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Introduction

The MetaTyper project provides a powerful approach to using runtime types in TypeScript and JavaScript code.

It is based on the principle of using classical objects or classes as a data schema for further validation and serialization. The goal of the project is to make runtime types as developer-friendly as possible.

More Facts:

• Works in Node.JS and all modern browsers.
• Automatically infers TypeScript types.
• Works with native JavaScript.
• It's tiny.
• Zero dependencies.
• Rich error details.
• Rich extensibility support.

Installation

npm install metatyper

or

yarn add metatyper

or

<script src="https://cdn.jsdelivr.net/npm/metatyper/lib/metatyper.min.js"></script>

<!-- or another cdn -->
<script src="https://www.unpkg.com/metatyper/lib/metatyper.min.js"></script>

In this case, you need to use this library through the MetaTyper global variable: MetaTyper.Meta({ /* ... */ })

Basic Usage

First, you can create a Meta object.

import { Meta, NUMBER } from 'metatyper'

const user = Meta({
  id: 0,
  username: 'some user name',
  stars: NUMBER({ min: 0, default: 0 })
})

user.id = 'some text' // type & validation error
user.stars = 'some text' // type & validation error

You can also simply validate different objects.

import { BOOLEAN, INTEGER, Meta, STRING } from 'metatyper'

const userSchema = {
  id: INTEGER(),
  username: STRING({ minLength: 3, regexp: '^[a-zA-Z0-9 _]+$' }),
  stars: 0, // any number

  someFlag: BOOLEAN({ optional: true })
}

Meta.validate(userSchema, {
  id: 1.1, // throw a validation error
  username: 'some user name',
  stars: 1
})

Meta.validate(userSchema, {
  id: 1,
  username: 'some user name'
  // throw a validation error, because stars is not optional field
})

Finally, you can work with classes instead of objects.

import { DATE, INTEGER, Meta, STRING } from 'metatyper'

import 'reflect-metadata'

@Meta.Class({ ignoreProps: ['someInstanceFlag', 'someClassFlag'] })
class User {
  @Meta.declare(INTEGER({ default: 0 }))
  id: number

  username = 'some user name'

  @Meta.declare({ default: 0, min: 0 })
  stars: number // for this you need `reflect-metadata`

  createdAt = DATE({
    default: new Date(),
    coercion: true // cast a string or number to a date (and vice versa)
  })

  someInstanceFlag = false

  static someClassFlag = true
  static someAnotherClassFlag = true
}

User.someClassFlag = 'string' as any
// ok, because the field is listed in the ignoreProps

User.someAnotherClassFlag = 'string' as any // type & validation error

const user = new User()

Meta.deserialize(user, {
  id: 2,

  username: 'some another user name',

  createdAt: 1704067200 * 1000,
  // this timestamp will cast to Date("2024-01-01")

  someInstanceFlag: 'no boolean'
  // ok, because the field is listed in the ignoreProps
})

Meta.deserialize(user, {
  id: 1.1 // validation error
})

Table of Contents

• Introduction
• Installation
• Basic Usage
• Table of Contents
• Documentation
  • Meta Objects
    • Meta
    • Meta args
    • Meta inheritance
    • Meta.Class decorator
    • Meta.declare decorator
    • Meta.isMetaObject
    • Meta.copy
    • Meta.rebuild
  • Meta Types
    • MetaType
    • MetaType Implementation
    • MetaTypeArgs
  • Built-in Meta Types
    • ANY
    • BOOLEAN
    • STRING
    • NUMBER
    • INTEGER
    • BIGINT
    • DATE
    • LITERAL
    • INSTANCE
    • UNION
    • ARRAY
    • TUPLE
    • OBJECT
    • Recursive structures
  • Validation
    • Validators
    • Disabling Validation
  • Serialization and Deserialization
    • Serializers and Deserializers
    • Disable Serialization
    • Types Coercion
      • BOOLEAN coercion
      • STRING coercion
      • NUMBER coercion
      • INTEGER coercion
      • BIGINT coercion
      • DATE coercion
  • Errors
    • MetaTypeSerializationError
    • MetaTypeSerializerError
    • MetaTypeDeSerializerError
    • MetaTypeValidationError
    • MetaTypeValidatorError
    • MetaTypeValidatorsArrayError
• Similar Libraries
• Change Log

Documentation

Meta Objects

Meta

To work with Meta types it is convenient to use Meta objects. A Meta object is a proxy object that changes the logic of reading and writing values to the object's properties.

Example:

const objA = {
  a: 1
}
const metaObjA = Meta(objA)

// throw a validation error because this property has been initialized number
metaObjA.a = 'str'

const metaObjB = Meta()
metaObjB.a = 'str'

// throw a validation error because this property has been initialized string
metaObjB.a = 2

Since classes are more often used to describe properties, this library provides Meta classes. The difference from Meta objects is that instances of the class will also be Meta objects.

Example:

class A {
  a = 'string'

  static staticA = 2
}

const MetaA = Meta(A) // similar to the @Meta.Class() decorator

// throw a validation error because this property was initialized number
MetaA.staticA = 'str' as any

const metaInstanceA = new MetaA()

// throw a validation error because this property was initialized 'string'
metaInstanceA.a = 1 as any

To get an original object from Meta object you can use Meta.proto(metaObject) method.

