Is there a method in typescript to guarantee that a function's return type covers all possibilities?

Question

Is there a method in typescript to guarantee that a function's return type covers all possibilities?

In the case of having a constant enum like:

enum Color {
  RED,
  GREEN,
  BLUE,
}

A common approach is to create a helper function accompanied by a switch statement, as shown below:

function assertNever(x: never): never {
    throw new Error(`Unexpected object: ${x}`)
}

function toString (key: Color): string {
  switch (key) {
    case Color.RED: return 'Red'
    case Color.GREEN: return 'Green'
    case Color.BLUE: return 'Blue'
    default: return assertNever(key)
  }
}

This design makes it necessary to update the toString implementation whenever changes are made to the Color enum.

However, when looking at the reverse scenario:

function fromString (key: string): Color {
  switch (key) {
    case 'Red': return Color.RED
    case 'Green': return Color.GREEN
    case 'Blue': return Color.BLUE
    default: throw new Error(`${key} is not a Color`)
  }
}

It becomes evident that keeping the fromString function updated with any modifications to the Color enum could be challenging.

Is there a method to guarantee that every Color type has a corresponding path in the function? How can we ensure that the function's output range remains within the boundaries of the Color enum?

typescript

Answer 1

Answer №1

There isn't a built-in feature that will automatically enforce this for you. It's not considered an issue if the actual return type of a function is more specific than the declared return type. For example, if a function is supposed to return a 'string' but always returns the exact string "hello", that's perfectly fine. The problem arises when the function is supposed to return "hello" but ends up returning a general 'string'.

To achieve something similar in TypeScript, you can let the compiler infer the return type of a function and then use a compile-time check to verify it matches your expectations. Here's an example:

// MutuallyExtends<T, U> only compiles if T extends U and U extends T
type MutuallyExtends<T extends U, U extends V, V=T> = true;

// Notice how the return type is not explicitly annotated
function fromString(key: string) {
  switch (key) {
    case 'Red': return Color.RED
    case 'Green': return Color.GREEN
    case 'Blue': return Color.BLUE
    default: throw new Error(`${key} is not a Color`)
  }
  // This line will generate an error if not exhaustive:
  type Exhaustive = MutuallyExtends<ReturnType<typeof fromString>, Color>
}

The above code snippet compiles successfully. However, the following code triggers an error because 'Color.BLUE' is missing:

function fromString(key: string) {
  switch (key) {
    case 'Red': return Color.RED
    case 'Green': return Color.GREEN
    default: throw new Error(`${key} is not a Color`)
  }
  type Exhaustive = MutuallyExtends<ReturnType<typeof fromString>, Color> // error!
//  Color does not satisfy constraint Color.RED | Color.GREEN ---> ~~~~~
}

This method serves as a workaround solution. It might be helpful for you or others facing similar issues. Best of luck!

Answer 2

There isn't a built-in feature that will automatically enforce this for you. It's not considered an issue if the actual return type of a function is more specific than the declared return type. For example, if a function is supposed to return a 'string' but always returns the exact string "hello", that's perfectly fine. The problem arises when the function is supposed to return "hello" but ends up returning a general 'string'.

To achieve something similar in TypeScript, you can let the compiler infer the return type of a function and then use a compile-time check to verify it matches your expectations. Here's an example:

// MutuallyExtends<T, U> only compiles if T extends U and U extends T
type MutuallyExtends<T extends U, U extends V, V=T> = true;

// Notice how the return type is not explicitly annotated
function fromString(key: string) {
  switch (key) {
    case 'Red': return Color.RED
    case 'Green': return Color.GREEN
    case 'Blue': return Color.BLUE
    default: throw new Error(`${key} is not a Color`)
  }
  // This line will generate an error if not exhaustive:
  type Exhaustive = MutuallyExtends<ReturnType<typeof fromString>, Color>
}

The above code snippet compiles successfully. However, the following code triggers an error because 'Color.BLUE' is missing:

function fromString(key: string) {
  switch (key) {
    case 'Red': return Color.RED
    case 'Green': return Color.GREEN
    default: throw new Error(`${key} is not a Color`)
  }
  type Exhaustive = MutuallyExtends<ReturnType<typeof fromString>, Color> // error!
//  Color does not satisfy constraint Color.RED | Color.GREEN ---> ~~~~~
}

This method serves as a workaround solution. It might be helpful for you or others facing similar issues. Best of luck!

Answer 3

Answer №2

If you're facing a challenge, there's a potential workaround that could be of help, assuming I grasp your desired outcome correctly.

