Angular Firestore - Get document data and assign to variable inside the component

I'm using Angular Firestore to get and filter the data base on the timestamp.

The only issue is whenever I get the doc.data(), I can't assign it to a variable so that I can use this variable anywhere inside the component.

Component.ts

joinData(toData, fromData){
    let toDate = toData
    let fromDate = fromData
    console.log("valueToDate : ", toDate)
    let dateTo = new Date(toDate).getTime() / 1000;
    let dateFrom = new Date(fromDate).getTime() / 1000;
    console.log("dateTo :", dateTo)


          let collection = this.fstore.firestore.collection('data_reports').where('timestamp', '>=', dateFrom).where('timestamp', '<=', dateTo);
          collection.get().then(function(querySnapshot) {
            querySnapshot.forEach(function(doc) {
              console.log("data: ", doc.data()) // ---> this line I can get the doc.data()
              this.wilsData = doc.data();       // ---> this line the value of wilsData is undefined
              console.log("wilsData: ", this.wilsData)
            })
          }); 
    
  }

I assigned the wilsData like this:

  private wilsData: any[];

I get undefined value when I log it.

So the question is, how can I assign the doc.data() to a variable?

Thanks for your help