Can TypeScript accommodate setting conditions within types or interfaces?

Question

Can TypeScript accommodate setting conditions within types or interfaces?

When working with TypeScript, I often set types like this:

type range = 1 | 2 | 3 | 4 | 5;

By doing this, I limit the valid values for range to numbers 1, 2, 3, 4, and 5.

I have been exploring whether it is possible to add conditions to types. Rather than specifying specific values as shown above, can I do something like this instead:

type range >= 0 && range <= 5

If this cannot be achieved using a type, perhaps an interface would work. My goal is to validate values based on conditions rather than listing out individual literals as seen earlier.

typescript types interface conditional-statements

Answer 1

Answer №1

The TypeScript language does not have a specific subtype of the `number` data type that corresponds perfectly to the values for which a particular function returns `true` as described below:

function validMode(mode: number): boolean {
    return mode > 0 && mode <= 5;
}
console.log(validMode(1), validMode(5), 
  validMode(2.5), validMode(Math.PI)); // true, true, true, true
console.log(validMode(0), validMode(6), 
   validMode(-1), validMode(Infinity)); // false, false, false, false

This is because JavaScript numbers like `2.5` and `Math.PI` are considered valid inputs due to JavaScript's double precision floating point numbers not being restricted to integers when the type of `mode === "number"`.

In order for TypeScript to support such a type, it would require refinement types or mathematical operations on numeric literal types, both of which are currently missing features in the language.

Therefore, creating this specific type is not possible at the moment within TypeScript.

Answer 2

The TypeScript language does not have a specific subtype of the `number` data type that corresponds perfectly to the values for which a particular function returns `true` as described below:

function validMode(mode: number): boolean {
    return mode > 0 && mode <= 5;
}
console.log(validMode(1), validMode(5), 
  validMode(2.5), validMode(Math.PI)); // true, true, true, true
console.log(validMode(0), validMode(6), 
   validMode(-1), validMode(Infinity)); // false, false, false, false

This is because JavaScript numbers like `2.5` and `Math.PI` are considered valid inputs due to JavaScript's double precision floating point numbers not being restricted to integers when the type of `mode === "number"`.

In order for TypeScript to support such a type, it would require refinement types or mathematical operations on numeric literal types, both of which are currently missing features in the language.

Therefore, creating this specific type is not possible at the moment within TypeScript.

Answer 3

Answer №2

If you're looking to achieve a specific outcome, consider implementing a custom type guard for TypeScript:

type RangeZeroToFive = number & {
  // Define additional properties for numbers between zero and five.
  example(): number;
};

function isInRangeZeroToFive(n: Number): n is RangeZeroToFive {
  return 0 <= n && n <= 5;
}

function bar(n: number) {
  if (isInRangeZeroToFive(n)) {
    console.log(n.example());
  }
  else {
    console.log(n); // Trying to access n.example() will throw an error here
  }
}

Try it out in the playground

Answer 4

If you're looking to achieve a specific outcome, consider implementing a custom type guard for TypeScript:

type RangeZeroToFive = number & {
  // Define additional properties for numbers between zero and five.
  example(): number;
};

function isInRangeZeroToFive(n: Number): n is RangeZeroToFive {
  return 0 <= n && n <= 5;
}

function bar(n: number) {
  if (isInRangeZeroToFive(n)) {
    console.log(n.example());
  }
  else {
    console.log(n); // Trying to access n.example() will throw an error here
  }
}

Try it out in the playground

Can TypeScript accommodate setting conditions within types or interfaces?

Answer №1

Answer №2

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