TypeScript icon, indicating that this package has built-in type declarations

    1.4.4 • Public • Published


    Quick and declarative object validation with TypeScript with simple objects


    When building APIs I will need to validate the request.body or request.query object to ensure it conforms to a type.
    I have done this in the past with varying amount of rigor, robustness, and verbosity.

    I wanted a clear, yet terse way to solve this problem that was still robust and not overly repetitive.


    TypeScript 4+


    > yarn add frisker


    Frisker can check for a complete or partial match of an object to a validator object.
    Validator objects are real objects declared using the as const syntax that closely resembles a TypeScript type or interface.
    The objects can be deeply nested and composed together using the object spread syntax.

    Objects are comprised the values:

    • Primitives
      • 'string'
      • 'number'
      • 'boolean'
    • Optional Primitives
      • 'string?'
      • 'number?'
      • 'boolean?'
    • A single element tuple with a primitive or an object which represents an array
      • [Primitive] | [Object]
    • An optional array of primitives
      • [OptionalPrimitive]
      • I.e. ['string?'] | ['number?'] | ['boolean?']
    • Objects containing all of the previous types of values
    • { [key: string]: Primtive | [Primitive] | [Object] | Object }
    • A tuple of string literals represents a string literal
      • [...string[]]
      • E.g. ['user', 'guest', 'admin']

    This is a fairly shallow example using all of the available "types"

    export const user = {
      id: 'string',
      name: { first: 'string', last: 'string', alias: 'string' },
      union: ['admin', 'user', 'guest'],
      age: 'number',
      isAdmin: 'boolean',
      permissions: ['string'],
      groups: [{ id: 'string', name: 'string' }],
      alias: 'string?', // Optional string
      previousAliases: ['string?'], // Optional array
    } as const
    export type User = UnwrapBody<typeof user>

    Type Guard

    function isValid(type: Validator, input: any)

    Frisker can be used as a Type Guard:

    // Example using an express handler:
    import { Request, Response } from 'express'
    import { isValid } from 'frisker'
    import { user } from './types'
    export function ({ body }: Request, res: Response) {
      if (!isValid(user, body)) {
        return res.status(400).json({ message: 'Bad request' })
      // body is now a User type which is inferred from the first parameter is isValid()
      if (!body.permissions.includes('users')) {
        return res.status(403).json({ message: 'Not allowed' })
      return res.json({ message: 'ok' })

    Type Assertion

    function assertValid(type: Validator, input: any, partial?: boolean)

    The assertValid function will throw if the input does not conform to the validator.
    When combined with an error handling middleware, the assertion helper can reduce boilerplate as seen in earlier examples:

    import { RequestHandler } from 'express'
    import { assertValid } from 'frisker'
     * A helper for passing errors to the error middleware
    function wrap(handler: RequestHandler) {
      const wrapped: RequestHandler = async (req, res, next) => {
        try { await handler(req, res, next) }
        catch (ex) { next(ex) }
      return wrapped
    const body = { username: 'string', password: 'string' } as const
    export const register = wrap((req, res) => {
      assertValid(body, req.body)
      // The req.body object will now be the type you expect and be safe to use
      await createAccount(req.body.username, req.body.password)
      res.json({ true })

    Partial Type Guard

    function isValidPartial(type: Validator, input: any)
    import { isValidPartial } from 'frisker'
    import { user } from './types' // From the previous example
    export function handler({ body, params }: Request, res: Response) {
      if (!isValidPartial(, body)) {
        return res.status(400).json({ message: 'Bad request' })
      await updateName(, body)
      res.json({ message: 'ok' })

    Configurable Validation

    function validateBody(type: Validator, input: any, options: Options)

    Using the validateBody() function you can get more detail information about the validation


    • notThrow: boolean: Defaults to false
      • the validateBody function throws by default with a message containing all of the errors
      • Provide true if you want an array of the errors
    • partial: boolean
      • Will only validate properties provided in the object provided for comparison


    npm i frisker

    DownloadsWeekly Downloads






    Unpacked Size

    84.5 kB

    Total Files


    Last publish


    • seikho