How to Create an Array of Objects from Another Class in Angular

Learn how to efficiently create an array of objects from one class in Angular using TypeScript. This guide explains common mistakes and provides step-by-step solutions for developers.
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This video is based on the question https://stackoverflow.com/q/69081597/ asked by the user 'Seyon Seyon' ( https://stackoverflow.com/u/4956735/ ) and on the answer https://stackoverflow.com/a/69081667/ provided by the user 'dom.macs' ( https://stackoverflow.com/u/5415153/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction: The Problem of Creating an Array of Objects in Angular

If you're working with Angular and TypeScript, you may encounter a common problem when trying to create an array of objects from another class. Specifically, you might have two classes that you would like to work with, but find yourself running into issues like errors or unexpected behaviors.

In this guide, we'll dive into a specific case involving two classes, A and B. We'll discuss how to correctly create an array of class B objects in class A, and the mistakes that commonly lead to errors in the code. Let’s walk through it!

Understanding the Classes

We'll start by examining the two classes involved in our example:

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The class B has two properties: id and Name. This serves as our template for the objects we’ll be creating in the array.

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In class A, we have an array property mygroupB designed to hold objects of class B. The aim is to populate mygroupB with data retrieved via an HTTP client request.

The Error Encountered

While trying to execute the code, you might encounter the following error:

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This often occurs due to incorrect instantiation of the mygroupB array.

Solution: Creating the Array of Objects Properly

To resolve this problem, we need to correctly instantiate mygroupB and ensure we are appropriately populating it with instances of class B. Let's break down the solution into clear steps.

Step 1: Instantiate the Array

Instead of mistakenly using new B[data.length] which does not work as intended, we will instantiate mygroupB as an empty array of type B.

Step 2: Iterate Through the Data

Next, we will loop through the incoming data and push new objects of class B into the mygroupB array.

Updated Code Example

Here’s the corrected implementation of the setData method:

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Explanation of Changes

Instantiation: The line this.mygroupB = new Array<B>(); initializes mygroupB as a properly instantiated array that can hold objects of type B.

Looping through Data: By using data.forEach, we iterate through the incoming data and create new objects that are pushed into mygroupB. Here, we're ensuring that we are specifically creating the required structure as per class B.

Conclusion

Creating an array of objects from another class in Angular can initially seem daunting, especially if you run into constructor errors. By understanding the proper instantiation and data handling in TypeScript, you can easily overcome these challenges.

The solution we discussed allows you to dynamically create and populate an array based on the retrieved data, enabling you to manage your classes effectively. Happy coding!

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