Reflow is the process where the browser recalculates the layout of the page, whereas repaint only redraws elements that have changed visually, without affecting the layout.
Example:
// Reflow happens when you modify layout-affecting properties like width/height
document.body.style.width = '500px'; // Triggers reflow
// Repaint happens when you modify visual properties like background-color
document.body.style.backgroundColor = 'red'; // Triggers repaintTags: advanced, JavaScript, Performance