Meta args

This is arguments for creating a Meta object.

function Meta<T extends object>(protoObject?: T, metaArgs?: MetaArgs): Meta<T>

Meta function has the following arguments:

type MetaArgs = {
  name?: string
  initialValues?: Record<string | symbol, any>

  ignoreProps?: (string | symbol)[] | ((propName: string | symbol) => boolean)

  validationIsActive?: boolean
  serializationIsActive?: boolean

  changeHandlers?: MetaChangeHandlerInfoType[]
  errorHandlers?: MetaErrorHandlerInfoType[]

  metaTypesArgs?: MetaTypeArgs | ((metaTypeImpl: MetaTypeImpl) => MetaTypeArgs)

  metaTypesResolver?: MetaTypesResolver
  autoResolveMetaTypes?: boolean
  dynamicDeclarations?: boolean

  metaInstanceArgs?: MetaArgs | 'same'
  buildMetaInstance?: boolean

  metaBuilder?: MetaObjectsBuilder
} & Record<string, any>

• name?: string - A string that overrides the default name of the Meta object. The name is used when displaying the Meta object. For example, if the default name is MetaObject, you can pass MyMetaObject as the name argument to change it.

• initialValues?: Record<string | symbol, any> - An object that defines the initial values of the properties of the Meta object. The default value is {}.

• ignoreProps?: (string | symbol)[] | ((propName: string | symbol) => boolean) - Specifies which properties of the Meta object should be ignored by the Meta object. The default value is []. It can be either:
  • An array of strings or symbols that represent the property names to ignore.
  • A function that takes a property name as an argument and returns a boolean value indicating whether to ignore it or not.

• validationIsActive?: boolean - A boolean that indicates whether the Meta object should perform validation on the Meta object or not. The default value is true.

• serializationIsActive?: boolean - A boolean that indicates whether the Meta object should perform serialization on the Meta object or not. The default value is true.

• changeHandlers?: MetaChangeHandlerInfoType[] - An array of handlers that handle changes in the Meta object. The default value is [].

• errorHandlers?: MetaErrorHandlerInfoType[] - An array of handlers that handle errors in the Meta object. The default value is [].

• metaTypesArgs?: MetaTypeArgs | ((metaTypeImpl: MetaTypeImpl) => MetaTypeArgs) - Defines the arguments for building or rebuilding the Meta types of the Meta object. It can be either:
  • An object that contains the properties and values of the Meta types arguments.
  • A function that takes a Meta type implementation as an argument and returns an object of Meta types arguments. The default value is {}.

• metaTypesResolver?: MetaTypesResolver - A function that resolves Meta types from values. It takes any value as an argument and returns a Meta type: (value: any, args?: MetaTypeArgs) => MetaTypeImpl. The default value is MetaTypeImpl.getMetaTypeImpl

• autoResolveMetaTypes?: boolean - A boolean that indicates whether the Meta object should automatically resolve the Meta types from a value or not. For example, the value 1 in the Meta({field: 1 }) object will be used to declare a NUMBER metatype. The default value is true.

• dynamicDeclarations?: boolean - A boolean that indicates whether the Meta object should allow new declarations of new properties or not. For example, you can define a new metatype like this: metaObject.anyField = NUMBER({ default: 1 }). The default value is true.

• metaInstanceArgs?: MetaArgs | 'same' - Defines the arguments for creating the Meta instance of the Meta class. It can be either:
  • An object that contains the properties and values of the Meta instance arguments.
  • The string 'same' that indicates that the same arguments as the Meta class should be used. The default value is 'same'.

• buildMetaInstance?: boolean - A boolean that indicates whether the Meta class should build the Meta instance or not. If true, the Meta class will use the metaInstanceArgs object to create the Meta instance. The default value is true.

• metaBuilder?: MetaObjectsBuilder - A Meta objects builder that is used to create the Meta object. The Meta objects builder is an object that implements the MetaObjectsBuilder interface. The default value is the global Meta objects builder (MetaObjectsBuilder.instance).

Record<string, any> - used if you want to use a custom builder

To get the Meta args of a Meta object you can use Meta.getMetaArgs(metaObject) method.

Meta inheritance

Meta objects support the extension of classic objects with an additional side effect.

Objects inheritance

import { BOOLEAN, Meta, MetaType, NUMBER, STRING } from 'metatyper'

const obj1: any = {
  a: 1,
  b: NUMBER({ optional: true })
}

const obj2: any = {
  c: 2,
  d: STRING({ optional: true })
}

const obj3: any = {
  e: 3,
  f: BOOLEAN({ optional: true })
}

Object.setPrototypeOf(obj2, obj1)

const metaObj2 = Meta(obj2)

// true,
// because obj1 is not a Meta object
// and there is no special logic for its properties
console.log(metaObj2.b instanceof MetaType)

// but this will create its own MetaObj2 property 'a'
// and add a Meta type declaration NUMBER({ default: 0 })
metaObj2.b = NUMBER({ default: 0 })
console.log(metaObj2.b === 0) // true

Object.setPrototypeOf(obj3, metaObj2)

obj3.e = '1' // ok, because obj3 is not a Meta object

// validation error,
// because this is a property of the MetaObj2 Meta object
// and there is special logic for it
obj3.c = '1'

The prototype of a Meta object is the original object: Object.getPrototypeOf(metaObj2) === obj2

Classes inheritance

import { Meta, NUMBER } from 'metatyper'

