The combination of arrays and array methods in intersection types may encounter difficulty in accessing all fields

Question

The combination of arrays and array methods in intersection types may encounter difficulty in accessing all fields

I have two different types, both in the form of arrays of objects with specified fields, combined into an intersection type in Typescript.

When I access an element from the array, I can retrieve the second field without any issues. However, when I try to use array methods such as forEach, map, or filter, the compiler throws errors.

What could be the reason behind this inconsistency between the two cases?

Below is a snippet of the sample code:

var obj: {
  foo?: string;
}[] & {
  bar?: boolean;
}[] = [{foo: "a", bar: true}];

obj.forEach((q) => q.bar); // bar does not exist
obj.map((q) => q.bar) // bar does not exist
obj[0].bar; // OK

TS Playground Link

arrays typescript union-types

Answer 1

Answer №1

According to Ryan Cavanaugh (a key member of the TypeScript team), combining two array types is not recommended.

This is due to the method in which forEach signatures are merged sequentially, while element access results in the recursive merging of property types. One could define a type annotation on an item, or create type foo3 = foo2 & foo1.

However, interpreting the type TU = T[] & U[] is ambiguous - is it a mixed array, similar to Array, or a combined array like Array? The fundamental assumption is that any operation x.y(z) should remain valid for x even if x becomes an intersection type comprising the original constituents. This would not hold true if we considered the array to be Array (as z would also need to be the intersected element type).

Therefore, it's generally not advisable to utilize the type T[] & U[] simultaneously.

It's recommended to use

Array<{ foo?: string;} & {bar?: boolean}>

instead.

Playground

Answer 2

According to Ryan Cavanaugh (a key member of the TypeScript team), combining two array types is not recommended.

This is due to the method in which forEach signatures are merged sequentially, while element access results in the recursive merging of property types. One could define a type annotation on an item, or create type foo3 = foo2 & foo1.

However, interpreting the type TU = T[] & U[] is ambiguous - is it a mixed array, similar to Array, or a combined array like Array? The fundamental assumption is that any operation x.y(z) should remain valid for x even if x becomes an intersection type comprising the original constituents. This would not hold true if we considered the array to be Array (as z would also need to be the intersected element type).

Therefore, it's generally not advisable to utilize the type T[] & U[] simultaneously.

It's recommended to use

Array<{ foo?: string;} & {bar?: boolean}>

instead.

Playground

Answer 3

Answer №2

To enhance clarity, it would be beneficial to categorize array types as follows:

type ItemFoo = {
  foo?: string;
};
type ArrFoo = ItemFoo[];

type ItemBar = {
  bar?: boolean;
};
type ArrBar = ItemBar[];


const obj: ArrFoo & ArrBar = [{foo: "a", bar: true}];

By doing so, it becomes apparent that ArrFoo possesses the

forEach(cb: (x: ItemFoo, i?) => void)

method, while ArrBar has the

forEach(cb: (x:ItemBar, i?) => void)

method. During the type merging process, only one signature is selected.

If limited to access only the ArrFoo and ArrBar types, form the intersection of item types as shown below:

const obj: (ArrFoo[0] & ArrBar[0])[] = [{foo: "a", bar: true}];

Alternatively, utilize the Helper Type:

type ArrIntersection<T1 extends Array<any>, T2 extends Array<any>> = (T1[0] & T2[0])[];

const obj: ArrIntersection<ArrFoo, ArrBar> = [{foo: "a", bar: true}];

Answer 4

To enhance clarity, it would be beneficial to categorize array types as follows:

type ItemFoo = {
  foo?: string;
};
type ArrFoo = ItemFoo[];

type ItemBar = {
  bar?: boolean;
};
type ArrBar = ItemBar[];


const obj: ArrFoo & ArrBar = [{foo: "a", bar: true}];

By doing so, it becomes apparent that ArrFoo possesses the

forEach(cb: (x: ItemFoo, i?) => void)

method, while ArrBar has the

forEach(cb: (x:ItemBar, i?) => void)

method. During the type merging process, only one signature is selected.

If limited to access only the ArrFoo and ArrBar types, form the intersection of item types as shown below:

const obj: (ArrFoo[0] & ArrBar[0])[] = [{foo: "a", bar: true}];

Alternatively, utilize the Helper Type:

type ArrIntersection<T1 extends Array<any>, T2 extends Array<any>> = (T1[0] & T2[0])[];

const obj: ArrIntersection<ArrFoo, ArrBar> = [{foo: "a", bar: true}];

The combination of arrays and array methods in intersection types may encounter difficulty in accessing all fields

Answer №1

Answer №2

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