JavaScript page lifecycle
JavaScript DOMContentLoaded, load events, async defer, and resource loading
Learn JavaScript page lifecycle events with focused lessons on DOMContentLoaded, window load, beforeunload, unload, document.readyState, script async, script defer, and resource loading onload/onerror.
Page lifecycle and resource loading lessons
DOMContentLoaded and load
Compare DOMContentLoaded, window load, beforeunload, unload, readyState, and browser page lifecycle timing.
Outcome: Run JavaScript at the right moment during page parsing, loading, and unload.
- DOMContentLoaded
- window load JavaScript
- beforeunload JavaScript
Script async and defer
Understand how script loading affects parsing, execution order, DOMContentLoaded, async scripts, and defer scripts.
Outcome: Choose script loading attributes that preserve page speed and predictable execution order.
- script async defer
- defer JavaScript
- async JavaScript
Resource loading onload and onerror
Handle image, script, iframe, and external resource load and error events with onload and onerror.
Outcome: Detect loaded and failed resources without confusing them with full page lifecycle events.
- onload onerror JavaScript
- resource loading JavaScript
- script onerror