Add a 'Add rel="nofollow" to link' checkbox to the WordPress link editor

add-rel-nofollow-checkbox.php

<?php

/**
 * Add a 'Add rel="nofollow" to link' checkbox to the WordPress link editor
 *
 * @see https://danielbachhuber.com/tip/rel-nofollow-link-modal/
 */
add_action( 'after_wp_tiny_mce', function(){
	?>
	<script>
		var originalWpLink;
		// Ensure both TinyMCE, underscores and wpLink are initialized
		if ( typeof tinymce !== 'undefined' && typeof _ !== 'undefined' && typeof wpLink !== 'undefined' ) {
			// Ensure the #link-options div is present, because it's where we're appending our checkbox.
			if ( tinymce.$('#link-options').length ) {
				// Append our checkbox HTML to the #link-options div, which is already present in the DOM.
				tinymce.$('#link-options').append(<?php echo json_encode( '<div class="link-nofollow"><label><span></span><input type="checkbox" id="wp-link-nofollow" /> Add rel="nofollow" to link</label></div>' ); ?>);
				// Clone the original wpLink object so we retain access to some functions.
				originalWpLink = _.clone( wpLink );
				wpLink.addRelNofollow = tinymce.$('#wp-link-nofollow');
				// Override the original wpLink object to include our custom functions.
				wpLink = _.extend( wpLink, {
					/**
					 * Fetch attributes for the generated link based on
					 * the link editor form properties.
					 *
					 * In this case, we're calling the original getAttrs()
					 * function, and then including our own behavior.
					 */
					getAttrs: function() {
						var attrs = originalWpLink.getAttrs();
						attrs.rel = wpLink.addRelNofollow.prop( 'checked' ) ? 'nofollow' : false;
						return attrs;
					},
					/**
					 * Build the link's HTML based on attrs when inserting
					 * into the text editor.
					 *
					 * In this case, we're completely overriding the existing
					 * function.
					 */
					buildHtml: function( attrs ) {
						var html = '<a href="' + attrs.href + '"';

						if ( attrs.target ) {
							html += ' target="' + attrs.target + '"';
						}
						if ( attrs.rel ) {
							html += ' rel="' + attrs.rel + '"';
						}
						return html + '>';
					},
					/**
					 * Set the value of our checkbox based on the presence
					 * of the rel='nofollow' link attribute.
					 *
					 * In this case, we're calling the original mceRefresh()
					 * function, then including our own behavior
					 */
					mceRefresh: function( searchStr, text ) {
						originalWpLink.mceRefresh( searchStr, text );
						var editor = window.tinymce.get( window.wpActiveEditor )
						if ( typeof editor !== 'undefined' && ! editor.isHidden() ) {
							var linkNode = editor.dom.getParent( editor.selection.getNode(), 'a[href]' );
							if ( linkNode ) {
								wpLink.addRelNofollow.prop( 'checked', 'nofollow' === editor.dom.getAttrib( linkNode, 'rel' ) );
							}
						}
					}
				});
			}
		}
	</script>
	<style>
	#wp-link #link-options .link-nofollow {
		padding: 3px 0 0;
		white-space: nowrap;
		overflow: hidden;
		text-overflow: ellipsis;
	}
	#wp-link #link-options .link-nofollow label span {
		width: 83px;
	}

	.has-text-field #wp-link .query-results {
		top: 223px;
	}
	</style>
	<?php
});

this is great! quick question - do you know of any documentation available for wplink.js, or did you just figure out how to use those methods by looking through the code?

may I suggest to change https://gist.github.com/danielbachhuber/4c5e0ce7b5ab6475a27e5389c93716ac#file-add-rel-nofollow-checkbox-php-L32 to attrs.rel = wpLink.addRelNofollow.prop( 'checked' ) ? 'nofollow' : null;.
In that case you will not face the problem that on each link update not selecting nofollow to true would cause rel="false"

Suggestion: https://gist.github.com/danielbachhuber/4c5e0ce7b5ab6475a27e5389c93716ac#file-add-rel-nofollow-checkbox-php-L66

Replace with
wpLink.addRelNofollow.prop( 'checked', editor.dom.getAttrib( linkNode, 'rel' ).includes('nofollow') );
to allow for the existence of other rel attributes (noopener).

Hi,
could you also add the option of rel="sponsored"?
So that it results in something like this:
rel="nofollow sponsored"

Thanks