@Meta.Class()
class A {
  static a = NUMBER({ optional: true })
  a = NUMBER({ optional: true })
}

class B extends A {
  static b = NUMBER({ optional: true })
  b = NUMBER({ optional: true })
}

@Meta.Class()
class C extends B {
  static c = NUMBER({ optional: true })
  c = NUMBER({ optional: true })
}

console.log(A.toString())
// [<meta> class A] { a: NUMBER = undefined }

console.log(B.toString())
// [<meta child> class B] { b = NUMBER }

console.log(C.toString())
// [<meta> class C] { c: NUMBER = undefined; [a]: NUMBER = undefined }
// brackets [a] mean that property is a property of the parent Meta class

const aInstance = new A()
const bInstance = new B()
const cInstance = new C()

console.log(aInstance.toString())
// [<meta> instance A] { a: NUMBER = undefined }

console.log(bInstance.toString())
// [<meta> object] { a: NUMBER = undefined; b: NUMBER = undefined }

console.log(cInstance.toString())
// [<meta> instance C] { a: NUMBER = undefined; b: NUMBER = undefined; c: NUMBER = undefined }

// There are no [a] brackets in instances,
// as these properties are intrinsic
// (you can learn how js instance creation works)

Static classes work as simple Meta objects.

Meta.Class decorator

Decorator does the same thing as Meta(A).

Example:

import { Meta } from 'metatyper'

@Meta.Class() // Meta.Class(args) has arguments as in Meta({}, args)
class MetaA {
  a = 'string'

  static a = 2
}

// throw a validation error because this property was initialized number
MetaA.a = 'str'

const metaInstanceA = new MetaA()

// throw a validation error because this property was initialized 'string'
metaInstanceA.a = 1

Meta.declare decorator

This decorator lets you specify the Meta type of your properties.

You can do this in different ways:

• Specify the Meta type explicitly:

class Test {
  @Meta.declare(NUMBER({ min: 0 }))
  a: number
}

• Let the decorator infer the Meta type from the property value.

class Test {
  @Meta.declare({ min: 0 })
  a: number = 0
}

• Use reflect-metadata to automatically resolve the Meta type from the property type:

class Test {
  @Meta.declare({ min: 0 })
  a: number
}

You need to import reflect-metadata before using the option. Otherwise, the Meta type will be ANY().

Meta.isMetaObject

If you need to check if an object is a Meta object, you can use this method: Meta.isMetaObject(obj).

Meta.copy

Sometimes you may need to copy a Meta object. You can do this by using the spread operator: { ...metaObject }. However, this will not copy the type declarations of the Meta object. To copy the type declarations as well, you can use Meta.copy:

const metaObjectCopy = Meta.copy(metaObject)

This method creates a copy of a Meta object and preserves its values, types, prototype and arguments.

Example:

import { Meta, STRING } from 'metatyper'

const origObject: any = { a: 1 }
const origMetaObject = Meta(origObject)

origMetaObject.a = 2
origMetaObject.b = STRING({ default: '' })

const metaObjectCopy = Meta.copy(origMetaObject)
metaObjectCopy.a === 2 // true
metaObjectCopy.b === '' // true

Meta.rebuild

You may also need to reset the meta object to its original state. The Meta.rebuild is useful for creating a new instance of a Meta object with its initial state and configuration.

const newMetaObject = Meta.rebuild(metaObject)

This method rebuilds a Meta object using the same original object and arguments that were used to create the Meta object.

Example:

import { Meta, STRING } from 'metatyper'

const origObject: any = { a: 1 }
const origMetaObject = Meta(origObject)

origMetaObject.a = 2
origMetaObject.b = STRING({ default: '' })

const newMetaObject = Meta.rebuild(origMetaObject)
newMetaObject.a === 1 // true
newMetaObject.b === undefined // true
// because `origObject` was used to create the new Meta object

Meta Types

MetaType

Meta types extend built-in types, but they have more features: validation and serialization. The basic logic of Meta types is in metaTypeImpl.

Example, how to create a new Meta type:

import { MetaType, StringImpl } from 'metatyper'

const newType1 = MetaType<string>(StringImpl, {
  /* metaTypeArgs */
})
const newType2 = MetaType<string>(
  StringImpl.build({
    /* metaTypeArgs */
  })
)

MetaType Implementation

Meta type implementation example:

import { MetaType, StringImpl } from 'metatyper'

class LowerCaseStringImpl extends StringImpl {
  static isCompatible(value: string) {
    if (!this.isCompatible(value)) {
      return false
    }

    return !/[A-Z]/.test(value)
  }
}

export function LowerCaseString() {
  return MetaType<LowerCaseString>(LowerCaseStringImpl)
}

type LowerCaseString = MetaType<Lowercase<string>, LowerCaseStringImpl>

import { Meta } from 'metatyper'

@Meta.Class()
class MyNewExample {
  str = LowerCaseString()
}

const instance = new MyNewExample()

instance.str = 'abc' // ok
instance.str = 'aBc' // type and validation error

To learn more about the principles of Meta types creation, you can explore the source code of the built-in Meta types.

