Recently, a coworker at Nx approaches me with a TypeScript problem
that we both thought "It seems simple" at first. We soon find that it's not as simple as we thought. In this blog post, I'll walk you through the problem, the seems to be solution, the solution, and the thought process behind them.
The Problem
declare function table(items: any[], fieldOptions: any[]): void;
-
We have a function that accepts some collection of
items
and a collection offieldOptions
that should be strongly typed to the type of each individualitems
declare function table(items: any[], fieldOptions: any[]): void; const items = [ { foo: "some foo", bar: 123, }, { foo: "some foo two", bar: 456, }, ];
-
From this usage,
items
has a type ofArray<{foo: string, bar: number}>
andfieldOptions
needs to be strongly typed against{foo: string, bar: number}
. Usage oftable()
can be as follow
const items = [ { foo: "some foo", bar: 123, }, { foo: "some foo two", bar: 456, }, ]; table(items, [ "foo", { field: "bar", mapFn: (val) => { // should return something. eg: a string }, }, ]);
Here, we can see that
fieldOptions
can accept each key of{foo: string, bar: number}
, aka'foo' | 'bar'
. In addition,fieldOptions
can also accept aFieldOption
object that has afield: 'foo' | 'bar'
as well asmapFn
callback that will be invoked with the value at{foo: string, bar: number}[key]
. In other words, when we passfield: "bar"
,mapFn
then needs to have type:(value: number) => string
becausebar
hasnumber
as its valuet type.
The "seems to be" Solution
At first glance, it seems easy. We'll go through each step.
-
First,
table()
needs to accept a generic to capture the type of each item initems
collection
- declare function table(items: any[], fieldOptions: any[]): void; + declare function table<TItem>(items: TItem[], fieldOptions: any[]): void;
In addition, we also like to constraint
TItem
to anobject
so the consumers can only pass in a collection of objects
- declare function table<TItem>(items: TItem[], fieldOptions: any[]): void; + declare function table<TItem extends Record<string, unknown>>(items: TItem[], fieldOptions: any[]): void;
TItem extends Record<string, unknown>
is the constraint -
Second, we need a type for
fieldOptions
. This type needs to accept a generic that is an object so that we can iterate through the object keys
+ type FieldOption<TObject extends Record<string, unknown>> = keyof TObject | { + field: keyof TObject; + mapFn: (value: TObject[keyof TObject]) => string; + } declare function table<TItem extends Record<string, unknown>>( items: TItem[], - fieldOptions: any[], + fieldOptions: FieldOption<TItem>[] ): void;
With the above type declaration,
FieldOption<{foo: string, bar: number}
is as follow
type Test = FieldOption<{ foo: string; bar: number }>; // ^? 'foo' | // 'bar' | // { // field: 'foo' | 'bar'; // mapFn: (value: string | number) => string; // } //
-
This seems correct but when we apply it, we find that the type isn't as strict as we like
const items = [{ foo: "some foo", bar: 123 }]; table(items, [ { field: "foo", mapFn: (value) => { // ^? value: string | number }, }, ]);
We like for
value
to have type ofstring
instead of a unionstring | number
because we specify the"foo"
forfield
. Hence,{foo: string, bar: number}['foo']
should bestring
-
The problem is that TypeScript cannot narrow down
mapFn
fromfield
because we cannot constraint them the way we currently have our type. Our next step is to try havingkeyof TItem
as a generic as well hoping that TypeScript can infer it fromfield
type FieldOption< TItem extends Record<string, unknown>, TKey extends keyof TItem = keyof TItem > = | TKey | { field: TKey; mapFn: (value: TKey) => string; };
But it still won't work because we can never pass a generic in for
TKey
andTKey
is always defaulted tokeyof TItem
which will always be'foo' | 'bar'
for our{foo: string, bar: number}
item type. So as of this moment, we're stuck.
We spent a good 15-20 minutes trying things out but to no avail.
