onKeyUp

onKeyUp is an event used in graphical user interfaces and web development to respond when a key is released after being pressed. It is commonly triggered by user input on keyboard-enabled elements, such as text fields and buttons, and is supported across many platforms and frameworks.

In web development, onKeyUp refers to the event that fires when the user releases a key while the focus is on a particular element. It can be attached as an inline HTML attribute (onkeyup) or registered via the DOM API (for example, element.addEventListener('keyup', handler)). The event object is typically a KeyboardEvent and exposes properties such as key (the character produced by the key), code (the physical key on the keyboard), and modifier flags like altKey, ctrlKey, and shiftKey. Some older code also references keyCode, which is deprecated in modern standards.

onKeyUp is distinct from onKeyDown (which fires when the key is pressed) and the older onKeyPress (deprecated in favor of keydown and keyup). In many scenarios, keyup is preferred for actions that depend on the final value of an input field, since it occurs after the character has been added to the field.

Cross-environment terminology varies: in Windows Forms and .NET, KeyUp corresponds to a similar event in KeyEventArgs; in Java, a similar effect is achieved with keyReleased in KeyListener. In React, the event is handled with onKeyUp, using a synthetic event object.

Best practices include using e.key or e.code for robust key handling and avoiding reliance on legacy keyCode values; for text inputs, consider combining onKeyUp with input or change events and debouncing to limit processing.