MetaTypeArgs

This represents the arguments for creating a Meta type.

type MetaTypeArgs<
  T = any,
  IsNullishT extends boolean = boolean,
  IsNullableT extends boolean = IsNullishT,
  IsOptionalT extends boolean = IsNullishT
> = {
  name?: string
  subType?: any
  default?: T | ((declaration?: MetaTypeImpl) => T)
  nullish?: IsNullishT
  nullable?: IsNullableT
  optional?: IsOptionalT
  coercion?: boolean
  validateType?: boolean
  noBuiltinValidators?: boolean
  noBuiltinSerializers?: boolean
  noBuiltinDeSerializers?: boolean
  validators?: (ValidatorType | ValidatorFuncType)[]
  serializers?: (SerializerType | SerializeFuncType)[]
  deserializers?: (DeSerializerType | DeSerializeFuncType)[]
} & Record<string, any>

name?: string - A string that overrides the default name of the Meta type. The name is used when displaying the Meta type.

subType?: any - A Meta type or a value that defines the type of the nested values in the value. For example, if the value is an array, you can use the subType to specify the type of the elements in the array.

default?: T | ((declaration?: MetaTypeImpl) => T) - A value or a function that returns a value that is used as the default value for the Meta type. The default value is used when the initial value is undefined.

nullish?: boolean - A boolean indicating whether the value can be null or undefined. If false, a NullableValidator and an OptionalValidator are added to the Meta type. The default value is false.

nullable?: boolean - A boolean indicating whether the value can be null. If false, a NullableValidator is added to the Meta type. If nullish and nullable are contradictory, the value of nullable will be chosen. Default value is the same as nullish.

optional?: boolean - A boolean indicating whether the value can be undefined. If false, an OptionalValidator is added to the Meta type. If nullish and optional are contradictory, the value of optional will be chosen. Default value is the same as nullish

coercion?: boolean - A boolean that indicates whether the value should be coerced to the expected type or not. If true, an CoercionSerializer is added to the Meta type, which tries to convert the main value to the appropriate type. For example, if the Meta type is a string, and the main value is a number, the number will be cast to a string.

validateType?: boolean - A boolean that indicates whether the value should be validated against the expected type or not. If true, a MetaTypeValidator is added to the Meta type, which checks that the main value matches the Meta type. Default value is true.

noBuiltinValidators?: boolean - A boolean that indicates whether the built-in validators should be disabled or not. If true, the Meta type will not use any of the default validators, like MetaTypeValidator or NullableValidator. Default value is false.

noBuiltinSerializers?: boolean - A boolean that indicates whether the built-in serializers should be disabled or not. If true, the Meta type will not use any of the default serializers, like CoercionSerializer. Default value is false.

noBuiltinDeSerializers?: boolean - A boolean that indicates whether the built-in deserializers should be disabled or not. If true, the Meta type will not use any of the default deserializers, like CoercionSerializer or ToLowerCaseSerializer (case argument in STRING). Default value is false.

validators?: (ValidatorType | ValidatorFuncType)[] - An array of validators that are used to check the value when it is assigned to an object property.

type ValidatorFuncType = (validateArgs: ValidatorArgsType) => boolean

type ValidatorType = {
  name?: string
  validate: ValidatorFuncType
}

You can read about validation and ValidatorArgsType in the following section: Validation

serializers?: (SerializerType | SerializeFuncType)[] - An array of serializers that change the value when it is retrieved from the object. For example, obj['prop'] or Meta.serialize(obj).

type SerializeFuncType = (serializeArgs: SerializerArgsType) => any

type SerializerType = {
  serialize: SerializeFuncType
  name?: string
  serializePlaces?: ('get' | 'serialize' | 'unknown')[] | string[]
}

You can read about serialization and SerializerArgsType in the following section: Serialization and Deserialization

deserializers?: (DeSerializerType | DeSerializeFuncType)[] - An array of deserializers that modify the value when it is set to an object property, prior to validation. For example, obj['prop'] = 'value' or Meta.deserialize(metaObject, rawObject).

type DeSerializeFuncType = (deserializeArgs: DeSerializerArgsType) => any

type DeSerializerType = {
  serialize: DeSerializeFuncType
  name?: string
  deserializePlaces?: ('init' | 'reinit' | 'set' | 'deserialize' | 'unknown')[] | string[]
}

You can read about deserialization and DeSerializerArgsType in the following section: Serialization and Deserialization

Built-in Meta Types

Each built-in Meta type has args?: MetaTypeArgs at the end of arguments. How to use it you can see below.

ANY

import { ANY, Meta } from 'metatyper'

const obj1 = Meta({
  a: ANY({ nullish: true })
}) // as { a: any }

obj1.a = 1
obj1.a = {}

BOOLEAN

import { BOOLEAN, Meta } from 'metatyper'

const obj = Meta({
  someField: BOOLEAN({
    default: false,

    // BooleanMetaTypeArgs
    trueValues: [1],
    // will replace 1 with true
    falseValues: [(value) => value === 0]
    // will replace 0 with false
  })
}) // as { someField: boolean }

obj.someField = true
obj.someField = 1 as boolean
obj.someField = 'true' // type & validation error

STRING

import { Meta, STRING } from 'metatyper'

const obj = Meta({
  someField: STRING({
    nullish: true,

    // StringMetaTypeArgs
    notEmpty: true, // == minLength: 0
    minLength: 0,
    maxLength: 10,

    regexp: '^[a-zA-Z]+$',
    // validate using this regular expression

    toCase: 'lower'
    // serialize to lowercase (or 'upper')
  })
}) // as { someField?: string | null | undefined }

obj.someField = 'STR' // will serialize to lowercase
obj.someField = 1 // type & validation error

