Borislav Hadzhiev
Tue Oct 19 2021·2 min read
Photo by Yoori Koo
The error "Property 'offsetWidth' does not exist on type 'Element'" occurs
when we try to access the offsetWidth
property on an element that has a type
of Element
. To solve the error, use a type assertion to type the element as
HTMLElement
before accessing the property.
This is the index.html
file for the examples in this article.
<!DOCTYPE html> <html> <head> <meta charset="UTF-8" /> </head> <body> <div id="box">Box 100</div> <script src="./src/index.ts"></script> </body> </html>
And here is an example of how the error occurs in the index.ts
file.
// 👇️ const box: Element | null const box = document.querySelector('#box'); if (box != null) { // ⛔️ Property 'offsetWidth' does not exist on type 'Element'.ts(2339) const offsetWidth = box.offsetWidth; console.log(offsetWidth); }
The reason we got the error is because the return type of the
document.querySelector
method is Element | null
and the
offsetWidth
property doesn't exist on the Element
type.
To solve the error, use a type assertion to type the element as an
HTMLElement
.
const box = document.querySelector('#box') as HTMLElement | null; if (box != null) { const offsetWidth = box.offsetWidth; console.log(offsetWidth); // 👉️ 1448 }
If you used the
document.getElementsByClassName
method, type the collection as HTMLCollectionOf<HTMLElement>
.
// 👇️ with getElementsByClassName // type as HTMLCollectionOf<HTMLElement> const boxes = document.getElementsByClassName( 'box', ) as HTMLCollectionOf<HTMLElement>; for (let i = 0; i < boxes.length; i++) { const offsetWidth = boxes[i].offsetWidth; console.log(offsetWidth); }
We could have also been more specific and typed the element as HTMLDivElement
,
because we're working with a div
in the example.
Type assertions are used when we have information about the type of a value that TypeScript can't know about.
box
variable stores anHTMLElement
or a null
value and not to worry about it.We used a
union type
to specify that the variable could still be null
, because if an HTML element
with the provided selector does not exist in the DOM, the querySelector()
method returns a null
value.
We used a simple if
statement that serves as a
type guard
to make sure the box
variable doesn't store a null
value before accessing
its offsetWidth
property.
const box = document.querySelector('#box') as HTMLElement | null; // 👉️ box has type of HTMLElement or null here if (box != null) { // 👉️ box has type of HTMLElement here const offsetWidth = box.offsetWidth; console.log(offsetWidth); // 👉️ 1448 }
box
variable has a type of HTMLElement
in the if
block and allows us to directly access its offsetWidth
property.It's always a best practice to include null
in the type assertion, because the
querySelector
method would return null
if no element with the provided
selector was found.
You might also use the
optional chaining (?.)
operator to short-circuit if the reference is equal to null
or undefined
const box = document.querySelector('#box') as HTMLElement | null; const offsetWidth = box?.offsetWidth; console.log(offsetWidth); // 👉️ 1448
The optional chaining operator short-circuits returning undefined
if the
reference is equal to null
or undefined
.
In other words, if the box
variable stores a null
value, we won't attempt to
access the offsetWidth
property on null
and get a runtime error.