How can I contain my binary in prime/ folder

Hi. I’m a new snapcraft user maintaining an electron app and in charge of snapping in these days. However, I cannot snap my app well, so I want you to help with my snapping and publishing.

what I want to do

  1. build my app by grunt pre-build
  2. snap this app including the built binary
  3. the command asmstnote kicks running the built binary
  4. publish it without errors

I assume the built file should be located in prime folder, but I’m not sure how to do it.

Here’s my snapcraft.yaml

name: asmstnote # you probably want to 'snapcraft register <name>'
version: '0.1' # just for humans, typically '1.2+git' or '1.3.2'
summary: Single-line elevator pitch for your amazing snap # 79 char long summary
description: |
  This is my-snap's description. You have a paragraph or two to tell the
  most important story about your snap. Keep it under 100 words though,
  we live in tweetspace and your description wants to look good in the snap

grade: devel # must be 'stable' to release into candidate/stable channels
confinement: devmode # use 'strict' once you have the right plugs and slots

    command: desktop-launch $SNAP/start.wrapper
     - browser-support
     - network
     - unity7
     - gsettings
     # - home
     # - pulseaudio
     # - opengl

    plugin: nodejs
    source: .
    prepare: npm install grunt npm@5.2.0 -g && grunt pre-build

    plugin: nil
      - libgconf-2-4
      - libnss3
      - libxss1
      - fontconfig-config

    plugin: nil
      - libappindicator1
      - libdbusmenu-glib4
      - libnotify4
      - libunity9

    plugin: copy
      start: start.wrapper
    after: [desktop-glib-only]

Thank you in advance for your help.