Node.js读取传入的数据

I started to learn node.js. But I am having a problem reading the incoming data. title, content, postId does not have any trouble while reading. But I can not read "categories[]"

// NEWPOST.JADE
  $("#editButton").click(function() {
      var formSerialize = $("form#newPost").serialize();
      $.ajax({
          type: 'PUT',
          url: 'auth/new-post',
          data: formSerialize
      }).done(function() {
          console.log("SUCCESS");
      }).fail(function() {
          console.log("FAIL");
      })
  });



// POST.JS
    router.put('/', upload.single('image'), function (req, res, next) {
      console.log(req.body);
      console.log("new selected categories:  " + req.body.categories);
      const promise = Post.findByIdAndUpdate(
        req.body.postId,
        req.body,
        {
            new: true
        }
      );
      promise.then((post) => {
        res.json(post);
      }).catch((err) => {
        res.send(500);
      })

    });


// RESULT CMD
    { title: 'post title',
      content: '<p>post content</p>
',
      'categories[]':
       [ '5a7310e7cfa4c52440957bde',
         '5a7310eccfa4c52440957bdf',
         '5a7310f0cfa4c52440957be0',
         '5a7310f5cfa4c52440957be1' ],
      postId: '5a7628be95d1d63668b9da31' }
    new selected categories:  undefined
    PUT /auth/new-post 200 209.076 ms - 2

Despite the arrival of new selected categories it seemed to undefined. how can i meet it.

Thank you in advance.

Your json field of categories 'categories[]': is wrong.

Use the following json string should work

{ 
    "title": "post title",
    "content": "<p>post content</p>
",
     "categories":
       [ "5a7310e7cfa4c52440957bde",
         "5a7310eccfa4c52440957bdf",
         "5a7310f0cfa4c52440957be0",
         "5a7310f5cfa4c52440957be1"],
      "postId": "5a7628be95d1d63668b9da31" 

}