How can I solve the error: Type 'undefined' is not assignable to type 'File'

I am trying to make my variable of type File have the parameter undefined but I get the error: Type 'undefined' is not assignable to type 'File'.

export class CreateProductoComponent implements OnInit {


  public producto:any = {};


  public file: File = undefined;
  
  
  constructor() { 

  }


  ngOnInit(): void {
    
  }

And when I want to use the variable in the method, I get an error saying that it cannot be undefined.

//Mètode obtenir imatge directe del input
  fileChangeEvent(event:any):void{
    //Validem imatge
    var file;

    if(event.target.files && event.target.files[0]){ //Validem si rebem imatge desde el input
      file = <File>event.target.files[0]; //Guardem l'arxiu en la variable file
      console.log(file);
    }else{  //Si no hi ha imatge
      iziToast.warning({
        title: 'ERROR',
        class: 'text-danger',
        color: 'red',
        position: 'topRight',
        message: 'No hay imágen'
      });
    }

    

    //Validem la mida de la imatge
    if(file.size <= 4000000){
      if(file.type ==){

      }
    }else{
      iziToast.warning({
        title: 'ERROR',
        class: 'text-danger',
        color: 'red',
        position: 'topRight',
        message: 'La imágen supera los 4MB'
      });
    }

  }

Make the attribute optional with a question mark, which lets it be undefined too. You may keep it simpler by not assigning undefined to the variable, if uninitialized it will be undefined already.

https://www.typescriptlang.org/docs/handbook/2/objects.html#optional-properties

export class CreateProductoComponent implements OnInit {


  public producto:any = {};


  public file?: File;
  
  
  constructor() { 

  }


  ngOnInit(): void {
    
  }

The error is in your fileChangeEvent function. There are multiple issues there, but the main one is the use of var file tells TypeScript to use any for the type of file. This is an antipattern for TypeScript.

The error specifically occurs when you reference file.size, because it's possible for file to undefined (it's technically possible for file to be any type).

The following will resolve the error you are facing related to Type 'undefined' is not assignable to type 'File':

Note that the important change here is to first check if file exists in your if (file) ... condition.

//Mètode obtenir imatge directe del input
  fileChangeEvent(event:Event):void{
    //Validem imatge
    let file : File | undefined;

    if(event.target.files && event.target.files[0]){ //Validem si rebem imatge desde el input
      file = <File>event.target.files[0]; //Guardem l'arxiu en la variable file
      console.log(file);
    }else{  //Si no hi ha imatge
      iziToast.warning({
        title: 'ERROR',
        class: 'text-danger',
        color: 'red',
        position: 'topRight',
        message: 'No hay imágen'
      });
    }

    

    //Validem la mida de la imatge
    if(file && file.size <= 4000000){
      // TODO: Complete this code
      // if(file.type ==){
      //
      // }
    }else{
      iziToast.warning({
        title: 'ERROR',
        class: 'text-danger',
        color: 'red',
        position: 'topRight',
        message: 'La imágen supera los 4MB'
      });
    }
  }