can't check if variable is undefined in typescript

Question

I'm trying to solve compiler error " Object is possibly 'undefined' "

const destinationColumnIndex = (): number => {
  if (typeof result.destination === 'undefined') {
    return 0;
  }
  return boardData.findIndex(
    (column) => column.id === Number(result.destination.droppableId)
  );
};

but typescript compiler still tells me that "result.destination" may be undefined.

I have tried also:

  if (result.destination === undefined) {
    return 0;
  }

and:

  if (!result.destination) {
    return 0;
  }

and:

   if (!result || typeof result.destination === 'undefined') {
    return 0;
  }

and nothing works. Even thought it may be some bug so i restarted VS Code but there are still the same error.

EDIT - MORE CODE:

  const onDragEnd = async (result: DropResult) => {
if (!result.destination) {
  return;
}

const sourceColumnIndex = (): number =>
  boardData.findIndex(
    (column) => column.id === Number(result.source.droppableId)
  );

const destinationColumnIndex = (): number => {
  if (typeof result === 'undefined' || result.destination === undefined) {
    return 0;
  }
  return boardData.findIndex(
    (column) => column.id === Number(result.destination.droppableId)
  );
};

it's function inside of react component

it say "This condition will always return 'false' since the types 'DraggableLocation | undefined' and 'string' have no overlap." When i do if(typeof result?.destination === 'undefined') same error as before. — Zaben
– Zaben, Commented Mar 31, 2021 at 15:06

WilsonPena · Accepted Answer · 2021-03-31 15:19:22Z

1

You should just do:

  if (result === undefined || result?.destination === undefined) {
    return 0;
  }

Checking typeof is not a good way to check for undefined.

or

  if (!result || result?.destination === undefined) {
    return 0;
  }

UPDATE

try this:

const onDragEnd = (result: DropResult) => {
  if (!result || !result.destination) {
    return;
  }

  const sourceColumnIndex = (): number =>
    boardData.findIndex(
      (column) => column.id === Number(result.source?.droppableId)
    );

  const destinationColumnIndex = (): number => {
    if (!result || !result.destination) {
      return 0;
    }
    return boardData.findIndex(
      (column) => column.id === Number(result.destination?.droppableId)
    );
  };
}

edited Mar 31, 2021 at 15:19

answered Mar 31, 2021 at 14:58

WilsonPena

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7 Comments

evolutionxbox Over a year ago

curious, why is using typeof not good for checking undefined?

Zaben Over a year ago

unfortunately none of these work for me. There is still an error.

WilsonPena Over a year ago

try changing the first to result?.destination. I edited in my answer

WilsonPena Over a year ago

because checking for the value itself is basically the same thing but more straight forward.

WilsonPena Over a year ago

where is the variable result declared? can you post the full thing?

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