TypeScript: Why Object.fromEntries does not accept array of tuple? [duplicate]

Question

I make a array of tuple
Then convert it to object with Object.fromEntries
Then I would assign to a <{ [formFieldId: string]: number }> type.

What is wrong here?

const [itemsWithSelection, setItemsWithSelection] = useState<{ [formFieldId: string]: number }>(itemsWithSelection2)
if (!itemsWithSelection2) {
  let aa = Object.keys(invoiceItems).forEach((key) =>
    [key, 1]
  )
  setItemsWithSelection(Object.fromEntries(aa))
}

and got this error:

./components/product/InvoiceItemsToDeliver.tsx:30:44
Type error: No overload matches this call.
  Overload 1 of 2, '(entries: Iterable<readonly [PropertyKey, number]>): { [k: string]: number; }', gave the following error.
    Argument of type 'void' is not assignable to parameter of type 'Iterable<readonly [PropertyKey, number]>'.
  Overload 2 of 2, '(entries: Iterable<readonly any[]>): any', gave the following error.
    Argument of type 'void' is not assignable to parameter of type 'Iterable<readonly any[]>'.

  28 |     [key, 1]
  29 |   )
> 30 |   setItemsWithSelection(Object.fromEntries(aa))
     |                                            ^
  31 | }

forEach doesn't return anything - aa is always undefined. — VLAZ
– VLAZ, Commented Sep 8, 2021 at 18:44
Also relevant: Function with forEach returns undefined even with return statement | JavaScript: Difference between .forEach() and .map() | What does return keyword mean inside forEach function? — VLAZ
– VLAZ, Commented Sep 8, 2021 at 18:49

Parzh · Accepted Answer · 2021-09-08 19:17:02Z

0

You likely want to use .map here instead of .forEach:

const [itemsWithSelection, setItemsWithSelection] = useState<{ [formFieldId: string]: number }>(itemsWithSelection2)
if (!itemsWithSelection2) {
  let aa = Object.keys(invoiceItems).map((key) =>
    [key, 1]
  )
  setItemsWithSelection(Object.fromEntries(aa))
}

The first produces a new array based on the contents of the callback (or what is often called "closure" in Swift); while the second is used primarily for side effects, and always returns undefined (a value similar to Swift's Void).

answered Sep 8, 2021 at 19:17

Parzh

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