Change Item Level permission for SitePages Library

updateWriteSecurity.js

//Update the WriteSecurity to change the item level permissions
//Set the Edit property to Create items and edit items that were created by the user
// 1 - Create and edit all items
// 2 - Create items and edit items that were created by the user
// 3 - None

import { sp, Web } from "@pnp/sp/presets/all";

(async () => {

  let web1 = await Web("https://contoso.sharepoint.com/sites/sitename");
  const list = await web1.lists.getByTitle("Site Pages").select('Title,ReadSecurity,WriteSecurity').get()
  console.log("Before Update: " + list.Title);
  console.log("************************************");
  console.log('Read Security: ' + list.ReadSecurity);
  console.log('Write Security: ' + list.WriteSecurity);
  console.log("\n");  

  const updateProperties = {
    WriteSecurity: 2
  };
  await web1.lists.getByTitle("Site Pages").update(updateProperties).then(async (l: any) => {
    // get the updated WriteSecurity
    const r = await l.list.select("Title", "Description", "ReadSecurity", "WriteSecurity")();
    console.log("After Update: " + r.Title);
    console.log("************************************")
    console.log('Read Security: ' + r.ReadSecurity);
    console.log('Write Security: ' + r.WriteSecurity);
  });
})().catch(console.log);