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Learn how to fix the issue of being unable to call functions on observable results in Angular. Discover the importance of object instantiation in TypeScript and RxJS.
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Solving the TypeError in Angular when Working with Observable Results

When developing applications using Angular and TypeScript, you might run into some unexpected runtime errors, especially when dealing with data retrieved from an API. One common problem developers encounter is the inability to call methods on objects that seem like they should support them. In this guide, we are going to dive deep into a specific issue related to TypeError that occurs when you try to filter an array of Car objects based on their color without proper object instantiation.

The Problem

Suppose you have defined an interface Vehicle and a class Car that implements this interface. The Car class has a method called isColorRed, which checks whether the car's color is red. Here’s a simplified view of the code:

[[See Video to Reveal this Text or Code Snippet]]

Now, imagine that you are fetching an array of Car objects from your backend service:

[[See Video to Reveal this Text or Code Snippet]]

However, you receive a TypeError indicating that isColorRed() is not a function.

So, why is this happening?

The Core of the Issue

This issue arises from the nature of how TypeScript and JavaScript actually handle objects at runtime. While TypeScript provides strong typing and allows you to define interfaces and classes, when data is retrieved from an API, you often receive plain JSON objects. They do not carry along any methods defined in the class.

Here’s what goes wrong:

When you fetch the data, what you actually get back are raw JSON objects.

The Car instances that you expect are not being created; thus, methods like isColorRed() are not available.

The Solution: Explicit Mapping

To solve this problem, you need to explicitly instantiate Car objects from the raw data returned by the API. This ensures that the methods you expect to work on those objects are indeed available. Here is the modified code that will work as intended:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Solution

Step-by-Step Explanation:

Fetch the Data: Use the httpClient to get the array of vehicles as before.

Map to Car Instances: Before filtering, transform each vehicle object into a Car object:

[[See Video to Reveal this Text or Code Snippet]]

Use the Method: After mapping, proceed to filter the cars:

[[See Video to Reveal this Text or Code Snippet]]

Benefits of This Approach:

Ensures that all methods defined in the Car class are available and behave as expected.

Maintains the integrity of object-oriented principles in your application.

Conclusion

In conclusion, always remember that while TypeScript provides a layer of safety with its type system, the runtime execution is based on JavaScript behavior, which may not always align with your expectations. By explicitly mapping the raw data to the appropriate class instances, you can effectively avoid runtime errors such as TypeError.

Next time you're fetching data in Angular, keep this in mind to streamline your development process and ensure smoother function calls on your observable results!