Filtering array with discriminated union?

[elfin ocean]

Good afternoon,

I've created an object with a discriminated union and attempting to filter an array that contains these objects and subsequently map over these objects.

Unfortunately after I've filtered the array for the discriminating property, the map function is no longer aware that the discriminating property was filtered for and it complains that the additional property does not exist.

How might I inform the map that this property does exist?

Example below.

Thanks

[brittle junco]

smashman#0

Playground Link

Preview:```ts
type Item = {name: string} & (
| {hasGetter?: false}
| {hasGetter: true; getter: () => string}
)

const items: Item[] = [
{name: "one"},
{name: "two", hasGetter: true, getter: () => "cake"},
]

const itemsFromGetters = items
.filter(item => item.hasGe
...```

You can choose specific lines to embed by selecting them before copying the link.

[opaque lark]

your filter callback needs to return a type predicate instead of a plain boolean

[elfin ocean]

How would I write a predicate for this example?

[opaque lark]

they can be inferred nowadays, but it's not in this case since your type isn't really a discriminated union (because the discriminant is optional in one of the variants)

if you're comfortable with in narrowing, you don't even need a DU:

[brittle junco]

mkantor#0

Playground Link

Preview:ts ... .filter(item => "getter" in ...

[elfin ocean]

oh interesting

Ah well in my actual code it's not just a hasGetter

I guess I might stil be able to get rid of that property

[opaque lark]

if not it also works the way you want when you make the property required:

[brittle junco]

mkantor#0

Playground Link

Preview:```ts
type Item = {name: string} & (
| {hasGetter: false}
| {hasGetter: true; getter: () => string}
)

const items: Item[] = [
{name: "one", hasGetter: false},
{name: "two", hasGetter: true, getter: () => "cake"},
]

const itemsFromGetters
...```

You can choose specific lines to embed by selecting them before copying the link.

[opaque lark]

and then yet another option is explicitly annotating the return type of that callback as a type predicate (the syntax for that is explained in the handbook link i shared above)

[wary bramble]

Doing item.hasGetter === true should fix the issue

const itemsFromGetters = items
  .filter((item) => item.hasGetter === true)
  .map((item) => ({ name: item.getter() })); // No error

[opaque lark]

oh nice, i didn't realize that would do the trick

[elfin ocean]

Ok, this isn't quite working on the original code as I am using Object.entries. Lemme whip up an example to share

I can make another question if you would prefer

[opaque lark]

FYI i'm heading out the door in a moment and probably won't be back on discord until tomorrow

yeah maybe a new thread would be best?

[elfin ocean]

Ok will do. Thanks