Typescript - specify type for destructed variable alias

Question

ngOnChanges(changes: SimpleChanges) {
    const {previousValue: prevDate, currentValue: currDate}: SimpleChange = changes.dateFilter;
}

In the above code snippet I want to specify type DateFilter to the prevDate and currDate variables. How can I achieve that? I tried like <DateFilter>prevDate and is not working.

Answer 1

You need to cast the type:

const prevDate: DateFilter = changes.dateFilte.previousValue as DateFilter

or:

const {previousValue: prevDate, currentValue: currDate} = changes.dateFilter as {previousValue: DateFilter; currentValue: DateFilter };

Answer 2

You can just type the parent object. Here you can see that types of object properties are inherited to destructored variables myFoo and myBar :

  obj: Obj = { foo: 4, bar: '1'};

  ngOnInit() {
    const {foo: myFoo, bar: myBar} = this.obj;

    if (myFoo === '3') {
      // his condition will always return 'false' since the types 'number' and '"3"' have no overlap.
    }
  }

And with SimpleChanges something like:

  ngOnChange(c: SimpleChanges) {
    const obj: Obj = c['dateFilter'].currentValue;
    const {foo: foo1, bar: bar1} = obj
  }