The filter function fails to work on undefined values

Question

The filter function fails to work on undefined values

My eyes have been fixed on this code for quite some time now...

I need assistance

I am currently working with a filter and trying to eliminate undefined objects from an array:

  .then((items) => {
    const filtered = items.map( i => {
      if (i !== undefined) {
        return i
      }
    })
    updateState(filtered, query)
  });

However, I encountered an error while using the filtered data:

Argument of type '({ service: Service; results: Card[]; } | undefined)[]' is not assignable to parameter of type 'SearchResult[]'.
  Type '{ service: Service; results: Card[]; } | undefined' is not assignable to type 'SearchResult'.
    Type 'undefined' is not assignable to type 'SearchResult'.

Why am I facing this issue when trying to pass it to the updateState() function?

Isn't the purpose of the filter to remove undefined items from the array.. What could be causing this problem?

EDIT: Even after filtering with type assertion, the same error persists..

const filtered: SearchResult[] = results.filter( i => i as SearchResult)

Getting the same outcome...

Thank you

typescript typescript-typings

Answer 1

Answer №1

It appears that Typescript is unable to deduce the return type in this scenario.

After an extensive search using various keywords, I stumbled upon this particular line of code:

const filtered: SearchResult[] = results.filter( (i): i is SearchResult => !!i)

I found it referenced in a helpful blog post:

Wishing everyone a wonderful day!

Answer 2

It appears that Typescript is unable to deduce the return type in this scenario.

After an extensive search using various keywords, I stumbled upon this particular line of code:

const filtered: SearchResult[] = results.filter( (i): i is SearchResult => !!i)

I found it referenced in a helpful blog post:

Wishing everyone a wonderful day!

Answer 3

Answer №2

Another useful technique is to use flatMap instead of map when filtering out undefined or other types of values.

Here's how you can do it:

const filtered = items.flatMap((item) => (item ? [item] : []));

This approach allows Typescript to correctly infer the type for your array.

You can also write it with the full syntax like this:

const filtered = items.flatMap((item) => {
  if (item !== undefined) return [item];
  else return [];
});

To make the code more concise, you can even eliminate the unnecessary square brackets:

const filtered = items.flatMap((item) => {
  if (item !== undefined) return item;
  else return [];
});

const filtered = items.flatMap((item) => (item ? item : []));

Answer 4

Another useful technique is to use flatMap instead of map when filtering out undefined or other types of values.

Here's how you can do it:

const filtered = items.flatMap((item) => (item ? [item] : []));

This approach allows Typescript to correctly infer the type for your array.

You can also write it with the full syntax like this:

const filtered = items.flatMap((item) => {
  if (item !== undefined) return [item];
  else return [];
});

To make the code more concise, you can even eliminate the unnecessary square brackets:

const filtered = items.flatMap((item) => {
  if (item !== undefined) return item;
  else return [];
});

const filtered = items.flatMap((item) => (item ? item : []));

Answer 5

Answer №3

In the event that the map function does not yield a value, it will default to returning undefined. Therefore, if your current code generates an undefined result, it will remain as such.

To achieve your intended outcome, consider utilizing filter instead of map, or ensure that your map operation produces a non-undefined default value.

Answer 6

In the event that the map function does not yield a value, it will default to returning undefined. Therefore, if your current code generates an undefined result, it will remain as such.

To achieve your intended outcome, consider utilizing filter instead of map, or ensure that your map operation produces a non-undefined default value.

The filter function fails to work on undefined values

Answer №1

Answer №2

Answer №3

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