import Header from '@editorjs/header';

var editor = EditorJS({
  ...
  tools: {
    ...
    header: Header,
  },
  ...
});

var editor = EditorJS({
  ...
  tools: {
    ...
    header: {
      class: Header,
      shortcut: 'CMD+SHIFT+H',
    },
  },
  ...
});

var editor = EditorJS({
  ...
  tools: {
    ...
    header: {
      class: Header,
      config: {
        placeholder: 'Enter a header',
        levels: [2, 3, 4],
        defaultLevel: 3
      }
    }
  }
  ...
});