TypeScript string union type inferences problem in composite types

Question

I have the following code which won't compile with TypeScript compiler 3.7.3

type Fruit = 'apple' | 'banana'

const val = 'apple'

// Works. TS remembers the value of `val`
const fruit: Fruit = val

type Basket = {
    fruit: Fruit
}

const obj = {
    fruit: 'apple' 
}

// Error: Type 'string' is not assignable to type 'Fruit'.
// TS probably discarded that `fruit` property has concrete value and treats it as any `string`.
const structuralBasket: Basket = obj

// This works 
const declaredBasket: Basket = {
    fruit: 'apple'
}

I need the obj to stay as is. What I cannot do and not looking for in the answer:

using enums
declaring obj as a Basket

Is this a limitation of TypeScript compiler?

If so, is there workaround ? Will this be addressed in the future ?

The only thing I know of is explicitly telling TS that fruit in obj will be something that's contained in Fruit by doing obj = { fruit: 'apple' as const }. I'm not if you can somehow automatically enable this sort of inference in the compiler, though - the fact that you've declared fruit to be 'apple' now doesn't mean it can't change later if somebody does obj.fruit = 'lemon'. — VLAZ
– VLAZ, Commented Jan 13, 2020 at 10:05

Maciej Sikora · Accepted Answer · 2020-01-13 11:29:53Z

2

The problem is that when you just declare obj its gets a wider type then Fruit is, so what you got is {fruit: string}, what is naturally not assignable to its sub-type. You can add as const

const obj = {
    fruit: 'apple' 
} as const

And such construct will be inferred as {fruit: 'apple'}, what is already assignable to Basket being its subtype.

You can also create such objects by value constructors. Consider:

const makeBasket = (fruit: Fruit): Basket => ({fruit})
const obj = makeBasket('apple'); // Basket object

answered Jan 13, 2020 at 11:29

Maciej Sikora

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