tslint / codelyzer / ng lint error: “for (… in …) statements must be filtered with an if statement”

Lint error message:

src/app/detail/edit/edit.component.ts[111, 5]: for (… in …)
statements must be filtered with an if statement

Code snippet (It is a working code. It is also available at angular.io form validation section):

for (const field in this.formErrors) {
      // clear previous error message (if any)
      this.formErrors[field] = '';
      const control = form.get(field);

      if (control && control.dirty && !control.valid) {
        const messages = this.validationMessages[field];
        for (const key in control.errors) {
          this.formErrors[field] += messages[key] + ' ';
        }
      }
    }

Any idea how to fix this lint error?