The secret of Captain America

PASTE THIS CODE ON YOUR SITE

<a href="https://developerslife.tech/en/wp-content/uploads/2015/03/captain-america.png"><img src="https://developerslife.tech/en/wp-content/uploads/2015/03/captain-america.png" alt="" title="" /></a><small>Source: <a href="https://developerslife.tech/en/">The Developer's Life</a></small>

Transcription ↓

Thor: My hammer is magical, but what about your shield?
Captain: I implemented a callback function!