请帮忙解释一下location?: string = "";无法赋值null，这个?代表什么东西？

export class ActivityResult {
  code?: number = 0;
  msg?: string = "";
  data?: Array<ActivityData> = new Array<ActivityData>();
}

export class ActivityData {
  atId?: number = -1;
  atName?: string = "";
  atUrl?: string = "";
  atTemplate?: number = -1;
  buttonText?: string = "";
  jumpUrl?: string = "";
  atType?: number = -1;
  atStartTime?: string = "";
  atEndTime?: string = "";
  sort?: number = -1;
  createTime?: string = "";
  status?: number = -1;
  location?: string = "";
}
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.