dialog.showSaveDialog force normalized extension

main.js

let { app, dialog } = require('electron')
let path = require("path")
let pathCompleteExtname = require("path-complete-extname")

app.whenReady().then(async () => {
  for (let defaultPath of ['change-me.txt', 'change-me.tar.gz', 'change-me.jpeg']) {
    let defaultExtname = pathCompleteExtname(defaultPath)
    let defaultBasename = path.basename(defaultPath, defaultExtname)

    let { canceled, filePath: selectedPath } = await dialog.showSaveDialog({
      defaultPath: defaultBasename,
    })
    if (canceled) return

    let selectedDirname = path.dirname(selectedPath)
    let selectedExtname = pathCompleteExtname(selectedPath)
    let selectedBaseName = path.basename(selectedPath, selectedExtname)

    let finalPath = path.format({
      dir: selectedDirname,
      name: selectedBaseName,
      ext: defaultExtname,
    })

    await dialog.showMessageBox({
      message: `Selected file path (normalized - dont let user change extension): ${finalPath}`
    })
  }
})

package.json

{
  "name": "wacky-notebook-innovate-xddrf",
  "productName": "wacky-notebook-innovate-xddrf",
  "description": "My Electron application description",
  "keywords": [],
  "main": "./main.js",
  "version": "1.0.0",
  "author": "Jamie",
  "scripts": {
    "start": "electron ."
  },
  "dependencies": {
    "path-complete-extname": "1.0.0"
  },
  "devDependencies": {
    "electron": "25.1.0"
  }
}