defer vs async

🏷️ Javascript, Web dev

defer

• fetch the script parallel as the parser executes
• execute the script after parsing is complete

async

• fetch the script parallel while parsing
• execute asap as it is available
• parsing is blocked while the script executes

https://www.patterns.dev/vanilla/third-party

https://javascript.info/script-async-defer

https://web.dev/articles/optimizing-content-efficiency-loading-third-party-javascript