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Undefined and void #8937
Description
Hi!
Proposal
Support the literal type undefined and distinguish undefined from void.
The proposal may be divided into two proposals:
add(done)undefinedtype (backward compatible)- make
voidtype the absence of value (breaking change)
Add undefined type
const x: undefined = undefined // ✔️
function f (x: undefined): void {}
f() // ❌ parameter is required
f(undefined) // ✔️
function g (): undefined { /* ❌ return is required */ }
function h (): undefined { return /* ❌ a value must be returned */ }
function j (): undefined { return undefined /* ✔️ */ } make void type the absence of value
const x: void // ❌ void cannot be used as variable type
function f (x: number | void): void {}
function f (x?: number): void {} // equivalent
f() // ✔️
f(undefined) // ❌
function g (): void { return undefined /* ❌ */ }
function g (): void { return /* ✔️ */ } Use case / Rationales
This makes Flow more compatible with TypeScript.
This allows to type a function with a required parameter that accepts undefined:
function g (x: number | undefined) {}
f() // ❌ parameter is required
f(undefined) // ✔️This makes the intent more clear: void is often associated with the absence of value. The use of void to denote undefined is misleading.
Activity
[-]Distinguish undefined from void[/-][+]Undefined and void[/+]