gatsby-ssr.js有什么作用？

  const React = require("react")
  const Layout = require("./src/components/layout")
  // Adds a class name to the body element
  exports.onRenderBody = ({ setBodyAttributes }, pluginOptions) => {

setBodyAttributes({

  className: "my-body-class",

})

  }

  const React = require("react")
  const Layout = require("./src/components/layout")
  // Wraps every page in a component
  exports.wrapPageElement = ({ element, props }) => {

return <Layout {...props}>{element}</Layout>

  }