What is DOM reflow?

Question

I was reading about the difference between two CSS properties display:none and visibility:hidden and encountered the DOM reflow term.

The statement was

display: none causes a DOM reflow whereas visibility: hidden does not.

So my question is:

What is DOM reflow and how does it work?

Answer 1

A reflow computes the layout of the page. A reflow on an element recomputes the dimensions and position of the element, and it also triggers further reflows on that element’s children, ancestors and elements that appear after it in the DOM. Then it calls a final repaint. Reflowing is very expensive, but unfortunately it can be triggered easily.

Reflow occurs when you:

• insert, remove or update an element in the DOM
• modify content on the page, e.g. the text in an input box
• move a DOM element
• animate a DOM element
• take measurements of an element such as offsetHeight or getComputedStyle
• change a CSS style
• change the className of an element
• add or remove a stylesheet
• resize the window
• scroll

For more information, please refer here: Repaints and Reflows: Manipulating the DOM responsibly