How to use the prosemirror-markdown.defaultMarkdownSerializer.serialize function in prosemirror-markdown

onAction: (action) => {
            self.$emit('_content-change-editor', action)
            self.content.editor = this.view.editor.state.doc
            self.content.markdown = defaultMarkdownSerializer.serialize(this.view.editor.state.doc)
          }
        })

changed (txn) {
    if (txn.docChanged) {
      let content = defaultMarkdownSerializer.serialize(this.view.state.doc)
      this.value = content
      this.props.onChange(content)
    }
  }

get content() {
    return defaultMarkdownSerializer.serialize(this.view.state.doc)
  }
  focus() { this.view.focus() }

toMarkdown: () => {
      const markdown = defaultMarkdownSerializer.serialize(view.state.doc);
      return markdown;
    },
    focus: () => {

toMarkdown: () => {
      const markdown = defaultMarkdownSerializer.serialize(view.state.doc);
      return markdown;
    },
    focus: () => {