Dynamically Assigning a Variable Using window and DOM Elements in TypeScript

Learn how to dynamically assign a variable based on DOM elements and window properties in TypeScript to avoid errors related to property types.
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Dynamically Assigning a Variable Using window and DOM Elements in TypeScript

In web development, especially when using TypeScript, you might encounter situations where you need to assign a variable based on certain conditions. This challenge becomes even more prevalent when dealing with dynamic elements on the page and the global window object. Today, we're going to explore a common issue many developers face and how to effectively address it.

The Problem

Imagine you want to get the current scroll position of a specific DOM element that you have selected from the document. However, if that element is not found, you want to fallback to the window's scroll position. The following code snippet highlights the challenge you might encounter:

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

The issue arises because the TypeScript compiler raises an error when attempting to access the scrollTop property on layoutContainer. The error typically states that the property scrollTop does not exist on the type Element | (Window & typeof globalThis). This discrepancy occurs because the querySelector method may return either an Element or null, and if it returns null, your variable layoutContainer ends up pointing to the window, which lacks the scrollTop property.

The Solution

To avoid this issue, you can rewrite your code more effectively. Let’s break it down step by step:

Step 1: Safely Access the DOM Element

Instead of checking the layoutContainer variable after it’s assigned, you can safely access the scrollTop property using optional chaining. This ensures that the property is only accessed if layoutContainer is an actual DOM element:

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

In this adjustment:

We utilize optional chaining (?.) to safely access scrollTop. This means that if the element is not found (and thus is null), the expression evaluates to undefined instead of throwing an error.

Step 2: Handling TypeScript Errors

To make sure TypeScript acknowledges that scrollTop can be accessed this way, you could rely on type assertion. Here’s how you can implement that:

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

In this final piece of code:

We use a type assertion to tell TypeScript that layoutContainer is of type Element. This informs the compiler that the element being accessed has the scrollTop property.

Key Takeaways

Dynamic Element Handling: Accessing DOM elements dynamically can lead to type-related errors in TypeScript.

Optional Chaining: Use optional chaining to prevent errors when accessing properties that may not exist on null objects.

Type Assertions: When dealing with TypeScript, type assertions help in explicitly defining what you expect the type to be, facilitating safer access to properties.

By following these steps, you can efficiently manage variable assignments based on DOM elements and the window object, allowing for smoother and error-free development processes. With a handy understanding of TypeScript's type system, you can build more robust web applications.