NUMBER

import { Meta, NUMBER } from 'metatyper'

const obj = Meta({
  someField: NUMBER({
    nullish: true,

    // NumberMetaTypeArgs
    min: 1, // value >= 1
    max: 9, // value <= 9
    greater: 0, // value > 0
    less: 10 // value < 10
  })
}) // as { someField?: number | null | undefined }

obj.someField = 1.2
obj.someField = 11 // validation error
obj.someField = 'str' // type & validation error

INTEGER

import { INTEGER, Meta } from 'metatyper'

const obj = Meta({
  someField: INTEGER({
    nullish: true,

    // NumberMetaTypeArgs
    min: 1, // value >= 1
    max: 9, // value <= 9
    greater: 0, // value > 0
    less: 10 // value < 10
  })
}) // as { someField?: number | null | undefined }

obj.someField = 1
obj.someField = 11 // validation error
obj.someField = 1.1 // validation error

BIGINT

import { BIGINT, Meta } from 'metatyper'

const obj = Meta({
  someField: BIGINT({
    nullish: true,

    // NumberMetaTypeArgs
    min: 1, // value >= 1
    max: 9, // value <= 9
    greater: 0, // value > 0
    less: 10 // value < 10
  })
}) // as { someField?: bigint | null | undefined }

obj.someField = 1n
obj.someField = 11n // validation error
obj.someField = 1 // type and validation error

DATE

import { DATE, Meta } from 'metatyper'

const obj = Meta({
  someField: DATE({
    nullish: true,

    // DateMetaTypeArgs
    min: 1, // value >= new Date(1)
    max: new Date(), // value <= new Date()
    greater: 0, // value > new Date(0)
    less: 10n // value < new Date(10)
  })
}) // as { someField?: Date | null | undefined }

obj.someField = new Date(1)
obj.someField = 1 // type and validation error

LITERAL

import { LITERAL, Meta } from 'metatyper'

const obj = Meta({
  someField: LITERAL(1, {
    nullish: true
  })
}) // as { someField?: 1 | null | undefined }

obj.someField = 1
obj.someField = 2 // type and validation error

INSTANCE

import { Meta, INSTANCE } from 'metatyper'

class A {
    a = 1
}

class B extends A {
    b = 2
}

const obj = Meta({
    someField: INSTANCE(B, {
        nullish: true

        // InstanceMetaTypeArgs
        allowChildren: false
        // disallow the use of children B, default: true
    })
})  // as { someField?: B | null | undefined }

obj.someField = new B() // ok
obj.someField = new A() // validation error
obj.someField = {} // type and validation error
obj.someField = B // type and validation error

UNION

import { BOOLEAN, Meta, STRING, UNION } from 'metatyper'

const obj = Meta({
  someField: UNION([BOOLEAN({ nullable: true }), STRING({ optional: true })])
})
// as { someField: (boolean | null) | (string | undefined) }

obj.someField = true // ok
obj.someField = new Date() // type and validation error

ARRAY

import { Meta, ARRAY, BOOLEAN, STRING } from 'metatyper'

const obj = Meta({

    someField: ARRAY(
        [
            BOOLEAN({ default: null, nullable: true }),
            STRING({ optional: true })
        ],
        {
            nullish: true,

            // ArrayMetaTypeArgs
            notEmpty: true, // == minLength: 0
            minLength: 0,
            maxLength: 10,
            freeze: true,
            // will create a frozen copy when deserializing
            serializeSubValues: false // default: true
        }
    )

    someField2: ARRAY(STRING(), { default: [] })

})
/*
as {
    someField:
      | readonly (boolean | null | string | undefined)[]
      | null
      | undefined
    someField2: string[]
}
*/

obj.someField = ['1', '2'] // ok
obj.someField = [1, '1'] // type and validation error

TUPLE

import { Meta, STRING, TUPLE } from 'metatyper'

const obj = Meta({
  someField: TUPLE([false, STRING({ optional: true })], {
    nullish: true,

    // TupleMetaTypeArgs

    freeze: true,
    // will create a frozen copy when deserializing

    serializeSubValues: false // default: true
  })
})
/* 
as { 
    someField: 
      | readonly [ boolean, string | undefined ] 
      | null 
      | undefined 
} 
*/

obj.someField = [true, '1'] // ok
obj.someField = ['1', true] // type and validation error

OBJECT

import { Meta, OBJECT, STRING, BOOLEAN } from 'metatyper'

const obj = Meta({

    someField: OBJECT({
        a: 1,
        b: 'string',
        c: BOOLEAN(),
        d: {
            e: STRING({ optional: true }),
            f: OBJECT({})
        }
    }, {
        nullish: true,

        // ObjectMetaTypeArgs

        freeze: true,
        // will create a frozen copy when deserializing

        required: ['a', 'b', 'c'],
        // by default all fields are required

        serializeSubValues: false // default: true
    })

    someField2: OBJECT(BOOLEAN(), { optional: true })

})
/*
as {
    someField: {
        readonly a: number
        readonly b: string
        readonly c: boolean
        readonly d?: {
            e?: string
            f: Record<string, any>
        }
    }

    someField2?: Record<string, boolean>
}
*/


obj.someField = {
    a: 2,
    b: 'str',
    c: false,
    d: {
        // e: 'optional field'
        f: {
            anyField: true
        }
    }
}

obj.someField = {
    a: 2,
    b: 'str',
    // type and validation error, `c` is not an optional field
}

obj.someField2 = {
    anyField: true
}

Recursive structures

Meta types like OBJECT, ARRAY, TUPLE and UNION inherited from StructuralMetaTypeImpl. So, it allows to create a recursive structures like this:

