ORIGINAL EXCEPTION: self.context.signup is not a function

<form [formGroup]="form" (ngSubmit)="signup()">
    <div class="form-group">
        <label for="username">Username</label>
        <input
            formControlName="username"
            id="username"
            type="text"
            name="username"
            class="form-control">

    </div>
    <div *ngIf="!form.controls.username.valid"
            class="alert alert-danger">Username is required.
    </div> {{ form.controls.username.errors | json }}

    <div class="form-group">
        <label for="password">Password</label>
        <input
            formControlName="password"
            id="password"
            name="password"
            type="text"
            class="form-control">
    </div>
    <button class="btn btn-primary" type="submit">Sign Up</button>
</form>

import {Component} from '@angular/core';
import { FormGroup, FormControl, Validators } from '@angular/forms';

@Component({
    selector: 'signup-form',
    templateUrl: './app/signup-form.component.html'
})
export class SignUpFormComponent {
   form = new FormGroup({
        username: new FormControl('', Validators.required),
        password: new FormControl('', Validators.required)
    });

    signup() {
        console.log(this.form.value);
    }
}