How to use Spread Operator in angular component template

It's unlikely that you're going to get this syntax to work in a template (there are many valid typescript constructs that don't work in templates).

You could write a helper method in the component instead, that takes the item as an argument, and then makes the appropriate call, as in, for example:

public doCommand(item: ToolbarItem): void {
  item.command(...item.commandParams);
}

and then change your template to:

<button (click)="doCommand(item)"...