off compared to the first method
Good: In TypeScript, this has 100% type safety
Good: Works in ECMAScript 3
Good: You only have to type the instance name once
Bad: You'll have to type the parameters twice
Bad: Doesn't easily work with variadic parameters

Answer 2

I think that you should move the result block into the subscribe callback associated the getPlaces method call:
showPosition(position) {
this.xml_Latitude = position.coords.latitude;
this.xml_Lang = position.coords.longitude;
this._http.getPlaces(this.xml_Latitude, this.xml_Lang).subscribe(
data => {
this.maps = data;
var result = this.maps.results; // <----------
var city = result[0].address_components[4].long_name + "," + result[0].address_components[6].long_name;
alert(city);
},
err => { this.cat_error = true }
);
}
It's because this.maps is undefined before the callback is called. And you try tyo get the result attribute before (this.maps.results).
Edit
I also see a problem at the line navigator.geolocation.getCurrentPosition. You could refactor your code this way:
if (navigator.geolocation) {
navigator.geolocation.getCurrentPosition((position) => { // <----
this.showPosition(position);
});
} else {
alert("Geolocation is not supported by this browser.");
}