Declaring string type with min/max length in typescript

After going through the docs, it seems there is no direct way to type check for min/max length of a string datatype.

But, is there a way to declare a string datatype using some custom types so that it checks whether the string length is with the given bounds?

Answer

You can achieve this using a type constructor and something called a “Phantom Type” (read a nice article about this here) which is a technique to ensure that a type can not be assigned to a value directly.

Here’s an example of a StringOfLength<Min,Max> type using these techniques:

type StringOfLength<Min, Max> = string & {
  StringOfLength: unique symbol // this is the phantom type
};

// This is a type guard function which can be used to assert that a string
// is of type StringOfLength<Min,Max>
const isStringOfLength = <Min extends number, Max extends number>(
  str: string,
  min: Min,
  max: Max
): str is StringOfLength<Min, Max> => str.length >= min && str.length <= max;
    
// type constructor function
export const stringOfLength = <Min extends number, Max extends number>(
  input: unknown,
  min: Min,
  max: Max
): StringOfLength<Min, Max> => {
  if (typeof input !== "string") {
    throw new Error("invalid input");
  }
    
  if (!isStringOfLength(input, min, max)) {
    throw new Error("input is not between specified min and max");
  }
    
  return input; // the type of input here is now StringOfLength<Min,Max>
};

// Now we can use our type constructor function
const myString = stringOfLength('hello', 1, 10) // myString has type StringOfLength<1,10>

// the type constructor fails if the input is invalid
stringOfLength('a', 5, 10) // Error: input is not between specified min and max

// The phantom type prevents us from assigning StringOfLength manually like this:
const a: StringOfLength<0, 10> = 'hello' // Type '"hello"' is not assignable to type { StringOfLength: unique symbol }

There are some limitations here – which are that you can’t prevent someone from creating an invalid type like StringOfLength<-1, -300> but you can add runtime checks that the min and max values passed to the stringOfLength constructor function are valid.

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