用户在线时发出发布请求，否则存储在本地存储中

Question

在单击提交按钮时，我想在用户在线时发出发帖请求。如果用户离线，我想将提交的数据存储在本地存储中。不幸的是，关闭wifi后单击“提交”按钮时出现500错误。当我从浏览器网络脱机时，它会起作用

onSubmit() {  
 let postData = [];
for (var groupId in this.deltasByGroupId) {
  if (!this.deltasByGroupId.hasOwnProperty(groupId)) { continue }

  for (var k in this.deltasByGroupId[groupId]) {
    if (!this.deltasByGroupId[groupId].hasOwnProperty(k)) { continue }

    let secs = this.deltasByGroupId[groupId][k];

    var item1 = secs["items"]
    for (var j in item1) {

      var item2 = item1[j]
      for (var l in item2) {

        var listItem = item2["items"]
        for (var li in listItem) {
          this.model = new MeasurementChange(listItem[li].id, listItem[li].percent_complete);
          postData.push(this.model);
        }
      }
    }
  }
}
if(navigator.onLine){
  this.pEntryService.postEntries(postData).subscribe(res => {
        alert("Data submitted successfully")
      })

} else{

this.storage.set('cashedData', postData).then(res => {
alert("Data successsfully stored locally")
})
}
}