TS2531: Object is possibly 'null'

问题:

I have the following function:-

uploadPhoto() {
    var nativeElement: HTMLInputElement = this.fileInput.nativeElement;

    this.photoService.upload(this.vehicleId, nativeElement.files[0])
        .subscribe(x => console.log(x));
}

however on the nativeElement.files[0], I am getting a typescript error, "Object is possibly 'null'". Anyone can help me solve this issue?

I tried to declare the nativeElement as a null value, however did not manage to succeed.

Thanks for your help and time.

回答1:

files is defined to be FileList | null so it can be null. You should either check for null (using a if) or use a not null assertion operator (!) if you are sure it is not null;

if(nativeElement.files != null) {
    this.photoService.upload(this.vehicleId, nativeElement.files[0])
        .subscribe(x => console.log(x));
}

//OR
this.photoService.upload(this.vehicleId, nativeElement.files![0])
    .subscribe(x => console.log(x));

Note The not null assertion operator, will not perform any runtime checks, it just tells the compiler you have special information and you know nativeElement.files will not be null at runtime. If nativeElement.files is null at runtime, it will generate at error. This is not the safe navigation operator of other languages.

回答2:

TypeScript 3.7 got released in 11/2019. Now "Optional Chaining" is supported, this is the easiest and most secure way of working with potentially null-able values:

You simply write:

nativeElement?.file?.name

Note the Question-Mark! They check for null/undefined and only return the value, if none of the properties (chained with dots) is null/undefined.

Instead of

if(nativeElement!=null && nativeElement.file != null) {
  ....
}

But imagine something more complex like this: crm.contract?.person?.address?.city?.latlang that would otherwise a lot more verbose to check.

回答3:

If you are sure that there is a file in all cases. You need make compiler to be sure.

(nativeElement.files as FileList)[0]