The Solution
Out of frustration, I asked for help in trashh_dev Discord and Tom Lienard provided the solution with a super neat trick. I'll attempt to go through the thought process to understand the solution
What we're stuck on is we're so hung up on the idea of FieldOption
needs to be an object type {field: keyof TItem, mapFn}
but in reality, what we actually need is as follow
// let's assume we're working with {foo: string, bar: number} for now instead of a generic to simplify the explanation
// what we think we need
type FieldOption = {
field: "foo" | "bar";
mapFn: (value: string | number) => string;
};
// what we actually need
type FieldOption =
| {
field: "foo";
mapFn: (value: string) => string;
}
| {
field: "bar";
mapFn: (value: number) => string;
};
Yes, we need a Mapped Union from our TItem
instead of a single object with union properties. The question is how we get to the Mapped Union. Well, it is a 2-step process
- We need to convert
TItem
into a Mapped Type
type FieldOption<TItem extends Record<string, unknown>> = {
[TField in keyof TItem]: {
field: TField;
mapFn: (value: TItem[TField]) => string;
};
};
type Test = FieldOption<{ foo: string; bar: number }>;
// ^? {
// foo: { field: 'foo'; mapFn: (value: string) => string };
// bar: { field: 'bar'; mapFn: (value: number) => string };
// }
- We need to map over the Mapped Type with keys of
TItem
to get the Mapped Union
type FieldOption<TItem extends Record<string, unknown>> = {
[TField in keyof TItem]: {
field: TField;
mapFn: (value: TItem[TField]) => string;
};
}[keyof TItem];
type Test = FieldOption<{ foo: string; bar: number }>;
// ^? | { field: 'foo'; mapFn: (value: string) => string }
// | { field: 'bar'; mapFn: (value: number) => string }
Now, let's try using table()
with our Mapped Union type to see if it works
type FieldOption<TItem extends Record<string, unknown>> =
| keyof TItem
| {
[TField in keyof TItem]: {
field: TField;
mapFn: (value: TITem[TField]) => string;
};
}[keyof TItem];
declare function table<TItem extends Record<string, unknown>>(
items: TTem[],
fields: FieldOption<TItem>[]
): void;
const items = [
{ foo: "string", bar: 123 },
{ foo: "string2", bar: 1234 },
];
table(items, [
{
field: "foo",
mapFn: (value) => {
// ^? value: string
return "";
},
},
{
field: "bar",
mapFn: (value) => {
// ^? value: number
return "";
},
},
]);
And that is our solution. So simple, yet so powerful trick. Here's the TypeScript Playground that you can play with.
If anyone knows the correct term for Mapped Union, please do let me know so I can update the blog post.
Top comments (1)
What you are looking at is two different concepts... the record-building and accessing the key, thereof, is its own thing.
The concept of
is reliant on "Discriminated Unions" (or tagged unions). That being a type union where every type in the union shares a common tag, and the value for each tag is unique to that type. Celebrity has "kind" and the value uniquely identifies Person and Persona, and if we add "Mascot" later, and "Influencer" and whatever else, that "kind" will continue to be the key we use to look up the unique properties of that particular type.
The concept of "tagged unions" requires everything to share a tag, but that's not the only benefit to unions.
It also applies in the case where you have many optional properties that are mutually inclusive.
You might have, I dunno, a color object that is trying to store information that gets turned into CSS or Canvas ImageData or whatever...
Instead of having a color type which can optionally handle RGB, HSL and HSV:
...which is bad, because you essentially have 2^7 (128) possible types, when you only meant to have 3...
you probably meant to do something like:
but I wouldn't need to use mode exclusively, here, TS knows that each type has keys that the other types must not have.
If these were all optional types (or each key was its own union decoupled from all of the other keys), this type of check might be impossible, or require a nonsensical level of
r && b && g || h && s && l || h && s && v
style checking. Discriminated Unions for the save.