Ensuring Type Safety for Collections in TypeScript

My code snippet looks like this:

  portfolioList: MatTableDataSource<Portfolio>;

  ngOnInit(): void {
    this.backend.getStatement().subscribe(
      list => {
        if(list as Portfolio[])
        this.portfolioList = new MatTableDataSource(list);
      }
    );
  }

Portfolio represents an interface.

An error message is showing:

Error: src/app/admin/statement/statement.component.ts:28:53 - error TS2345: Argument of type 'Object' is not assignable to parameter of type 'Portfolio[]'.
  The 'Object' type is only compatible with a few other types. Did you mean to use the 'any' type?
    Type 'Object' is missing key properties from type 'Portfolio[]': length, pop, push, concat, and more.

28         this.portfolioList = new MatTableDataSource(list);

Why is this error occurring and how can I resolve it?
I've attempted using typeof() and instanceof instead of as, but haven't been successful in fixing the error.

Answer №1

Before proceeding, it's crucial to verify that the variable list is actually an array and that each element within the array is of type Portfolio.

if (Array.isArray(list) && list.every(item => item instanceof Portfolio)) {
  this.portfolioList = new MatTableDataSource(list);
}

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