Encountering TypeScript error 2345 when attempting to redefine a method on an Object Property

Question

Encountering TypeScript error 2345 when attempting to redefine a method on an Object Property

This question is related to

Object Property method and having good inference of the function in TypeScript

Fortunately, the code provided by @jcalz is working fine;

const P = <T,>(x: T) => ({ "foo": <U,>(R: (x: T) => U) => R(x) });

I have now written an advanced code that avoids creating a new object on every function call.

The approach is to first create a base object with _P(undefined) and then reuse the object by changing the property value using Object.defineProperty.

It almost works, but there is a type error 2345 being produced:

Argument of type '(a: number) => number' is not assignable to parameter of type '(x: undefined) => number'. Types of parameters 'a' and 'x' are incompatible. Type 'undefined' is not assignable to type 'number'.(2345)

const f: (a: number) => number

const _P = <T,>(x: T) =>
  ({ "foo": <U,>(R: (x: T) => U) => R(x) });

const P =
  (obj =>
    <T,>(x: T) =>
      Object.defineProperty(obj, "foo", {
        configurable: true, //property value changes
        value: <U,>(R: (x: T) => U) => R(x)
      })
  )(_P(undefined));

const f = (a: number) => a + 1;

const b = P(99)['foo'](f); //const b: number  //good
//however, `f` is marked as type error 2345

console.log(b);//100

TypeScript playground link

https://i.sstatic.net/AYaaG.png

Any suggestions on how to resolve this error? Thank you!

typescript

Answer 1

Answer №1

The reason behind this issue is that typescript cannot properly determine the changed type of a field when using Object.defineProperty. It is recommended to avoid using defineProperty in this scenario and stick to normal assignment for simplicity. Here's a simplified version of the code:

// Unsure why an immediately invoked function was used here,
// but let's simplify it
const obj = _P(undefined)
const P =
  <T>(x: T) => {
    // Avoid using `defineProperty` as it serves no purpose
    // Also, rename argument for `R` function to prevent confusion
    // about what `x` represents
    obj.foo = <U>(R: (arg: T) => U) => R(x)
  };

In reality, defineProperty masked the actual error message:

Type '<U>(R: (arg: T) => U) => U' is not assignable to 
  type '<U>(R: (arg: undefined) => U) => U'

By using defineProperty, the type checking was circumvented, making it difficult to identify the root cause of the error.

As for finding a solution, without clarity on the objective, it is challenging to offer specific advice. Previously, when P created a new object with each call, multiple callers could obtain unique instances of the object and its associated foo function, each tailored to accept different argument types. However, in the current implementation, there is only one shared {foo: ...} object across the application, limiting its ability to handle various argument types. Consider how this limitation will impact functionality given a scenario like the following:

const b = P('abc')
const c = P(99)

With this setup, after creating b, the object becomes:

// (1)
{ 
  foo: <U>(R: (arg: string) => U) => R('abc'),
}

Subsequently, upon creating c, the same object transforms into:

// (2)
{ 
  foo: <U>(R: (arg: number) => U) => R(9),
}

Consider the implications when calling b.foo(a => a.repeat(2)). Despite expecting the input to be 'abc' and output 'abcabc', due to the altered foo function, the actual outcome would differ, resulting in a runtime error. This approach may not achieve the desired results effectively.

Additionally, the rationale behind "avoiding object creation per call" remains unclear. Typically, the performance impact of creating objects in JavaScript is minimal unless dealing with highly demanding operations or operating at extreme scales. Furthermore, excessive object mutation, as seen in this context, can lead to complexities and challenges in management. Hence, exercise caution before proceeding further with the implementation.

Answer 2

The reason behind this issue is that typescript cannot properly determine the changed type of a field when using Object.defineProperty. It is recommended to avoid using defineProperty in this scenario and stick to normal assignment for simplicity. Here's a simplified version of the code:

// Unsure why an immediately invoked function was used here,
// but let's simplify it
const obj = _P(undefined)
const P =
  <T>(x: T) => {
    // Avoid using `defineProperty` as it serves no purpose
    // Also, rename argument for `R` function to prevent confusion
    // about what `x` represents
    obj.foo = <U>(R: (arg: T) => U) => R(x)
  };

In reality, defineProperty masked the actual error message:

Type '<U>(R: (arg: T) => U) => U' is not assignable to 
  type '<U>(R: (arg: undefined) => U) => U'

By using defineProperty, the type checking was circumvented, making it difficult to identify the root cause of the error.

As for finding a solution, without clarity on the objective, it is challenging to offer specific advice. Previously, when P created a new object with each call, multiple callers could obtain unique instances of the object and its associated foo function, each tailored to accept different argument types. However, in the current implementation, there is only one shared {foo: ...} object across the application, limiting its ability to handle various argument types. Consider how this limitation will impact functionality given a scenario like the following:

const b = P('abc')
const c = P(99)

With this setup, after creating b, the object becomes:

// (1)
{ 
  foo: <U>(R: (arg: string) => U) => R('abc'),
}

Subsequently, upon creating c, the same object transforms into:

// (2)
{ 
  foo: <U>(R: (arg: number) => U) => R(9),
}

Consider the implications when calling b.foo(a => a.repeat(2)). Despite expecting the input to be 'abc' and output 'abcabc', due to the altered foo function, the actual outcome would differ, resulting in a runtime error. This approach may not achieve the desired results effectively.

Additionally, the rationale behind "avoiding object creation per call" remains unclear. Typically, the performance impact of creating objects in JavaScript is minimal unless dealing with highly demanding operations or operating at extreme scales. Furthermore, excessive object mutation, as seen in this context, can lead to complexities and challenges in management. Hence, exercise caution before proceeding further with the implementation.

Encountering TypeScript error 2345 when attempting to redefine a method on an Object Property

Answer №1

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