SharePoint JSOM: Check permissions of current page

_spPageContextInfo.sample.json

 _spPageContextInfo 
{
    webServerRelativeUrl : "/ProjectWeb",
    webLanguage : 1033,
    currentLanguage : 1033,
    webUIVersion : 4,
    pageListId : "{c1d7b89f-f07b-4e2e-b89c-76c315831d59}",
    pageItemId : 5,
    userId : 68,
    alertsEnabled : true,
    siteServerRelativeUrl : "/",
    allowSilverlightPrompt : "True"
} 

jsom_checkperms.js

(function(){
  var ctx = SP.ClientContext.get_current();
  var web = ctx.get_web();
  var list = web.get_lists().getById(_spPageContextInfo.pageListId);
  var li = list.getItemById(_spPageContextInfo.pageItemId);
  
  ctx.load(li, 'EffectiveBasePermissions');
  
  ctx.executeQueryAsync(function(){
    var perms = li.get_effectiveBasePermissions();
    
    // PermissionKind values: https://msdn.microsoft.com/en-us/library/office/ee556747(v=office.14).aspx
    if (perms.has(SP.PermissionKind.editListItems)) {
          console.log('can edit');
    }
  }, function onError(request, errorArgs){
    console.log(errorArgs.get_message());
  });
  
})();