Use argument to create a REF

import { Meta, OBJECT } from 'metatyper'

OBJECT((selfImpl) => {
  type MyObjectType = {
    // ... any fields
    self?: MyObjectType
  }

  return {
    // ... any fields
    self: selfImpl as any
  } as MyObjectType
})
// OBJECT(g4dv1h)<{ self: REF<g4dv1h> }>

Be careful, selfImpl is of type ObjectImpl

Use a variable to create a REF

import { Meta, OBJECT } from 'metatyper'

type MyType = {
  // ... any fields
  self?: MyType
}

const myType: OBJECT<MyType> = OBJECT(() => {
  return {
    // ... any fields
    self: myType
  }
})
// OBJECT(g4dv1h)<{ self: REF<g4dv1h> }>

Use recursive structures to create a REF

import { Meta, OBJECT } from 'metatyper'

type MyType = {
  // ... any fields
  self?: MyType
}

const myTypeSchema: MyType = {
  /* any fields */
}
myTypeSchema.self = myTypeSchema

OBJECT(myTypeSchema)
// OBJECT(g4dv1h)<{ self: REF<g4dv1h> }>

And of course you can create more complex structures like this

import { Meta, OBJECT, STRING, TUPLE } from 'metatyper'

const myObjectType = OBJECT((selfImpl) => {
  type MyTuple = [MyObjectType, string, MyTuple]
  type MyObjectType = {
    a: number
    b: { selfImpl: MyObjectType }
    c: MyObjectType[]
    d: MyTuple
  }

  const myObjectSchema: any = {
    a: 1,
    b: { selfImpl },
    c: null,
    d: TUPLE((selfImpl) => [myObjectType, STRING(), selfImpl])
  }

  myObjectSchema.c = [myObjectSchema, selfImpl, myObjectType]

  return myObjectSchema as MyObjectType
})

console.log(myObjectType.toString())
/*
OBJECT(n6f76)<{ 
    a: NUMBER, 
    b: OBJECT(jqrb3)<{ selfImpl: REF<n6f76> }>, 
    c: ARRAY(pugop)<
        UNION(ljwth)<
            REF<n6f76> | REF<n6f76> | REF<n6f76>
        >[]
    >, 
    d: TUPLE(zwgxz)<[ REF<n6f76>, STRING, REF<zwgxz> ]> 
}>
*/

REF - another optional type that works like a proxy.

Recursive value is not available now. You need to provide undefined value instead of recursive reference.

e.g.

// OBJECT( (myObjImpl) => ({ myObj: myObjImpl }) )

const myObj = { myObj: { myObj: { myObj: undefined } } }

Validation

Meta objects come with built-in validation capability. Validators specific to each Meta type are utilized during the validation process. If validation fails, an exception is raised. For more information on validation errors, refer to the Errors section.

Validators for Meta types are categorized into:

• Built-in Validators: For example, STRING Meta type uses MinLengthValidator, configurable via the minLength argument.

• Runtime Validators: These are provided as arguments at runtime to the Meta type.

For more details on the arguments accepted by Meta types, see the MetaTypeArgs section.

Validation occurs automatically when assigning new values to a Meta object. Additionally, you can explicitly validate another object using the method:

Meta.validate(
    metaObjectOrSchema: Meta<object> | object,
    rawObject: object,
    validateArgs?: {
        safe?: boolean,
        stopAtFirstError?: boolean
    }
) => boolean

Example:

import { Meta } from 'metatyper'

const schema = {
  id: 0,
  name: STRING({
    validators: [
      /* validators here */
    ]
  })
}

Meta.validate(schema, { id: '351', name: null })
// throws MetaTypeValidatorError

Validators

A validator is an object that contains a validate method:

type ValidatorType = {
  name?: string
  validate: (args: ValidatorArgsType) => boolean
}

type ValidatorArgsType = {
  value: any
  metaTypeImpl?: MetaTypeImpl
  propName?: string | symbol
  targetObject?: object
  baseObject?: object

  safe?: boolean
  stopAtFirstError?: boolean
} & Record<string, any>

• value: The value to be validated.
• metaTypeImpl: The Meta type implementation invoking this validator.
• propName: Specified when using Meta objects for validation.
• targetObject: The object that needs to be validated.
• baseObject: The object that contains the Meta type declaration with this validator.
• safe: Determines whether a validation error should throw an exception.
• stopAtFirstError: Specifies if validation should cease after the first error. Defaults to true.

Disabling Validation

Validation can be disabled using the methods below:

Method 1

Meta.validationIsActive(metaObj) === true

Meta.disableValidation(metaObj)

Meta.validationIsActive(metaObj) === false

metaObj.myProp = 0 // This is now allowed.

Meta.enableValidation(metaObj)

Method 2

@Meta.Class()
class MyClass2 {
  myProp = MyType({
    validators: [],
    // Set empty array

    noBuiltinValidators: true
    // Disables the built-in validators like MetaTypeValidator or MinLengthValidator
  })
}

Serialization and Deserialization

Serialization and deserialization of values are handled by Meta type's serializers and deserializers. Serialization is performed when retrieving a property’s value, whereas deserialization occurs during value assignment. Direct invocation of serialization and deserialization is also supported through specific methods:

Meta.serialize = (
    metaObject: Meta<T> | T,
    serializeArgs?: {
        metaArgs?: MetaArgs
    }
): { [key in keyof T]: any }

You can specify the serialize result type: Meta.serialize<{ a: number }>(...)

