Update array value inside promise ionic 4

Ionic Framework
1

Hello everyone. I want to get API data and set to localStorage(after convert image from the server to base64). When I get data from API, I create a simple loop for update image URL to base64 format but this.users[key].picture["large"] = dataUrl; don’t work. Because of dataUrl value don’t work outside of a function.
How can I update the row object?
What can I do?

this.http.get<User[]>('https://randomuser.me/api?result=100').subscribe(data => {
      this.users = data['results'];

      for (let key in this.users) {
        let value = this.users[key].picture["large"];

        this.toDataURL('//cors-anywhere.herokuapp.com/' + value)
          .then((dataUrl) => {
            this.users[key].picture["large"] = dataUrl;
          });
      }

      this.storage.set('users', "");//clear
      this.storage.set('users', this.users);
      console.log(this.users);
    });

my toDataUrl function:

public toDataURL(url) {
    return fetch(url)
      .then(response => response.blob())
      .then(blob => new Promise((resolve, reject) => {
        const reader = new FileReader();
        reader.onloadend = () => resolve(reader.result);
        reader.onerror = reject;
        reader.readAsDataURL(blob);
      }));
  }
2

I already solved this problem.
this