parsoid-example.js

 $.ajax( {
		// Get this URL
		url: '/api/rest_v1/page/html/' + mw.config.get( 'wgPageName' ),
		// Interpret the result as HTML (as opposed to XML or JSON or something)
		dataType: 'html'
	} )
	.done( function( html ) {
		// Parse the result as HTML
		var doc = new DOMParser().parseFromString( html, 'text/html' ),
			// You can now use jQuery to find stuff using $( 'selector', doc )
			// (you have to specify doc because $('p') finds paragraphs on the current page)
			// For example, the below finds all paragraphs
			$paragraphs = $( 'p', doc );
		// $paragraphs is now a list of all <p> tags in doc

		// Insert <b>YAY</b> before every paragraph
		$paragraphs.before( '<b>YAY</b>' );

		$.ajax( {
			url: '/api/rest_v1/transform/html/to/wikitext/Main_Page',
			method: 'POST',
			dataType: 'text',
			data: {
				html:'<b>Yay</b>'
			}
		} )
		.done( function( wikitext ) {
			// wikitext is the result of the conversion
			console.log( wikitext );
			$( '#wpTextbox1' ).val( wikitext );
		} );
	} );