The correlation of types in Typescript typing

I'm working with these specific types and conversion function:

type LeftRightField = null |
    { left: null, right: number } |
    { left: number, right: null } |
    { left: number, right: number }

type LeftRightArray = [null, number] |
    [number, null] |
    [number, number] |
    null

const fmtField = function (field: LeftRightField): LeftRightArray {
    const rightField = field?.right ?? null
    const leftField = field?.left ?? null
    return (leftField == null && rightField == null) ? null : [leftField, rightField]
}

Live Demo

This however fails with an error message stating:

Type 'number | null' is not assignable to type 'null'.
Type 'number' is not assignable to type 'null'

Can you suggest the correct approach for this conversion?

typescript

Answer №1

This situation seems to stem from TypeScript struggling with keeping track of what I refer to as correlated types. In the scenario presented, both leftField and rightField fall under the union type number | null. Unfortunately, the compiler does not treat them as a cohesive unit during type guards, viewing them as separate entities rather than linked. As a result, the array [leftField, rightField] is considered to have a type of [number | null, number | null], even though having [null, null] as a subtype is impossible.

The best course of action here, aside from resorting to asserting that your actions are safe like so:

return (leftField === null && rightField === null) ? null :
 [leftField, rightField] as LeftRightArray;

would be to restructure the code into potentially redundant separate paths that the compiler can validate using control flow analysis:

return (leftField === null) ? (
 rightField === null ? null : [leftField, rightField]
) : [leftField, rightField];

If your use case only involves two fields, this approach works well. However, if more fields are in play, the redundancy may become excessive, prompting you to assert and proceed.

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