Angular2 FormBuilder and nested object property returns undefined

Question

I'm just getting my feet wet with TypeScript and Angular2. I'm working with an API that has a nested object structure. I would like my model to mirror the resource from the API. The model/resource, "Inquiry", as defined in TypeScript:

// inquiry.ts
export class Inquiry {
  name: {
    first: string,
    last: string
  };
  email: string;
  phone: string;
  question: string;
}

My form component is as such:

import { Component, OnInit } from '@angular/core';
import { FormBuilder, FormGroup, Validators } from '@angular/forms';
import { InquireService } from './inquire.service';
import { Inquiry } from './inquiry';

@Component({
  selector: 'inquire-form',
  templateUrl: './inquire-form.component.html'
})
export class InquireFormComponent implements OnInit {
  inquiryForm: FormGroup;
  inquiry = new Inquiry;

  constructor(
    private formBuilder: FormBuilder,
    private inquireService: InquireService) {}

  ngOnInit(): void {
    this.buildForm();
  }

  buildForm(): void {
    this.inquiryForm = this.formBuilder.group({
      'firstName': [
        this.inquiry.name.first, [
          Validators.required,
          Validators.minLength(2),
          Validators.maxLength(50)
        ]
      ], ...
  }
}

The error I get when I hit my route is:

Error: Uncaught (in promise): TypeError: Cannot read property 'first' of undefined

When I log this.inquiry.name it is indeed undefined, but I'm not understanding why. What am I doing wrong?

You merely stated what the type signature of the name property is but haven't actually initialized it. — Jeff Mercado
– Jeff Mercado, Commented Sep 16, 2016 at 21:19
Hmm. Is it not initialized when I created the new Inquiry instance (above the constructor)? — Brandon Taylor
– Brandon Taylor, Commented Sep 16, 2016 at 23:11
As far as I understand, you just declared that an Inquiry object contains 4 properties, a name of type {first:string,last:string} and email, phone, and question of type string. None of the properties are explicitly initialized so they just have their default values (undefined). — Jeff Mercado
– Jeff Mercado, Commented Sep 16, 2016 at 23:15
I see. I'm new to Typescript. Should I / can I provide a default value of an empty string for those properties in the class definition? Or should I pass the default values in when I create the instance? — Brandon Taylor
– Brandon Taylor, Commented Sep 16, 2016 at 23:27
I'm not quite understanding why I have to provide a value for name but not the other properties. — Brandon Taylor
– Brandon Taylor, Commented Sep 16, 2016 at 23:48

Jeff Mercado · Accepted Answer · 2016-09-17 03:10:24Z

2

The problem is that none of your properties on the Inquiry object are initialized so they're all the default value undefined. And since the one that matters name isn't initialized, attempts to access any of its properties (first or last) will fail.

Either set it in a constructor:

constructor() {
    this.name = {} as any; // too lazy to give first/last properties
}

or initialize it in the declaration:

class Inquiry {
    name = {} as {
        first: string,
        last: string
    };
    email: string;
    phone: string;
    question: string;
}

or if you want to explicitly keep the type in the declaration and not rely on type inference:

class Inquiry {
    name: {
        first: string,
        last: string
    } = {} as any;
    email: string;
    phone: string;
    question: string;
}

edited Sep 17, 2016 at 3:10

answered Sep 16, 2016 at 23:50

Jeff Mercado

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4 Comments

Brandon Taylor Over a year ago

Ah. The second example makes more sense. I'll give that a try as soon as I get back to my computer. Thank you very much for the help.

Brandon Taylor Over a year ago

I implemented the second example and my formBuilder is working again. Thank you! Obviously I need to brush up on my TypeScript.

Jeff Mercado Over a year ago

I wouldn't say this is necessarily a typescript thing, just general OOP. The difference here is that you're describing a pattern of an object and not giving it an explicit name. You declared a variable/property to an object with certain properties. But you still need to initialize that variable to some value as you would in say Java or C#.

Brandon Taylor Over a year ago

Gotcha. It's been about 10 years since I've worked with C# :)

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