Paste into Dev Tools while running Plex to automatically work through your library and assign default album art.

albumArtSelector.js

var albumArt = (() => {
  // Helpers
  // (e.g. DOM shortcuts and class-checking utilities)
  var qsa = ( container, selector ) => [ ...container.querySelectorAll( selector ) ];
  var nodeHasClass = ( node, str ) => node.classList.toString().indexOf( str ) > -1;
  var whereNodeHasClass = ( str ) => ( node ) => nodeHasClass( node, str );

  var wherePropertyLike = ( obj, str ) => {
    const key = Object.keys( obj ).find( key => key.indexOf( str ) > -1 );
    return key ? obj[ key ] : null;
  };

  // Inspectors
  // (e.g. find things out about the DOM)
  var needsArt = ( node ) => qsa( node, 'i' ).reduce( ( hasPlaceholder, node ) => (
    hasPlaceholder || nodeHasClass( node, 'placeholderIcon' )
  ), false );
  var getReactInstance = node => {
    const key = Object.keys(node).filter(key => key.indexOf('react')>-1)[0];
    return node[ key ];
  };

  // Interactions
  // (e.g. do things to the page)

  var press = ( node ) => {
    const rII = getReactInstance( node );
    const ownerInstance = rII._currentElement._owner._instance;
    ownerInstance.props.onPress();
  };

  var openModal = ( item ) => {
    const editButton = qsa( item, 'button' ).filter( whereNodeHasClass( 'editButton' ) )[ 0 ];
    press( editButton );
  };

  var selectPosterTab = () => document.querySelector('a.poster-btn').click();

  var selectArtwork = () => {
    const artList = document.querySelector('.artwork-options-list');
    const art = artList && artList.querySelector('a.artwork-option');
    art && art.click();
    return ! ! art;
  };

  var save = () => document.querySelector('.save-btn').click();
  var cancel = () => document.querySelector('.cancel-btn').click();

  var execute = fn => () => new Promise( resolve => {
    try {
      fn();
      resolve();
    } catch(e) { reject( e ); }
  });

  var wait = delay => () => new Promise( resolve => setTimeout( resolve, delay ) );

  var waitFor = selector => () => {
    const test = () => document.querySelector( selector ) !== null;
    return new Promise( ( resolve, reject ) => {
      // Nothing should take more than 5 seconds (I hope)
      const rejectionTimeout = setTimeout( reject, 5000 );
      const pollID = setInterval( () => {
        if ( test() ) {
          clearInterval( pollID );
          clearTimeout( rejectionTimeout );
          resolve();
        }
      }, 17 );
    } );
  };

  var log = msg => () => console.log( msg );

  // Main Logic

  var scroller = qsa( document, 'div' ).filter( whereNodeHasClass( 'MetadataListPageContent-metadataListScroller' ) )[0];
  var scrollTop = scroller.scrollTop;

  var getVisibleItems = () => [ ...scroller.children[0].children ];

  var runInSequence = arrOfPromises => arrOfPromises
    .reduce(( lastStep, nextStep ) => lastStep.then( () => nextStep() ), Promise.resolve() );

  var getAmountToScroll = () => {
    const { min, max } = getVisibleItems().reduce(( minMax, item ) => {
      const boundingRect = item.getBoundingClientRect();
        return {
        min: Math.min( boundingRect.top, minMax.min ),
        max: Math.max( boundingRect.bottom, minMax.max ),
        };
    }, {
      min: Infinity,
      max: -Infinity,
    });
    return max - min - 200; // -200 to avoid skipping items
  };

  let run = true;
  var populateArt = () => {
    if ( ! run ) {
      console.log('Processing Cancelled');
      return;
    }
    const visibleItems = getVisibleItems();
    const artlessItems = visibleItems.filter( needsArt );
    var process = item => Promise.resolve( openModal( item ) )
      .then( log( 'Modal open' ) )
      .then( waitFor( 'a.poster-btn' ) )
      .then( log( 'Modal Ready' ) )
      .then( wait( 1000 ) )
      .then( selectPosterTab )
      .then( log( 'Selected "Poster" tab' ) )
      .then( wait( 200 ) )
      .then( () => {
        var hasPosters = selectArtwork();
        console.log( hasPosters );
        if ( hasPosters ) {
          return Promise.resolve()
            .then( log( 'Artwork Selected' ) )
            .then( wait( 1500 ) )
            .then( save )
            .then( log( 'Artwork saved!' ) );
        }
        return Promise.resolve()
          .then( cancel )
          .then( log( 'No artwork available' ) );
      } )
      .then( wait( 1000 ) )
      .catch( err => console.error( err ) );

    // return runInSequence( artlessItems.map( item => () => process( item ) ) )
    const fnsReturningPromises = artlessItems.map( item => () => process( item ) );
    return runInSequence( fnsReturningPromises )
      .then(() => scroller.scrollTop += getAmountToScroll())
      .then( wait( 1000 ) )
      .then( populateArt );
  };

  return {
    start: () => {
      run = true;
      populateArt();
    },
    runOne: () => {
      run = true;
      populateArt();
      run = false;
    },
    stop: () => {
      run = false;
    },
  };
})();

edit-season-title.js

/**
 * Run in console with a TV show season open to add an input
 * which can be used to specify a custom title.
 */
(() => {
  const row = document.createElement('div');
  row.classList.add('row');
  row.innerHTML = `
<div class="col-md-12">
  <div class="form-group selectize-group">
    <label for="season-title">Season Title</label>
    <div class="input-group">
      <input class="form-control" id="season-title" name="title" placeholder="Season Title">
    </div>
  </div>
</div>
`;
  $('.edit-metadata-form').prepend(row);
})();