Struggling to narrow down the type of an object property even after verifying it with a type guard

Question

Struggling to narrow down the type of an object property even after verifying it with a type guard

Flavor is a distinct union, represented as a value of an Object.

While attempting to execute this idea, it functions in js, however, TypeScript does not approve. ts playground link

Desired Outcome: For TypeScript to comprehend discriminated unions within loops, much like it comprehends them independently

type Flavor = ({ 
    natural: true,
    naturalComponent : string
}) | ({ 
    natural: false,
    artificialComponent: string
}) 

type IceCreamType = Record<string, Flavor>

const IceCreams: IceCreamType = {
    CokeIceCream: {
        natural:false,
        artificialComponent: 'Coke'
    },
    Vanilla: {
        natural: true,
        naturalComponent: 'Vanilla Extract'
    },
    Mango: {
        natural: true,
        naturalComponent: 'Mango Fruit'

    }
}


const iceCreamKeys = Object.keys(IceCreams)

iceCreamKeys.forEach( item => {
    if(IceCreams[item].natural){
    console.log(IceCreams[item].naturalComponent) // TypeScript reports "Property doesn't exist.."
    }
})


if(IceCreams.Mango.natural){
    console.log(IceCreams.Mango.naturalComponent) // This works without an issue
    }

typescript discriminated-union

Answer 1

Answer №1

The issue lies in the fact that the compiler struggles with performing narrowing on an object property such as IceCreams[item] when you are using a key with an unspecified type that is not a specific literal type. TypeScript only considers the type of the index, not the exact value. In this case, the type of item is string. For instance, if you have variables item1 and item2, both being of type string, evaluating IceCreams[item1] does not provide any insight into IceCreams[item2]. Since TypeScript cannot differentiate between item1 and

item2</code, or between <code>item

and item, it cannot narrow down the type. This limitation in TypeScript has been acknowledged and reported on microsoft/TypeScript#10530. While a solution may be developed in the future, a simple workaround is available:

By creating a new variable and copying the value into it, the problematic indexing is done only once:

iceCreamKeys.forEach(item => {
    const x = IceCreams[item];
    if (x.natural) {
        console.log(x.naturalComponent)  // this is fine
    }
})

Link to code demo in TypeScript Playground

Answer 2

The issue lies in the fact that the compiler struggles with performing narrowing on an object property such as IceCreams[item] when you are using a key with an unspecified type that is not a specific literal type. TypeScript only considers the type of the index, not the exact value. In this case, the type of item is string. For instance, if you have variables item1 and item2, both being of type string, evaluating IceCreams[item1] does not provide any insight into IceCreams[item2]. Since TypeScript cannot differentiate between item1 and

item2</code, or between <code>item

and item, it cannot narrow down the type. This limitation in TypeScript has been acknowledged and reported on microsoft/TypeScript#10530. While a solution may be developed in the future, a simple workaround is available:

By creating a new variable and copying the value into it, the problematic indexing is done only once:

iceCreamKeys.forEach(item => {
    const x = IceCreams[item];
    if (x.natural) {
        console.log(x.naturalComponent)  // this is fine
    }
})

Link to code demo in TypeScript Playground

Answer 3

Answer №2

Consider storing the item in a separate variable instead of directly accessing it with the index. This approach allows TypeScript to recognize the correct type:

iceCreamKeys.forEach( item => {
    const c = IceCreams[item]
    if(c.natural){
    console.log(c.naturalComponent) 
    }
})

(see it in action on the TS Playground)

Answer 4

Consider storing the item in a separate variable instead of directly accessing it with the index. This approach allows TypeScript to recognize the correct type:

iceCreamKeys.forEach( item => {
    const c = IceCreams[item]
    if(c.natural){
    console.log(c.naturalComponent) 
    }
})

(see it in action on the TS Playground)

Answer 5

Answer №3

loopThroughIceCreams.forEach( flavor => {
    if(IceCreams.flavor.organic){
    console.log(IceCreams.flavor.organicComponent) // This method of accessing also functions properly
    }
})

Recently discovered that this approach also yields successful results.

Answer 6

loopThroughIceCreams.forEach( flavor => {
    if(IceCreams.flavor.organic){
    console.log(IceCreams.flavor.organicComponent) // This method of accessing also functions properly
    }
})

Recently discovered that this approach also yields successful results.

Struggling to narrow down the type of an object property even after verifying it with a type guard

Answer №1

Answer №2

Answer №3

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