microsoft / microsoft/TypeScript
Missing arguments in a callback function within a decorator result in error
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Since Jun 12, 2023.
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Description
Bug Report
In TS 5.1.3 when writing decorators that use the experimentalDecorators setting,
and writing a decorator that receives a callback function in the factory, which it calls with the function this context and arguments, then TS fails with Unable to resolve signature of method decorator when called as an expression. when the callback function has not defined the argument in its callback, but works when it is present even if unused.
This even happens if the callback function is explicitly typed to also accept a version that does not take arguments.
🔎 Search Terms
Decorators, experimentalDecorators
🕗 Version & Regression Information
TS 5.1.3
experimentalDecorators enabled
⏯ Playground Link
💻 Code
type LogFn<C, A extends any[]> = (this: C, ...args: A) => string
function LoggingDecorator<C, A extends any[]>({ log }: { log: LogFn<C, A> }) {
return function (
_target: C,
_methodName: string,
descriptor: TypedPropertyDescriptor<(this: C, ...args: A) => Promise<any>>
) {
const originalMethod = descriptor.value
descriptor.value = function (this: C, ...args: A) {
const result = log.apply(this, args)
console.log(result)
return originalMethod!.apply(this, args)
}
}
}
class NumberService {
private number = 0
// This errors
@LoggingDecorator({
log() {
return `test-${this.number}`
},
})
async setNumber(value: number): Promise<number> {
this.number = value
return this.number
}
// This works fine
@LoggingDecorator({
log(_number) {
return `test-${this.number}`
},
})
async setNumber2(value: number): Promise<number> {
this.number = value
return this.number
}
}
🙁 Actual behavior
TS errors with
Unable to resolve signature of method decorator when called as an expression.
Argument of type 'TypedPropertyDescriptor<(value: number) => Promise<number>>' is not assignable to parameter of type 'TypedPropertyDescriptor<(this: NumberService) => Promise<any>>'.
Types of property 'value' are incompatible.
Type '((value: number) => Promise<number>) | undefined' is not assignable to type '((this: NumberService) => Promise<any>) | undefined'.
Type '(value: number) => Promise<number>' is not assignable to type '(this: NumberService) => Promise<any>'.ts(1241)
🙂 Expected behavior
TS accepts the function even without arguments.
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Assessment
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