I would like to create helper function that will ensure that value is wrapped in array.

• if value is null or undefined return empty array
• if value is array, leave it as is
• else wrap value in array

Typescript sandbox.

What I don't understand is what is typescript complains about, how to fix it and is there a way to get more descriptive error message:

function ensureArray<T>(
  value: T,
): T extends undefined | null ? [] : T extends Array<any> ? T : T[] {
  if (value === undefined || value === null) {
    // Type '[]' is not assignable to type 'T extends null | undefined ? [] : T extends any[] ? T : T[]'.(2322)
    return []; // should I just use as any?
  }

  // Type '(T & any[]) | [T]' is not assignable to type 'T extends null | undefined ? [] : T extends any[] ? T : T[]'.
  //  Type 'T & any[]' is not assignable to type 'T extends null | undefined ? [] : T extends any[] ? T : T[]'.(2322)
  return Array.isArray(value) ? value : [value];
}


const x1: number[] = ensureArray(1)
const x2: number[] = ensureArray([1, 2])
const x3: string[] | [] = ensureArray('str' as string | null)

CodePudding user response：

You don't need the conditional type !

function ensureArray<T>(
  value: T | Array<T> | undefined | null,
): T[] {
  if (value === undefined || value === null) {
    return []; // should I just use as any?
  }

  return Array.isArray(value) ? value : [value];
}


const x1: number[] = ensureArray(1)
const x2: number[] = ensureArray([1, 2])
const x3: string[] = ensureArray('str' as string | null)

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