Using global variable in anonymous function

Question

In html

<input type="file" multiple="multiple" (change)="getFileSizeandName($event)"/>

in file.ts

I have global variable

imageStringBase64: Array<string>;

and function call when event change trigger

getFileSizeandName(args) {
    let files = args.target.files;

    for (let i = 0; i < files.length; i++) {
      let reader = new FileReader();
      reader.onload = (function (f) {
        return function(e) {
          console.log(e.target.result);
          this.imageStringBase64 = e.target.result;
        }
      })(files[i]);
      reader.readAsDataURL(files[i]);
    }
  }
}

When i use conole.log(e.target.result) in console printed stringBase64 Assign this.imageStringBase64 = e.target.result and have unexpected result, this.imageStringBase64 is undefined. How use global var in anonymous function?