Returned type of a function is not inferred when it's declared as generic from the function's argument

user_8954711 0
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The problem

I have the following two types.

interface SchemaItem {
  name: string,
  value: string
}

type Result<T extends SchemaItem[]> = {
  [K in T[number] as K['name']]: K['value']
}

And I have a function that is supposed to create a value of the Result type from the SchemaItem[] argument.

function createResult<T extends SchemaItem[]>(schemaItems: T): Result<T> {
  // implementation doesn't matter for this question
  return {} as Result<T>
}

The Result type works as expected when I test it like this.

// ResultType = { foo: "Foo", bar: "Bar" }
type ResultType = Result<[
  { name: 'foo', value: "Foo" },
  { name: 'bar', value: "Bar" },
]>

But when I test the function with the same data the returned value is not inferred properly.

// result is of type Result<{ name: string; value: string }[]>
const result = createResult([
  { name: 'foo', value: "Foo" },
  { name: 'bar', value: "Bar" },
])

Is it possible to make TypeScript infer the type of the result as { foo: "Foo", bar: "Bar" } or it's a design limitation?

TS playground

What I have tried

I thought maybe the problem is that name property of SchemaItem is declared as string and here is what I come up with.

interface SchemaItem<T extends string = string> {
  name: T,
  value: string
}

type ArrayToUnion<T> = T extends Array<infer R> ? R : never

type Result<T extends SchemaItem[]> = ArrayToUnion<T> extends SchemaItem<infer R> ? { [key in R]: string } : never

But it didn't help and the result value was of type { [x: string]: string } which is no better.

typescript type-inference types
1 Answers

You can extend your generics from readonly array types, and then supply a readonly array of object literal types to the function by using the as const assertion on the input. (Using an as const assertion tells the compiler to infer the type literally.)

TS Playground

interface SchemaItem {
  name: string,
  value: string
}

type Result<T extends readonly SchemaItem[]> = {
  [K in T[number] as K['name']]: K['value']
}

type ResultType = Result<[
  { name: 'foo', value: "Foo" },
  { name: 'bar', value: "Bar" },
]>

function createResult<T extends readonly SchemaItem[]>(schemaItems: T): Result<T> {
  return {} as Result<T>
}

const result = createResult([
  { name: 'foo', value: "Foo" },
  { name: 'bar', value: "Bar" },
] as const);

result.bar; // "Bar"
result.foo; // "Foo"
user_4382730
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