OnLoad

OnLoad refers to a trigger or handler that runs when a component, document, or interface has finished loading. It is used to perform initialization tasks that require all resources to be available, such as rendering data, attaching event handlers, or starting animations.

In web browsers, the onload event fires after the entire document and all dependent resources have loaded. It can be wired either with the onload attribute in HTML, or by registering a listener for the load event in JavaScript (for example window.addEventListener('load', ...)). Because onload fires after images, scripts, and subframes have loaded, it often occurs later than the DOMContentLoaded event, which signals that the document structure is ready but not necessarily all resources.

Best practices encourage using DOMContentLoaded or similar events to initialize UI quickly, reserving onload for tasks that genuinely require all resources. Avoid heavy work in onload, and consider async loading and deferred resources to reduce perceived load time.

Beyond the browser, OnLoad is used in various GUI frameworks as a conventional name for initialization code that runs when a component is displayed. In Microsoft .NET, for example, a Form or Page may expose a Load or OnLoad method that developers override to set up data and state.

See also DOMContentLoaded, page load, ready event.