What is the purpose of type casting in Typescript?

As a TS newcomer, I have a question that surprisingly lacks a clear explanation. What is the main difference between specifying the type to TypeScript in these two ways:

const ul = document.querySelector('#nav') as HTMLUListElement;

and this way:

const ul: HTMLUListElement = document.querySelector('#nav');

I've noticed that TypeScript seems to recognize the type and provide IntelliSense in both scenarios. So why should I use type casting here as recommended by other developers?

Thank you in advance.

typescript

Answer №1

“The method querySelector will either return an Element or null.”

When using the keyword ‘as’, you are explicitly converting the value on the right side to a specific type.

However, the use of “:HTMLUListElement” is only valid when this type is a subclass of Element. You can refer to the structure of htmlelement at https://developer.mozilla.org/en-US/docs/Web/API/HTMLUListElement

Since queryselector returns Element, we can specify “: HTMLUListElement” If it returns something else like AppleClass, then this method cannot be used. In such cases, you would need to use 'as', but it can be risky if you are unsure about the exact return type. Using 'as' to cast an object from AppleClass to HTMLElement may result in missing necessary properties.

Based on the context provided, I believe that 'as' may not be suitable for your scenario and another approach might work better.

Edit: Using " : HTMLUListElement" denotes down casting.

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