google sign-in with angular2 and typescript - where to get gapi?

Question

I'm trying to use google sign-in with angular2 by following this question: Google Sign-In for Websites and Angular 2 using Typescript

But I'm getting an error:

ORIGINAL EXCEPTION: ReferenceError: gapi is not defined
ORIGINAL STACKTRACE:
ReferenceError: gapi is not defined
    at LoginAppComponent.ngAfterViewInit (http://localhost:3000/app/login.component.js:33:9)
    at DebugAppView._View_AppComponent0.detectChangesInternal (AppComponent.template.js:46:68)
    at DebugAppView.AppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12143:18)
    at DebugAppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12247:48)
    at DebugAppView.AppView.detectViewChildrenChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12169:23)
    at DebugAppView.AppView.detectChangesInternal (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12154:18)
    at DebugAppView.AppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12143:18)
    at DebugAppView.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:12247:48)
    at ViewRef_.detectChanges (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:10397:69)
    at eval (http://localhost:3000/node_modules/@angular/core//bundles/core.umd.js:9911:88)

Evidently gapi isn't defined - which I can understand as I seem to only be declaring an empty var.

My current code is as below:

import {Component, NgZone} from "@angular/core";

declare var gapi: any;

@Component({
    selector: "login",
    templateUrl: "templates/login-template.html"
})
export class LoginAppComponent {
  googleLoginButtonId = "google-login-button";
  userAuthToken = null;
  userDisplayName = "empty";

  constructor(private _zone: NgZone) {
    console.log(this);
  }

  // Angular hook that allows for interaction with elements inserted by the
  // rendering of a view.
  ngAfterViewInit() {
    // Converts the Google login button stub to an actual button.
    gapi.signin2.render(
      this.googleLoginButtonId,
      {
        "onSuccess": this.onGoogleLoginSuccess,
        "scope": "profile",
        "theme": "dark"
      });
  }

  // Triggered after a user successfully logs in using the Google external
  // login provider.
  onGoogleLoginSuccess = (loggedInUser) => {
    this._zone.run(() => {
        this.userAuthToken = loggedInUser.getAuthResponse().id_token;
        this.userDisplayName = loggedInUser.getBasicProfile().getName();
    });
}
}

The template loads fine, it is just the gapi bit.

So my question is: what am I missing? How do I need to be defining gapi for it to work?

Here is my main app.component code:

import { Component } from '@angular/core';
import { LoginAppComponent } from './login.component'

@Component({
  selector: 'my-app',
  template: `<script src="https://apis.google.com/js/platform.js?onload=onLoadCallback" async defer></script>
            <script>
            window.onLoadCallback = function(){
              gapi.auth2.init({
                  client_id: 'filler_text_for_client_id.apps.googleusercontent.com'
                });
            }
            </script>
             <h1>Angular2 P.O.C.</h1>
             <login></login>`,
  directives: [LoginAppComponent]
})
export class AppComponent { }

Answer 1

Have you included the Google platform API script?

<script src="https://apis.google.com/js/platform.js"></script>

See this question for instructions on how to wait for the GAPI script to load before executing your code.