ResizeObserver: ResizeObserver() constructor
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2020.
The ResizeObserver constructor creates a
new ResizeObserver object, which can be used to report changes to the
content or border box of an Element or the bounding box of an
SVGElement.
Syntax
new ResizeObserver(callback)
Parameters
callback-
The function called whenever an observed resize occurs. The function is called with two parameters:
entries-
An array of
ResizeObserverEntryobjects that can be used to access the new dimensions of the element after each change. observer-
A reference to the
ResizeObserveritself, so it will definitely be accessible from inside the callback, should you need it. This could be used for example to automatically unobserve the observer when a certain condition is reached, but you can omit it if you don't need it.
The callback will generally follow a pattern along the lines of:
jsfunction callback(entries, observer) { for (const entry of entries) { // Do something to each entry // and possibly something to the observer itself } }
Examples
The following snippet is taken from the resize-observer-text.html (see source) example:
const resizeObserver = new ResizeObserver((entries) => {
for (const entry of entries) {
if (entry.contentBoxSize) {
if (entry.contentBoxSize[0]) {
h1Elem.style.fontSize = `${Math.max(
1.5,
entry.contentBoxSize[0].inlineSize / 200,
)}rem`;
pElem.style.fontSize = `${Math.max(
1,
entry.contentBoxSize[0].inlineSize / 600,
)}rem`;
} else {
// legacy path
h1Elem.style.fontSize = `${Math.max(
1.5,
entry.contentBoxSize.inlineSize / 200,
)}rem`;
pElem.style.fontSize = `${Math.max(
1,
entry.contentBoxSize.inlineSize / 600,
)}rem`;
}
} else {
h1Elem.style.fontSize = `${Math.max(
1.5,
entry.contentRect.width / 200,
)}rem`;
pElem.style.fontSize = `${Math.max(1, entry.contentRect.width / 600)}rem`;
}
}
console.log("Size changed");
});
resizeObserver.observe(divElem);
Specifications
| Specification |
|---|
| Resize Observer # dom-resizeobserver-resizeobserver |
Browser compatibility
| desktop | mobile | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
ResizeObserver() constructor
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