Description
I am trying to avoid typing null checks for properties that are formerly optional on the outer interface, but are then assigned with default values for the whole context of the function body.
I used a type guard for it, the problem is that the guarded type is lost in a function, but not lost in an arrow function, even though it can't be hoisted before the type guard.
I don't want to introduce new variables for every function parameter in a similar manner and I don't want to call other functions with casting as printMenu(options as DefinedOptions)
.
TypeScript Version: 3.7.3, 3.8.0-beta
Search Terms:
typescript type guard function arrow function
Code
export interface Options {
header?: string
border?: boolean
pageSize?: number
helpMessage?: string
showKeypress?: boolean
}
interface DefinedOptions extends Options {
pageSize: number
helpMessage: string
}
const isType = <T>(arg: any): arg is T => true
export default async function menu(options: Options) {
options // Options
options.pageSize = options.pageSize ?? 0
options.helpMessage = options.helpMessage ?? 'Default message'
if (!isType<DefinedOptions>(options)) return null
options // DefinedOptions
return new Promise((resolve, reject) => {
function handleMenuKeypress(key: any) {
options // Options - should be DefinedOptions
printMenu(options) // error
}
const candleMenuKeypress = (key: any) => {
options // DefinedOptions
printMenu(options) // no error
}
})
}
function printMenu(options: DefinedOptions) {
}
Expected behavior:
Type should be DefinedOptions
in both.
Actual behavior:
Type is Options
in function but DefinedOptions
in arrow function.
Related:
#10927