ResizeObserver: observe() method
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 observe() method of the
ResizeObserver interface starts observing the specified
Element or SVGElement.
Syntax
observe(target)
observe(target, options)
Parameters
target-
A reference to an
ElementorSVGElementto be observed. optionsOptional-
An options object allowing you to set options for the observation. Currently this only has one possible option that can be set:
box-
Sets which box model the observer will observe changes to. Possible values are:
content-box(the default)-
Size of the content area as defined in CSS.
border-box-
Size of the box border area as defined in CSS.
device-pixel-content-box-
The size of the content area as defined in CSS, in device pixels, before applying any CSS transforms on the element or its ancestors.
Return value
None (undefined).
Exceptions
None.
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) {
// Checking for chrome as using a non-standard array
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 {
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);
An observe() call with an options object would look like so:
resizeObserver.observe(divElem, { box: "border-box" });
Specifications
| Specification |
|---|
| Resize Observer # dom-resizeobserver-observe |
Browser compatibility
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