Check whether js_ReascriptAPI is installed.lua

function checkJSAPI(requiredVersion)
  if reaper.JS_ReaScriptAPI_Version and reaper.JS_ReaScriptAPI_Version() >= requiredVersion then
    return true
  end

  local button = reaper.MB(string.format([[
This script requires js_ReaScriptAPI %s or newer. Do you want to install it now?

Choose [Yes] to open ReaPack's package browser.
  ]], requiredVersion), "js_ReaScriptAPI not found", 4)

  if button == 6 then
    reaper.ReaPack_BrowsePackages('js_ReascriptAPI')
  end

  return false
end

if not checkJSAPI(0.961) then
  return reaper.defer(function() end)
end

doSomething()