Meta.deserialize = (
    metaObjectOrProto: Meta<T> | T,
    rawObject: object,
    deserializeArgs?: {
        metaArgs?: MetaArgs
    }
): Meta<T>

Example:

import { Meta } from 'metatyper'

const objToSerialize = { id: '351', date: new Date(123) }

const objToDeSerialize = Meta.serialize<{
  id: string
  date: number
}>({ id: '', name: DATE({ coercion: true }) }, objToSerialize)

// objToDeSerialize now equals { id: '351', date: 123 }

Meta.deserialize({ id: '', name: DATE({ coercion: true }) }, objToDeSerialize) // returns Meta({ id: '351', date: new Date(123) })

Serializers and Deserializers

A serializer is an object with a serialize method, and a deserializer likewise has a deserialize method:

type SerializerArgsType = {
  value: any
  metaTypeImpl?: MetaTypeImpl
  propName?: string | symbol
  targetObject?: object
  baseObject?: object
  place?: SerializePlaceType
}

type SerializerType = {
  serialize: (serializeArgs: SerializerArgsType) => any

  name?: string
  serializePlaces?: SerializePlaceType[]
}

type DeSerializerArgsType = {
  value: any
  metaTypeImpl?: MetaTypeImpl
  propName?: string | symbol
  targetObject?: object
  baseObject?: object
  place?: DeSerializePlaceType
}

type DeSerializerType = {
  deserialize: (deserializeArgs: DeSerializerArgsType) => any

  name?: string
  deserializePlaces?: DeSerializePlaceType[]
}

Each field's purpose in both serializers and deserializers closely aligns with those in validators, specifying the object context and invoking Meta type implementation.

Serialization and deserialization processes can be adjusted depending on their specific use cases with the help of SerializePlaceType and DeSerializePlaceType. This allows for a more precise control over how and where these processes occur. The term "place" refers to the specific scenario in which serialization occurs.

SerializePlaceType indicates various contexts where serialization can happen:

• get: This is used when retrieving a property, for example, accessing a property like obj.prop.
• serialize: This context is applied during the serialization of an object, such as when using Meta.serialize(obj).
• unknown: This default setting is used for custom serialization logic that does not fit the other predefined contexts.

Similarly, DeSerializePlaceType outlines different scenarios for deserialization:

• init: Indicates the initialization of a new type declaration, like starting a Meta object with Meta({ prop: 'value' }).
• reinit: Used when re-initializing type declarations by defining a new Meta type, for example, changing a property's type with obj.prop = NUMBER().
• set: Applies when setting a new property value, such as obj.prop = 1.
• deserialize: This context is for deserializing an object, done like Meta.deserialize(obj, rawData).
• unknown: The default setting for custom deserialization logic that doesn't align with the specified contexts.

Disable Serialization

To disable serialization or deserialization:

Method 1

Meta.serializationIsActive(metaObject) === true

Meta.disableSerialization(metaObject)

Meta.serializationIsActive(metaObject) === false

metaObject.myProp = 0 // Deserialization does not occur.

Meta.enableSerialization(metaObject)

Method 2

@Meta.Class()
class MyClass2 {
  myProp = MyType({
    serializers: [],
    // Empty array disables serializers

    deserializers: [],
    // Empty array disables deserializers

    noBuiltinSerializers: true,
    // Disables all built-in serializers like Coercion

    noBuiltinDeSerializers: true
    // Disables all built-in deserializers like Coercion
  })
}

Types Coercion

Meta types have coercion capabilities upon serialization and deserialization. This is particularly handy for handling various value types such as dates in JSON data.

undefined and null values will not be converted

These built-in metatypes support coercion:

BOOLEAN coercion

BOOLEAN({ coercion: true })

Meta.deserialize will cast value to !!value.

STRING coercion

STRING({ coercion: true })

Meta.deserialize will cast any value to string.

For the date value, value.toISOString() will be used

NUMBER coercion

NUMBER({ coercion: true })

Meta.deserialize will cast value depending on the type of the value:

• Date -> value.getTime()
• bigint -> Number(value)
• string -> Number(value)
• boolean -> Number(value)

INTEGER coercion

INTEGER({ coercion: true })

Meta.deserialize will cast value depends on the type of the value:

• Date -> value.getTime()
• bigint -> Number(value)
• string -> Number(value)
• boolean -> Number(value)
• number -> Math.trunc(value)

BIGINT coercion

BIGINT({ coercion: true })

Meta.deserialize will cast value depends on the type of the value:

• Date -> BigInt(value.getTime())
• string -> BigInt(value)
• boolean -> BigInt(value)
• number -> BigInt(Math.trunc(value))

Meta.serialize will cast value to string.

DATE coercion

DATE({ coercion: true })

Meta.deserialize will cast value depends on the type of the value:

• bigint -> new Date(Number(value))
• number -> new Date(value)
• string -> new Date(value)

Meta.serialize will cast value to timestamp (value.getTime()).

Errors

When working with Meta objects, you may encounter a range of errors. These can be broadly categorized into standard errors, such as TypeError('Cannot assign to read only property ...'), and specialized errors unique to the handling of Meta objects. Understanding these errors and their hierarchy is crucial for effectively managing exceptions and maintaining robust code.

