How to use document.getElementsByClassName() in TypeScript


Borislav Hadzhiev

Fri Apr 01 20222 min read

Use document.getElementsByClassName() in TypeScript #

To use the document.getElementsByClassName() method in TypeScript, use a type assertion to type the collection of elements correctly, e.g. const boxes = document.getElementsByClassName('box') as HTMLCollectionOf<HTMLElement>;.

This is the index.html file for the examples in this article.

<!DOCTYPE html> <html> <head> <meta charset="UTF-8" /> </head> <body> <div class="box">Box 1</div> <div class="box">Box 2</div> <div class="box">Box 3</div> <script src="./src/index.ts"></script> </body> </html>

And here is the related TypeScript code.

const boxes = document.getElementsByClassName( 'box', ) as HTMLCollectionOf<HTMLElement>; Array.from(boxes).forEach(box => { console.log(box.innerText); });

The document.getElementsByClassName method has a return type of HTMLCollectionOf<Element>.

The Element interface does not have many of the properties you might need to use, e.g. style or innerText, so we used a type assertion to type the collection of elements as HTMLCollectionOf<HTMLElement>.

Type assertions are used when we have information about the type of a value that TypeScript can't know about.

We effectively tell TypeScript that each element in the collection will be of type HTMLElement and not to worry about it.

We could have also been more specific and used the HTMLDivElement type.

const boxes = document.getElementsByClassName( 'box', ) as HTMLCollectionOf<HTMLDivElement>; Array.from(boxes).forEach(box => { console.log(box.innerText); });

Now we typed the boxes variable as a collection of div elements.

The types are consistently named as HTML***Element. Once you start typing HTML.., your IDE should be able to help you with autocomplete.

Some commonly used types are: HTMLInputElement, HTMLButtonElement, HTMLAnchorElement, HTMLImageElement, HTMLTextAreaElement, HTMLSelectElement, etc.

Note that the document.getElementsByClassName method doesn't return an array, it returns an array-like object of all child elements which have the provided class name.

If you need to access array-specific methods on the collection, you have to convert it to an array first.

const boxes = Array.from( document.getElementsByClassName('box') as HTMLCollectionOf<HTMLElement>, ); boxes.forEach(box => { console.log(box.innerText); });

We used the Array.from method to convert the array-like object to an array, so we can use the forEach method to iterate over it.

The type of the box variable in each iteration is HTMLElement, because we used a type assertion with the getElementsByClassName method.

Note that if you have to access element-specific properties, like the value property on an input element, you'd have to type the collection as a collection of that specific element type, e.g. HTMLCollectionOf<HTMLInputElement>.

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