One approach is to create a string literal type encompassing all possible color strings. When modifying the enum, you'll first need to update the toString function. This necessitates adding another value to the color names type to accommodate the new color. Consequently, this will disrupt the functionality of the fromString function, prompting necessary updates for successful compilation. The modified code would appear as follows:

enum Color {
  RED,
  GREEN,
  BLUE
}

type ColorName = 'Red' | 'Green' | 'Blue';

function assertNever(x: never): never {
  throw new Error(`Unexpected object: ${x}`);
}

function toString (key: Color): ColorName {
  switch (key) {
    case Color.RED: return 'Red';
    case Color.GREEN: return 'Green';
    case Color.BLUE: return 'Blue';
    default: return assertNever(key);
  }
}

function assertNeverColor(x: never): never {
  throw new Error(`${x} is not a Color`);
}

function fromString (key: ColorName): Color {
  switch (key) {
    case 'Red': return Color.RED;
    case 'Green': return Color.GREEN;
    case 'Blue': return Color.BLUE;
    default: return assertNever(key);
  }
}

Answer 4

If you're facing a challenge, there's a potential workaround that could be of help, assuming I grasp your desired outcome correctly.

One approach is to create a string literal type encompassing all possible color strings. When modifying the enum, you'll first need to update the toString function. This necessitates adding another value to the color names type to accommodate the new color. Consequently, this will disrupt the functionality of the fromString function, prompting necessary updates for successful compilation. The modified code would appear as follows:

enum Color {
  RED,
  GREEN,
  BLUE
}

type ColorName = 'Red' | 'Green' | 'Blue';

function assertNever(x: never): never {
  throw new Error(`Unexpected object: ${x}`);
}

function toString (key: Color): ColorName {
  switch (key) {
    case Color.RED: return 'Red';
    case Color.GREEN: return 'Green';
    case Color.BLUE: return 'Blue';
    default: return assertNever(key);
  }
}

function assertNeverColor(x: never): never {
  throw new Error(`${x} is not a Color`);
}

function fromString (key: ColorName): Color {
  switch (key) {
    case 'Red': return Color.RED;
    case 'Green': return Color.GREEN;
    case 'Blue': return Color.BLUE;
    default: return assertNever(key);
  }
}

Answer 5

Answer №3

Suppose all the values are unique (as mapping otherwise would be challenging), you can create a typed map from the enum to the corresponding string. Then, establish a reverse lookup for this map to use it in a type-safe manner.

Here is an illustration:

enum Color {
  RED,
  GREEN,
  BLUE,
}

const ENUM_COLOR_TO_STRING: Record<Color, string> = {
  [Color.RED]: 'Red',
  [Color.GREEN]: 'Green',
  [Color.BLUE]: 'Blue',
}

const COLORS: Color[] = Object.keys(ENUM_COLOR_TO_STRING) as unknown as Color[];

const STRING_COLOR_TO_ENUM: Record<string, Color> = COLORS.reduce<any>((acc, colorEnum) => {
  const colorValue = ENUM_COLOR_TO_STRING[colorEnum]

  acc[colorValue] = colorEnum;

  return acc
}, {})

function fromString(key: string): Color {
  const color: Color | undefined = STRING_COLOR_TO_ENUM[key];

  if (!color) {
    throw new Error(`${key} is not a Color`)
  }

  return color
}

Check out the live demo on Typescript Playground

Answer 6

Suppose all the values are unique (as mapping otherwise would be challenging), you can create a typed map from the enum to the corresponding string. Then, establish a reverse lookup for this map to use it in a type-safe manner.

Here is an illustration:

enum Color {
  RED,
  GREEN,
  BLUE,
}

const ENUM_COLOR_TO_STRING: Record<Color, string> = {
  [Color.RED]: 'Red',
  [Color.GREEN]: 'Green',
  [Color.BLUE]: 'Blue',
}

const COLORS: Color[] = Object.keys(ENUM_COLOR_TO_STRING) as unknown as Color[];

const STRING_COLOR_TO_ENUM: Record<string, Color> = COLORS.reduce<any>((acc, colorEnum) => {
  const colorValue = ENUM_COLOR_TO_STRING[colorEnum]

  acc[colorValue] = colorEnum;

  return acc
}, {})

function fromString(key: string): Color {
  const color: Color | undefined = STRING_COLOR_TO_ENUM[key];

  if (!color) {
    throw new Error(`${key} is not a Color`)
  }

  return color
}

Check out the live demo on Typescript Playground

Is there a method in typescript to guarantee that a function's return type covers all possibilities?

Answer №1

Answer №2

Answer №3

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