Typescript Universal method decorator returning Promise as value

Question

I'm trying to implement a universal method decorator that can be used with both prototype and instance methods.

Based off: Typescript decorators not working with arrow functions

As you can see from the code below, instanceMethod() is returning a Promise.

Is there a way I can return the correct value?

Decorator code:

export function trace(verbose: boolean = false) {

    return (target: any, prop: string, descriptor?: TypedPropertyDescriptor<any>): any => {
        let fn
        let patchedFn

        if (descriptor) {
            fn = descriptor.value
        }

        return {
            configurable: true,
            enumerable: false,
            get() {
                if (!patchedFn) {
                    patchedFn = async (...args) => {
                        const ret = fn.call(this, ...args)
                        console.log(`typeof ret: ${typeof ret}`)
                        console.log(`return value: ${ret}`)
                        return ret
                    }
                }
                console.log(`get() patchedFn: ${patchedFn}`)
                return patchedFn
            },
            set(newFn) {
                console.log(`set() newFn: ${newFn}`)
                patchedFn = undefined
                fn = newFn
            },
        }

    }
}

Test class & test code:

class Greeter {

    @trace()
    public instanceMethod(s: string): string {
        return s
    }

    @trace()
    public async instanceAsyncMethod(s: string): Promise<string> {
        return s
    }

    @trace()
    public instanceArrowMethod = (s: string): string => {
        return s
    }

}

async function test() {
    const greeter = new Greeter()
    const result1 = greeter.instanceMethod('1')
    console.log(`* instanceMethod: ${result1}`) // should return '1', instead returns a Promise
    const result2 = await greeter.instanceAsyncMethod('2')
    console.log(`* instanceAsyncMethod: ${result2}`)
    const result3 = await greeter.instanceArrowMethod('3')
    console.log(`* instanceArrowMethod: ${result3}`)

}

test()

Output:

set() newFn: (s) => {
            return s;
        }
get() patchedFn: (...args) => __awaiter(this, void 0, void 0, function* () {
                        console.log(`typeof fn: ${typeof fn}`);
                        const ret = fn.call(this, ...args);
                        console.log(`typeof ret: ${typeof ret}`);
                        console.log(`return value: ${ret}`);
                        return ret;
                    })
typeof ret: string
return value: 1
* instanceMethod: [object Promise] <<<<<<<<<< The instance method is returning a Promise
get() patchedFn: (...args) => __awaiter(this, void 0, void 0, function* () {
                        console.log(`typeof fn: ${typeof fn}`);
                        const ret = fn.call(this, ...args);
                        console.log(`typeof ret: ${typeof ret}`);
                        console.log(`return value: ${ret}`);
                        return ret;
                    })
typeof ret: object
return value: [object Promise]
* instanceAsyncMethod: 2
get() patchedFn: (...args) => __awaiter(this, void 0, void 0, function* () {
                        console.log(`typeof fn: ${typeof fn}`);
                        const ret = fn.call(this, ...args);
                        console.log(`typeof ret: ${typeof ret}`);
                        console.log(`return value: ${ret}`);
                        return ret;
                    })
typeof ret: string
return value: 3
* instanceArrowMethod: 3

Answer 1

An async function will always return a promise.

You have patchedFn defined as an async function, you will either have to modify to declare it as normal function, or await at the caller if it is intended.

Answer 2

Based on Edward's answer above, I fixed the issue with the following code.

I've tested it on arrow function, instance and static methods.

export function trace(verbose: boolean = false) {

    return (target: any, prop: string, descriptor?: TypedPropertyDescriptor<any>): any => {
        let fn
        let patchedFn

        if (target instanceof Function) {
            // for static methods return function itself
            return target
        }

        if (descriptor) {
            fn = descriptor.value
        }

        return {
            configurable: true,
            enumerable: false,
            get() {
                if (!patchedFn) {
                    patchedFn = (...args) => {
                        const ret = fn.apply(this, args)
                        if (ret instanceof Promise) {
                            // execute the promise
                            ret.then((data) => {
                                return data
                            }).catch((error) => {
                                console.log(error)
                            })
                        }
                        return ret
                    }
                }
                return patchedFn
            },
            set(newFn) {
                patchedFn = undefined
                fn = newFn
            },
        }

    }
}