How to push selected objects from an array of objects into one array of objects

Question

I have this object which has many elements such as a couple other objects within the array.

There are Two Arrays:

array of objects1:

Inside that is this:

Subform and Section

the hierarchy is:

[0]--
title
     |
     section [
             properties|
                       [[***target***]]
                                       |
                                       title
     ]
     subform [
             properties|
                       [[***target***]]
                                       |
                                       title
     ]
[1]--
     |
     section [
             properties|
                       [[***target***]]
                                       |
                                       title
     ]
     subform [
             properties|
                       [[***target***]]
                                       |
                                       title
     ]

What I'm trying to do is selectively PICK OUT the titles like so:

I instantiate private obj like so:

private obj = {
  data: [{
    subform: '',
    section: '',
    field: ''
  }]
};

and run a for/next loop to populate the obj OBJECT

    for (let i = 0; i < this.dataTableJSON.length; i++) {
      this.obj.data[i].subform = this.dataTableJSON[i].subform.properties.title;
      this.obj.data[i].section = this.dataTableJSON[i].section.properties.title;
      this.obj.data[i].field = this.dataTableJSON[i].title;
    }

What happens is, it goes around 1 TIME and comes back for "1" and says in the error console:

Cannot set property 'subform' of undefined.

the result I seek is simply this:

private obj = {
  data: [{
    subform: 'Title 1',
    section: 'Section Title 1',
    field: 'Field Title 1'
  },{
    subform: 'Title 2',
    section: 'Section Title 2',
    field: 'Field Title 2'
  }]
};

Why is it DYING on the second time around? This is so simple... I'm using TYPESCRIPT

UPDATE FOR DAI:

Dai, I'm 99% there...

When implementing the code, I wrote the following enum.

export interface DataItem {
   subform: string;
   section: string;
   field: string;
};

export type ObjType = {
  data: DataItem[]
};

This is the import of the above

import {DataItem, ObjType} from '../../../services/datatables-integration-services/datatables-datatypes-enum';

and in the actual component.ts file I've put this:

 private readonly obj: ObjType = {
    data: []
 };

And I implemented the for/next loop as you said.

    for (let i = 0; i < this.dataTableJSON.length; i++) {

      const dataCopy: DataItem = {
        subform = this.dataTableJSON[i].subform.properties.title,
        section = this.dataTableJSON[i].section.properties.title,
        field   = this.dataTableJSON[i].title
      };

      this.obj.data[i] = dataCopy;
    }

The below is the error I'm getting and to overcome that error I needed to change

This

  subform = this.dataTableJSON[i].subform.properties.title,
  section = this.dataTableJSON[i].section.properties.title,
  field   = this.dataTableJSON[i].title

To THIS

  subform: this.dataTableJSON[i].subform.properties.title,
  section: this.dataTableJSON[i].section.properties.title,
  field: this.dataTableJSON[i].title

Now testing

And it WORKS! BOOYAH!

Thank you Dai!

UPDATE FOR DAI:

I have an minor mistake:

The FINAL JSON needs to look like this

  {
    "data": [
      {
        field: "Social Security number"
        required: true
        section: "Employee Information"
        subform: "Personal Information"
      },
      {
        field: "Country of issuance"
        required: true
        section: "Eligibility Information"
        subform: "Employment Eligibility"
      }
    ]
  }

Sorry, Dai... thanks again

You can only assign to a member of an indexed-element in an array if a correctly-typed object instance exists at that index. — Dai
– Dai, Commented Aug 31, 2020 at 4:35
OK, great, telling me that is fantastic... how do I get what I want and fix it, please! Also, this may in-fact be dynamic. Meaning everything after data[] may not be section.title, subform.title, or title. I tried push but that doesn't result in anything except and {} as 0, the 1: title1, 2: title 2 NOT inside the {} — Peter The Angular Dude
– Peter The Angular Dude, Commented Aug 31, 2020 at 4:36
My comment explains what's wrong with your code. If you need help understanding what I wrote then please ask to me elaborate - otherwise I feel that my comment does explain how to fix it. Please don't be sarcastic. — Dai
– Dai, Commented Aug 31, 2020 at 4:37
Dai, I'm sorry you took it that way. I thought you understood that if you make a suggestion, it's a given that you'd provide a solution that I can VOTE for you — Peter The Angular Dude
– Peter The Angular Dude, Commented Aug 31, 2020 at 4:38
I didn't provide a full answer in my comment - it was more of a hint or a nudge in the right direction. But I've provided a fuller answer now. — Dai
– Dai, Commented Aug 31, 2020 at 4:40

Dai · Accepted Answer · 2020-08-31 05:31:38Z

Change this:

    for (let i = 0; i < this.dataTableJSON.length; i++) {
      this.obj.data[i].subform = this.dataTableJSON[i].subform.properties.title;
      this.obj.data[i].section = this.dataTableJSON[i].section.properties.title;
      this.obj.data[i].field = this.dataTableJSON[i].title;
    }

To this:

    // Put these `type` declarations somewhere appropriate in your codebase:

    interface DataItem = {
        subform: string;
        section: string;
        field  : string;  
    };

    type ObjType = {
        data: DataItem[]
    };

    // Change your `obj` member to this:
    private readonly obj: ObjType = {
        data: []
    };

    // And change your for loop to this:
    for (let i = 0; i < this.dataTableJSON.length; i++) {
        
        const dataCopy : DataItem = {
            subform: this.dataTableJSON[i].subform.properties.title,
            section: this.dataTableJSON[i].section.properties.title,
            field  : this.dataTableJSON[i].title;
        };

        this.obj.data[i] = dataCopy;
    }

You can also do this (which is syntactically shorter, but hides type information which may confuse people new to TypeScript and JavaScript who may not understand what's going-on):

    for (let i = 0; i < this.dataTableJSON.length; i++) {
        
        this.obj.data[i] = {
            subform: this.dataTableJSON[i].subform.properties.title,
            section: this.dataTableJSON[i].section.properties.title,
            field  : this.dataTableJSON[i].title;
        };
    }

This can be syntactically simplified to this:

    for (let i = 0; i < this.dataTableJSON.length; i++) {
        const d = this.dataTableJSON[i];
        this.obj.data[i] = {
            subform: d.subform.properties.title,
            section: d.section.properties.title,
            field  : d.title;
        };
    }

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