Specialized errors fall under the umbrella of MetaError and include:

MetaTypeSerializationError

MetaTypeSerializationError serves as an extended version of MetaError, focusing specifically on the identification and handling of serialization errors. Its main purpose is to allow developers to pinpoint serialization issues distinctively using instanceof checks. This differentiation is crucial for separating serialization errors from others, like validation errors, enhancing debugging efficiency.

MetaTypeSerializerError

When it comes to serialization of Meta type data, encountering errors is a possibility. The MetaTypeSerializerError is designed to throw an exception in such cases. This error aims to simplify the debugging process and error handling by offering in-depth information about where and why the failure occurred.

Key fields of the MetaTypeSerializerError:

• serializer: This property provides a direct link to the SerializerType instance responsible for the error. This allows developers to easily identify which serializer was involved in the process and potentially inspect its configuration or state at the time of failure.

• serializerErrorArgs: Holding the type SerializerErrorArgsType, this property delivers a detailed look at the arguments fed into the serialization function at the error's occurrence. These arguments cover a range of information, from the value being serialized, the property's name, the target object, to additional options affecting serialization. Within serializerErrorArgs, there's a subError field holding an Error instance, shedding light on the precise cause of the serialization failure. This layered error reporting strategy significantly aids in debugging by providing a clear context of the error beyond merely indicating its occurrence.

type SerializerErrorArgsType = {
  value: any

  subError?: Error

  propName?: string | symbol
  targetObject?: object
  baseObject?: object
  place?: SerializePlaceType

  metaTypeImpl?: MetaTypeImpl
} & Record<string, any>

MetaTypeDeSerializerError

Mirroring the MetaTypeSerializerError, the MetaTypeDeSerializerError addresses errors during the deserialization of Meta type data. This exception is crucial for developers aiming to resolve issues arising from converting serialized data back into a usable form within the application.

Key fields of the MetaTypeDeSerializerError:

• deserializer: Reflecting the serializer attribute in MetaTypeSerializerError, this field links to the deserializer instance that encountered the error, facilitating an understanding of which deserialization logic didn't succeed.

• deserializerErrorArgs: As DeSerializerErrorArgsType, this attribute documents the arguments present at the deserialization function during the error event. Providing a comprehensive context, these arguments include the value being deserialized, relevant property names, and the objects involved, among others.

type DeSerializerErrorArgsType = {
  value: any

  subError?: Error

  propName?: string | symbol
  targetObject?: object
  baseObject?: object
  place?: DeSerializePlaceType

  metaTypeImpl?: MetaTypeImpl
} & Record<string, any>

MetaTypeValidationError

MetaTypeValidationError functions as an enhanced variant of MetaError, specifically dedicated to identifying and managing validation errors. Its primary role is to enable developers to efficiently distinguish validation errors from other types, such as serialization or deserialization errors, through instanceof checks. This specificity is vital for streamlined error handling and debugging processes.

MetaTypeValidatorError

During the validation process of Meta type data, it's plausible to encounter failures. The MetaTypeValidatorError is thrown in such scenarios to indicate a validation issue. The uniqueness of this error, compared to MetaTypeValidatorsArrayError, is determined by the function arguments during validation. Specifically, setting stopAtFirstError: false in the function arguments collects all encountered validation errors, encapsulating them within a MetaTypeValidatorsArrayError containing multiple MetaTypeValidatorError instances.

Key fields of the MetaTypeValidatorError:

• validator: This field connects directly to the ValidatorType that generated the error, allowing developers to identify and investigate the specific validator causing the issue.

• validatorErrorArgs: Holding the ValidatorErrorArgsType, this attribute provides a detailed account of the circumstances leading to the validation error. Information includes the value under validation, property names, the involved objects, and optionally, a subError detailing the underlying cause of failure, if applicable. Additionally, it may indicate whether the validation was set to stop at the first error or continue to gather all errors.

type ValidatorErrorArgsType = {
  value: any
  subError?: Error

  propName?: string | symbol
  targetObject?: object
  baseObject?: object

  metaTypeImpl?: MetaTypeImpl
  stopAtFirstError?: boolean
} & Record<string, any>

MetaTypeValidatorsArrayError

Mirroring scenarios in which MetaTypeValidatorError arises, the MetaTypeValidatorsArrayError is thrown when multiple validation errors are encountered during the checking of Meta type data. This exception is particularly generated when the validation function’s argument stopAtFirstError is set to false, allowing the accumulation of all validation errors into a single MetaTypeValidatorsArrayError. This error then contains an array of MetaTypeValidatorError instances, providing a comprehensive overview of all validation issues detected.

Key fields of the MetaTypeValidatorsArrayError:

• validatorsErrors: This attribute is an array of MetaTypeValidatorError instances, each detailing a specific validation failure encountered during the process. This collection offers a broad perspective on the nature and extent of validation issues, facilitating thorough error resolution.

Similar Libraries

There are many other popular good libraries that perform similar functions well. If you require specific functionality, you can explore various validation libraries (eg Zod) or type collections (eg Type-fest). Moreover, you have the flexibility to integrate the best features from these libraries alongside MetaTyper to achieve best outcomes.

These libs are worth a look:

• class-validator
• io-ts
• joi
• ow
• runtypes
• ts-toolbelt
• type-fest
• yup
• zod

Change Log

Stay updated with the latest changes and improvements: GitHub Releases.