Try using below code:

```
url: `${this.props.siteurl}/_api/web/lists/getbytitle('Templates')/items?$select=Title,CurrentLink,HistoricalLink,DataGroupTemplate/Template&$expand=DataGroupTemplate&$filter=DataGroupTemplate/Template eq '${a}'`
```

**Update**:

Instead of `${this.props.siteurl}`, try using `${reactHandler.props.siteurl}`.