Narrow typeof x === 'object' to Record<string | number | symbol, unknown> | null | unknown[]

Search Terms

typeof object narrow record

Suggestion

When narrowing a variable with typeof x === 'object', narrow to Record<string | number | symbol, unknown> | null | unknown[] instead of object | null.

Use Cases

When testing to see if something is an object, you almost always are starting out with unknown or similarly near-top level type and trying to narrow down to a lower “useful” type. When you use the typeof x === 'object' narrowing operation, currently TypeScript over narrows to the extremely constrained {} | null type. This suggestion would change that behavior to narrow down to the much wider Record<string | number | symbol, unknown> | null | unknown[], which the user can then choose to narrow more if they like, or they can choose to work with it as-is.

Examples

declare const apple: unknown
if (typeof apple !== 'object') throw new Error() // actual: object | null; desired: Record<string|number|symbol, unknown> | null | unknown[]

// uncomment this line and comment out the lines above to see desired behavior
// declare const apple: Record<string|number|symbol, unknown> | null | unknown[]

if (apple === null) throw new Error() // actual: object; desired: Record<string|number|symbol, unknown> | unknown[]
if (Array.isArray(apple)) throw new Error() // actual: object; desired: Record<string|number|symbol, unknown>
for (const key in apple) {
    // Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{}'.
    //   No index signature with a parameter of type 'string' was found on type '{}'.
    apple[key] // actual: error; desired: unknown
}

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

[RyanCavanaugh]

The great thing about users is that they write code you’d never expect.

Anyway one example would be

type IsArray<T> = T extends unknown[] ? true : false;

IsArray<typeof something_narrowed_to_object> is false today, the proposed change would make it true | false

[KotlinIsland]

@MicahZoltu You should update the title to: “Narrow typeof x === 'object' to Record<string | number | symbol, unknown> | null | unknown[]”