Copy & paste demo

by Comandeer

HTML

<textarea id="input" readonly>Copy me!</textarea>

<textarea id="output">Paste inside me!</textarea>

JavaScript

document.querySelector( '#input' ).addEventListener( 'copy', ( evt ) => {
	evt.preventDefault();
	
	evt.clipboardData.setData( 'text/plain', 'Not what you think you copied' );
} );

document.querySelector( '#output' ).addEventListener( 'paste', ( evt ) => {
	evt.preventDefault();
	
	evt.target.value = 'Not what you think you pasted';
} );