Restrict the scope of 'unknown' to an object containing only a string-field without resorting to 'any'

Question

Restrict the scope of 'unknown' to an object containing only a string-field without resorting to 'any'

Currently, I am working on validating the data that is being received by my application. To illustrate, consider the following scenario:

function extractField(data: unknown): string {
    if (typeof data !== 'object') {
        throw new Error('e1');
    }

    if (data == null) {
        throw new Error('e2');
    }

    const { field } = data;

    if (typeof field !== 'string') {
        throw new Error('e3');
    }

    return field;
}

My objective is to extract a string field from the data and ensure that the code throws an error if the data's structure is invalid. However, I am facing an issue with type checking. The code currently results in an error stating, "Property 'field' does not exist on type '{}'." I am aware that adding any can resolve this issue, but I am determined to find a type-safe solution without relying on any.

typescript

Answer 1

Answer №1

There has been a proposal to allow the in type guard to confirm the existence of properties, but this feature has not been implemented (GH)

If you need to check for the existence of a property, you can either use a type assertion like this (

const { field } = data as { field: unknown };

), or you can create a custom type guard:

function hasProp<K extends PropertyKey>(data: object, prop: K): data is Record<K, unknown> {
    return prop in data;
}
function getField(data: unknown): string {
    if (typeof data !== 'object') {
        throw new Error('e1');
    }

    if (data == null) {
        throw new Error('e2');
    }
    if (!hasProp(data, 'field')) {
        throw new Error('field does not exist');
    }
    const { field } = data;

    if (typeof field !== 'string') {
        throw new Error('e3');
    }

    return field;
}

Playground Link

Note: If you are throwing exceptions, consider using the new assertion syntax: Playground Link

Answer 2

There has been a proposal to allow the in type guard to confirm the existence of properties, but this feature has not been implemented (GH)

If you need to check for the existence of a property, you can either use a type assertion like this (

const { field } = data as { field: unknown };

), or you can create a custom type guard:

function hasProp<K extends PropertyKey>(data: object, prop: K): data is Record<K, unknown> {
    return prop in data;
}
function getField(data: unknown): string {
    if (typeof data !== 'object') {
        throw new Error('e1');
    }

    if (data == null) {
        throw new Error('e2');
    }
    if (!hasProp(data, 'field')) {
        throw new Error('field does not exist');
    }
    const { field } = data;

    if (typeof field !== 'string') {
        throw new Error('e3');
    }

    return field;
}

Playground Link

Note: If you are throwing exceptions, consider using the new assertion syntax: Playground Link

Answer 3

Answer №2

Expanding on Titian's inspiration, we have modified the code to allow for a broader range of inputs. In this version, the object can be of type unknown and the key can be either null or undefined:

function hasProp<K extends PropertyKey>(obj: unknown, key: K | null | undefined): obj is Record<K, unknown> {
    return key != null && obj != null && typeof obj === 'object' && key in obj;
}

Answer 4

Expanding on Titian's inspiration, we have modified the code to allow for a broader range of inputs. In this version, the object can be of type unknown and the key can be either null or undefined:

function hasProp<K extends PropertyKey>(obj: unknown, key: K | null | undefined): obj is Record<K, unknown> {
    return key != null && obj != null && typeof obj === 'object' && key in obj;
}

Answer 5

Answer №3

One exciting development is the use of in as a type guard:

function extractField(data: unknown): string {
  if (typeof data !== 'object') {
    throw new Error('e1');
  }

  if (data == null) {
    throw new Error('e2');
  }

  if (!('field' in data) || typeof data.field !== 'string') {
    throw new Error('e3');
  }

  return data.field;
}

Answer 6

One exciting development is the use of in as a type guard:

function extractField(data: unknown): string {
  if (typeof data !== 'object') {
    throw new Error('e1');
  }

  if (data == null) {
    throw new Error('e2');
  }

  if (!('field' in data) || typeof data.field !== 'string') {
    throw new Error('e3');
  }

  return data.field;
}

Restrict the scope of 'unknown' to an object containing only a string-field without resorting to 'any'

Answer №1

Answer №2

Answer №3

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