A TypeScript function that converts a value into an array if it is not already an array, ensuring the correct type is output

I'm attempting to develop a function that wraps a value in an array if it is not already an array.

export function asArray<T extends Array<any>>(value: T): T
export function asArray<T>(value: T): T[]
export function asArray(value: any) {
  return Array.isArray(value) ? value : [value]
}

However, I've encountered an issue when the value has a union type.

type A = { a: 1 } | { a: 1 }[]

const fn = (v: A) => {
  const b = asArray(v)
  return b[0].a
}

This results in an error:

Property 'a' does not exist on type 'A'.
  Property 'a' does not exist on type '{ a: 1; }[]'.ts(2339)

Answer №1

It appears that the type of your union is not an array, which means it is utilizing the second declaration:

function asArray<{ a: number } | { a: number }[]>(value: T): Array<{ a: number } | { a: number }[]>

As a result, when the compiler encounters b[0], it sees that it can be either a { a: number } or an array containing it. This ambiguity prevents the compiler from being certain about the presence of an a property.

A better approach would be to redefine your function like this:

export function asArray<T>(value: T | T[]): T[] {
  return ([] as T[]).concat(value)
}

This modification eliminates the need for an overload.

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