Understanding Why Array.map in TypeScript Returns JSX.Element[] Instead of typeof CustomComponent[]

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The Problem Explained

Consider the following code snippet:

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

When you run this code, items will return an array of type JSX.Element[]. You might question, "Why doesn’t it return typeof CustomComponent[] instead?" This is a common point of confusion for developers.

Why the Confusion?

When you call the map function on an array of values:

Immediate Rendering: The function is immediately creating and returning JSX.Element instances for each element in the original array. CustomComponent is instantiated as a JSX.Element, not as an instance of CustomComponent. This is why the result is JSX.Element[].

TypeScript Behavior: TypeScript recognizes that what you are working with is JSX, which is essentially a syntax that produces elements for the React framework, therefore its type is naturally JSX.Element.

The Solution: Manipulating Data Before Rendering

If your goal is to sort, filter, or otherwise manipulate the data before you render your components, you need to approach this problem differently. Instead of directly manipulating the array after calling map, you should handle data operations before the rendering step.

Filtering Data Before Render

Here’s how you can structure your code to achieve that:

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

Breakdown of Steps:

Filter Data: Use the filter method to pre-process your data. This allows you to remove any elements that do not meet your criteria.

Map to Components: Once the data is filtered, you can safely map over the filtered array and create CustomComponent instances.

Why This Matters

By separating the filtering logic from the rendering, you ensure that your components only render the relevant data. Additionally, operations like sorting will become much simpler without leading to type errors, as you'll have a cleaner dataset to work with.

Key Takeaway