When Does Reflow Occur in a DOM Environment?

Reflow in a DOM Environment

Reflow refers to the process of recalculating the dimensions and positions of elements within the DOM, often triggered by changes made to the document structure or style. In this context, different activities can initiate reflow, as described below:

Adding or Removing DOM Nodes:
When you add or remove elements from the DOM, the browser must readjust the layout of the page to accommodate the changes. This involves recalculating the dimensions and positions of affected elements, resulting in reflow.

Applying Style Dynamically:
Modifying an element's style dynamically, such as by changing its width or height using the element.style property, triggers reflow. The browser needs to update the layout based on the new style information.

Retrieving Calculated Measurements:
Accessing certain properties, such as offsetWidth, clientHeight, or computed CSS values via getComputedStyle(), can also initiate reflow. These properties require the browser to perform calculations to determine the current dimensions and positions of elements.

Contrary to the belief that measurements alone trigger reflow, both articles cited suggest that reflow only occurs when these measurements are taken during a time when DOM changes are already queued up to be made. This indicates that the specific timing of the measurement is crucial.

To minimize the potential impact of reflow, it's recommended to take measurements only when necessary and to avoid excessive retrieval of calculated values during periods of active DOM changes. By consciously optimizing DOM manipulation and measurement activities, developers can improve page performance and reduce unnecessary browser reflows.

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