WordPress (admin) posts search GET request filter

    function getPostsByCategorySlug(slug) {
  return site.categories().slug( slug )
    .then(function( matchingCats ) {
      // matchingCats will be an array of the one post that matches
      // the provided slug
      if ( ! matchingCats.length ) {
        throw new Error( `No category found for slug "${slug}"` );
      }
      var catID = matchingCats[ 0 ].id;
      return site.posts().categories(catID);
    });
}

getPostsByCategorySlug( 'some-category-slug' )
  .then(function( posts ) {
    // Do something with your posts
  })
  .catch(function( err ) {
    // Could not get categories
    console.error(err);
  });