Types in plain JavaScript

Let’s explore how we can validate types and precise constraints with convenient error messages in JavaScript. For example:

function updateCardDecimals(cardId, decimals) {
  isCard(cardId)
  isValidDecimal(decimals)
  setCard(cardId, CF.decimals, decimals)
}

That function is for formatting UI Drafter’s formulas.

isCard

Let’s ensure that cardId is a validly formatted string, and assert the card exists in the collection.

function isCard(cardId) {
  if (!/^[a-z]+$/i.test(cardId) ||
      !Object.hasOwn(cardsCollection, cardId))
    throw new CardNotFound(cardId)
}

class CardNotFound extends ReferenceError {}

isValidDecimal

In addition to the integer check, we can verify that the value is in the allowed numeric range.

function isValidDecimal(value) {
  if (!Number.isInteger(value) ||
      value < 0 || 
      value > MAX_FRACTION_DIGITS)
    throw RangeError(`Invalid Decimal "${value}"`)
}

BTW, the JavaScript Error Types:

EvalError, RangeError, ReferenceError, SyntaxError, TypeError, URIError, AggregateError

Server Side Example

In the following function we’ll send a Bad Request HTML response when the number of fields in the payload, or their names, don’t match the expectation. Also, we’ll examine the values beyond their data type.

const AuthError = 1
const ValidationError = 2

async function resetPasswordPost(request, response) {
  try {
    const body = await jsonBody(request)

    if (!(
      body
      && Object.keys(body).length === 3
      && isPassword(body.password)
      && isId(body.userId)
      && isId(body.token)
    ))
      throw ValidationError

    if (!await passwordResetTokenExists(body.token, body.userId))
      throw AuthError

    // …
    sendOk(response)
  }

  catch (error) {
    switch (error) {
      case AuthError:       sendUnauthorized(response); break;
      case ValidationError: sendBadRequest(response);   break;
      default:              sendInternalServerError(response);
    }
  }
}

function isId(value) {
    return check(value, String)
    && value.length === StandardIdCharLength
    && /^[\w-]*$/.test(value)
}

Open Source

Check out this library: type-check

check('a', String)
check(100, Number)
check(new Int8Array([1, 2, 3]), Int8Array)