Custom validation does not work in Angular

Question

I have a form builder in Angular with a custom validation but I am not able to get type of the file after I read it in my custom validation.

Here is the stackblitz:

https://stackblitz.com/edit/angular-ivy-atwqqc?file=src%2Fapp%2Fapp.component.ts

TS File

function checkFileType(
  control: AbstractControl
): { [key: string]: any } | null {
  const files: File = control.value;
  const errors: string[] = [];

  if (files) {
    console.log(files);
    if (files.type === "txt") {
      errors.push(`${files[0].name} has an invalid type of unknown\n`);
    }
    console.log(files.type); //This is always null. How can I get file type

    return errors.length >= 1 ? { invalid_type: errors } : null;
  }

  return null;
}


  onSelection($event) {
    const fileReader = new FileReader();
      const file = $event.target.files[0];
      fileReader.readAsDataURL(file);
      fileReader.onload = () => {
        this.reactiveForm.patchValue({
          file: fileReader.result
        });
      };  
  }

Answer 1

The problem is coming from the readAsDataURL(). It is encoding it into a base 64 string, which means it doesn't have a property type to look at. In fact it doesn't have any types to look at because it is a string and not a File. If you remove this, and set your file variable to your form, you get the properties you want

function checkFileType(
  control: AbstractControl
): { [key: string]: any } | null {
  const file: File = control.value;
  const errors: string[] = [];

  if (file) {
    console.log(file);
    if (file.type === "txt") {
      errors.push(`${file.name} has an invalid type of unknown\n`);
    }
    console.log(file.type); //This is always null. How can I get file type

    return errors.length >= 1 ? { invalid_type: errors } : null;
  }

  return null;
}


  onSelection($event) {
    const fileReader = new FileReader();
      const file = $event.target.files[0];
      fileReader.onload = () => {
        this.reactiveForm.patchValue({
          file: file
        });
      };  
  }