How come Typescript claims that X could potentially be undefined within useMemo, even though it has already been defined and cannot be undefined at this stage

I am facing an issue with the following code snippet:

  const productsWithAddonPrice = useMemo(() => {
    const addonsPrice = addonsSelected
      .map(id => {
        if (addons === undefined) { return 0}
        return addons.find(addon => addon.id === id).price})
      .reduce((a, b) => a + b, 0);
    return [...products].map(p => {
      const productCopy = { ...p };
      productCopy.price += addonsPrice;
      return productCopy;
    });
  }, [addonsSelected, products, addons]);

The code works fine in non-typescript environment and 'addons' is always initialized as an empty array, so it should never be undefined.

function useProductAndAddon(products: any[], addons: AddonType[] = []) {

But I'm getting a typescript error on this line:

return addons.find(addon => addon.id === id).price})

The error says:

Object is possibly 'undefined'.ts(2532)

This seems incorrect since I'm already checking for undefined on the line above it:

if (addons === undefined) { return 0}

This issue appears to be related to Typescript can't tell I'm returning early if param is undefined. However, I'm struggling to find a solution. Any clarification or guidance on this matter would be greatly appreciated.

Answer №1

If the addon with that specific id cannot be located, the result will be undefined

The find() function retrieves the value of the first element in the given array that meets the conditions specified by the testing function. If no elements meet the criteria, undefined is returned.

Refer to MDN

You can consider implementing something like this:

     .map(id => {
        const addon = addons?.find(a => a.id === id);
        return addon?.price ?? 0;
        })

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