The parameter does not accept a singular string value, rather it requires a union of string constants

Question

The parameter does not accept a singular string value, rather it requires a union of string constants

Here I am once again, looking for some advice on working with Typescript heuristics. I am facing challenges when trying to write a type guard because Typescript seems to be too restrictive when it comes to comparisons.

Let's consider the following scenario (or check out the Typescript Playground):

const FOO = 'foo' as const;
const BAR = 'bar' as const;

const ALL_THINGS = [FOO, BAR];
type AllThingsType = typeof ALL_THINGS[number];

function isAllThings(value: unknown): value is AllThingsType {
  return typeof value === 'string' && ALL_THINGS.includes(value);
}

The error message you will encounter is:

Argument of type 'string' is not assignable to parameter of type '"foo" | "bar"'.ts(2345)

There is a workaround available:

function isAllThingsWorkaround(value: unknown): value is AllThingsType {
  return typeof value === 'string' && (ALL_THINGS as string[]).includes(value);
}

Is there something that I might be missing in how I should approach this? The code snippet provided is a simplified version, but imagine if ALL_THINGS actually consisted of nearly 25 constants. How can I enhance this without resorting to the workaround?

Thank you for your assistance!

typescript typescript-typings

Answer 1

Answer №1

To achieve this, you can implement a method that does not utilize the .includes function.

const FOO = 'foo' as const;
const BAR = 'bar' as const;

const ALL_THINGS = [FOO, BAR];
type AllThingsType = typeof ALL_THINGS[number];

function isAllThings(value: unknown): value is AllThingsType {
  return typeof value === 'string' && ALL_THINGS.some(a => a === value);
}

console.log(isAllThings("cat")); // false
console.log(isAllThings("foo")); // true

This approach eliminates the need for type casting and allows you to define the criteria for what constitutes inclusion, instead of relying on javascript's default behavior.

Answer 2

To achieve this, you can implement a method that does not utilize the .includes function.

const FOO = 'foo' as const;
const BAR = 'bar' as const;

const ALL_THINGS = [FOO, BAR];
type AllThingsType = typeof ALL_THINGS[number];

function isAllThings(value: unknown): value is AllThingsType {
  return typeof value === 'string' && ALL_THINGS.some(a => a === value);
}

console.log(isAllThings("cat")); // false
console.log(isAllThings("foo")); // true

This approach eliminates the need for type casting and allows you to define the criteria for what constitutes inclusion, instead of relying on javascript's default behavior.

The parameter does not accept a singular string value, rather it requires a union of string constants

Answer №1

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