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.

Why not make the previousValue property of type DateFilter in SimpleChanges ? — adiga
– adiga, Commented Nov 14, 2019 at 6:25
@adiga SimpleChanges class is provided by angular, we don't have control over it — Kishore Barik
– Kishore Barik, Commented Nov 14, 2019 at 8:41
Isn't changes.dateFilter itself not of type DateFilter? prevDate and currentValue get their type from the destructured properties previousValue and currentValue belonging to changes.dateFilter. — ford04
– ford04, Commented Nov 14, 2019 at 10:06
@ford04 changes is of type SimpleChanges so changes.dateFilter becomes of type SimpleChange which has properties previousValue and currentValue. Both of them represent angular @Input() dateFilter: DateFilter. But in ngOnChanges() when fetched from changes.dateFilter.previousValue, the type is not set, we have explicitly set it. Which I can do in multi-line, but wanted to know if possible while de-structuring — Kishore Barik
– Kishore Barik, Commented Nov 14, 2019 at 12:04

Wandrille · Accepted Answer · 2019-11-14 06:23:46Z

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 };

answered Nov 14, 2019 at 6:23

Wandrille

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