Angular 2 Cannot read property 'x' of null

Question

I am learning promises and I am trying to get a simple example to work but I get the error. The promises role is just to check whether a certain name has been entered and produce a boolean error on call back

Cannot read property 'shouldBeUnique' of null

Here is my component

import {Component, Inject} from '@angular/core';

import {FormGroup, Validators, FormBuilder} from '@angular/forms';

import {UsernameValidators} from './usernameValidators'



@Component({

    selector: 'signup-form',

    templateUrl: 'app/signup-form.component.html'

})

export class SignUpFormComponent {



    form: FormGroup;



    constructor(@Inject(FormBuilder) fb: FormBuilder) {

        this.form = fb.group({

            username: ['', Validators.compose([

                Validators.required,

                UsernameValidators.cannotContainSpace

            ]), UsernameValidators.shouldBeUnique],

            password: ['', Validators.required]

        })

    }



    get username(): any {return this.form.get('username');}

    get password(): any {return this.form.get('password');}



}

Here is my component.html

<form [formGroup]="form" (ngSubmit)="signup()">

    <div class="form-group">

        <label for="username">Username</label>

        <input 

                id="username"

                type="text"

                class="form-control"

                formControlName="username" placeholder="Username"

                >

        <div *ngIf="username.touched && username.errors">

            <div *ngIf="username.errors.shouldBeUnique" class="alert alert-danger">This username is already taken</div>

        </div>

</form>

Here is my validator class where the promise is being made

import {FormControl} from '@angular/forms';



export class UsernameValidators {



    static shouldBeUnique(control: FormControl) {

        return new Promise((resolve, reject) => {

            setTimeout(function(){

                if (control.value == "mosh")

                    resolve({ shouldBeUnique: true});

                else

                    resolve(null);

            }, 1000)

        });

    }

}

Thanks

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Answer 1

Try using the safe navigation operator (?.) to guard against null and undefined values in property paths.

<div *ngIf="username.touched && username.errors">
    <div *ngIf="username.errors?.shouldBeUnique" class="alert alert-danger">This username is already taken</div>
</div>

This should resolve the error you are currently running into. Read more in the Angular 2 docs here:

https://angular.io/docs/ts/latest/guide/template-syntax.html#!#safe-navigation-operator

Angular 2 Cannot read property 'x' of null

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