Event

/ˈɒnˌprəʊɡrɛs/

noun … “an event handler for tracking incremental data transfer.”

onprogress is an event handler used to observe the ongoing progress of a long-running operation, most commonly data transfer over a network. Instead of waiting for completion or failure, it provides continuous feedback while bytes are still moving.

In web environments, onprogress is most often encountered during network requests and streaming operations. It is closely associated with browser networking primitives exposed through standardized APIs. As data is received or uploaded in chunks, progress events are emitted, allowing applications to react in near real time.

The core utility of onprogress is visibility. Without it, applications operate in a binary state … idle, then suddenly finished. With progress events, software can surface loading bars, percentage indicators, throughput estimates, or adaptive behaviors such as lowering resolution or pausing dependent work. This dramatically improves perceived responsiveness.

From a systems perspective, onprogress fits naturally into asynchronous execution models built around async operations and Promise-based workflows. While a promise represents eventual completion, progress events represent the journey. The two complement each other rather than overlap.

Progress handling is especially important for large payloads, media streaming, and file synchronization. Operations like uploads, downloads, and replication often involve unpredictable latency and bandwidth variation. By emitting progress updates, the runtime exposes internal state without blocking execution, aligning with event-driven design principles.

There is also a performance and correctness nuance. Progress events may fire frequently and unevenly depending on buffer sizes, transport layers, and implementation details. Code handling onprogress must therefore be lightweight and tolerant of partial information. It should never assume linearity or precise timing.

In modern web applications, onprogress is often paired with structured data exchange formats and runtime environments such as Node.js, where streaming abstractions extend the same idea beyond the browser. Whether client-side or server-side, the principle remains constant … long work should reveal its shape while it happens.

Conceptually, onprogress represents a philosophical shift away from opaque computation. Instead of treating time as a black box, it treats execution as something observable. That observability is not just cosmetic … it enables smarter interfaces, better error handling, and more humane software.

Used well, onprogress turns waiting into understanding. Used poorly, it becomes noise. Like most event hooks, its value lies in restraint, clarity, and respect for the asynchronous nature of the systems it observes.

/ˈɒnˌriːsaɪz/

noun … “an event handler triggered when dimensions change.”

onresize is an event handler used in interactive computing environments to detect when the size of a rendering context changes. Most commonly, this refers to changes in the browser window or viewport, but the underlying idea applies to any system where layout depends on dynamic dimensions.

In web environments, onresize is tightly coupled to the browser’s rendering pipeline and the DOM. When a resize occurs, the browser recalculates layout, reflows elements, and may repaint the screen. The onresize handler provides a hook into this moment, allowing code to react immediately after the geometry changes.

This reaction is often necessary when layout behavior cannot be expressed purely with CSS. While modern responsive design handles many cases declaratively, some logic requires computation … recalculating canvas dimensions, adjusting chart scales, or modifying interaction logic based on available space. These behaviors live at the intersection of presentation and control, where onresize becomes essential.

From a programming perspective, onresize is part of an event-driven model exposed through browser APIs. Instead of polling for size changes, the system emits a resize event only when change occurs. This mirrors the same architectural pattern used by onload and onerror, where execution responds to signals rather than assumptions.

Performance is the hidden danger zone. Resize events can fire repeatedly and rapidly while a user drags a window edge or rotates a device. Expensive computations inside an onresize handler can easily overwhelm the main thread, especially in JavaScript runtimes like Node.js when similar event patterns are used server-side. For this reason, resize logic is often throttled or debounced to limit execution frequency.

Modern interfaces also blur the line between window resizing and component resizing. While onresize traditionally watches the global viewport, newer layout systems focus on container-aware responsiveness, reflecting a broader shift in UI architecture. Still, the core idea remains the same … geometry changes, and behavior must adapt.

Conceptually, onresize is a reminder that spatial assumptions are fragile. Screens rotate, windows snap, displays scale, and user environments mutate constantly. Software that ignores this reality feels brittle. Software that listens for resize events feels alive.

Used with restraint and intention, onresize enables interfaces that respond fluidly to changing conditions. Used carelessly, it becomes a performance hazard. As with many low-level hooks, its real power lies not in reacting to every change, but in reacting only when change actually matters.

/ˈɒnˌɛrər/

noun … “an event handler for error conditions.”

onerror is an event handler used in web and programming environments to detect and respond to errors at runtime. It acts as a kind of early-warning system … when something fails, breaks, or refuses to load, onerror is where control flows next.

In the browser world, onerror most commonly appears in two related contexts: global JavaScript error handling and resource-loading errors. Both serve the same philosophical role … catching failures before they disappear into silence.

At the global level, onerror can be attached to the window object. When an uncaught JavaScript exception occurs … a syntax error, a reference to an undefined variable, or a runtime crash … the handler is triggered. This allows developers to log diagnostic data, display fallback UI, or report failures to monitoring systems rather than letting the application fail invisibly.

Resource handling is the second major domain. HTML elements such as images, scripts, audio, and video can define an onerror handler that fires when loading fails. A missing image file, a blocked script, or a network interruption all surface through this mechanism. Instead of showing a broken icon or silently failing, the application can react intelligently … load a fallback asset, retry, or notify the user.

Conceptually, onerror is part of the event-driven programming model. Rather than checking for failure after every operation, the system emits an error event when something goes wrong. The handler listens for that event and responds asynchronously. This fits naturally with non-blocking systems and complements constructs like async, await, and promises.

One important subtlety is scope. A global onerror handler catches only errors that escape local control. Errors that are already handled inside a try/catch block never reach it. This makes onerror a safety net, not a replacement for proper error handling logic.

Security and privacy also shape how onerror behaves. Browsers intentionally limit the information exposed for certain cross-origin errors. Instead of detailed stack traces, the handler may receive only a generic message. This prevents sensitive internal details from leaking across trust boundaries, even though it can make debugging more challenging.

Outside the browser, similar ideas exist in other runtimes. Server-side JavaScript environments expose comparable hooks for uncaught exceptions and fatal errors, though the exact APIs differ. The shared principle remains the same … centralized observation of failure.

Philosophically, onerror acknowledges an uncomfortable truth of computing: things will fail. Networks drop packets. Files go missing. Assumptions collapse. Rather than pretending perfection is possible, onerror provides a structured place to respond when reality intrudes.

Used thoughtfully, onerror turns crashes into data and confusion into recovery paths. Used carelessly, it can mask serious bugs by swallowing failures without fixing root causes. Like most powerful tools, its value lies not in its existence, but in how deliberately it is applied.