yup

What is yup?

The yup npm package is a JavaScript schema builder for value parsing and validation. It enables developers to construct schemas for objects, arrays, strings, numbers, booleans, and more, to validate the shape and content of data against those schemas. It is often used in the context of form validation on both the client and server sides.

What are yup's main functionalities?

Object Shape Validation

Validates the shape of an object, ensuring that it has specific fields with data types and constraints like strings being required, numbers being positive integers, and strings matching email or URL patterns.

{"const schema = yup.object().shape({ name: yup.string().required(), age: yup.number().required().positive().integer(), email: yup.string().email(), website: yup.string().url() }); schema.isValid({ name: 'John', age: 24, email: '[e-mail]', website: 'http://example.com' }).then(valid => { console.log(valid); // true });"}

Array Validation

Validates arrays, ensuring that they contain elements of a specific type and adhere to constraints, such as all elements being positive numbers.

{"const schema = yup.array().of(yup.number().positive()).required(); schema.isValid([1, 2, 3]).then(valid => { console.log(valid); // true });"}

String Validation

Validates strings, ensuring they meet criteria such as being required, having a minimum length, and not exceeding a maximum length.

{"const schema = yup.string().required().min(2).max(10); schema.isValid('Hello').then(valid => { console.log(valid); // true });"}

Number Validation

Validates numbers, ensuring they are positive, integers, and below a certain value.

{"const schema = yup.number().positive().integer().lessThan(100); schema.isValid(50).then(valid => { console.log(valid); // true });"}

Date Validation

Validates dates, ensuring they fall within a specified range.

{"const schema = yup.date().min(new Date(2000, 0, 1)).max(new Date(2020, 12, 31)); schema.isValid(new Date(2010, 6, 15)).then(valid => { console.log(valid); // true });"}

Custom Validation

Allows for custom validation functions to be defined, providing flexibility for more complex validation scenarios.

{"const schema = yup.string().test('is-john', 'The name must be John', value => value === 'John'); schema.isValid('John').then(valid => { console.log(valid); // true });"}

Other packages similar to yup

joi

Joi is a powerful schema description language and data validator for JavaScript. It offers a similar API to yup but with a broader feature set and more detailed error messages. Joi is often considered more heavyweight compared to yup.

ajv

Ajv is a JSON schema validator that supports draft-06/07/2019-09 of JSON Schema. It is known for its performance and is typically used for validating JSON data structures. Unlike yup, Ajv is based on pre-defined JSON schemas rather than a programmatic API.

validator

Validator is a library of string validators and sanitizers. It is more focused on string validation and does not provide object schema validation like yup. It's useful for simple validation tasks without the need for defining complex schemas.

class-validator

Class-validator works with TypeScript and uses decorators to define validation rules within class models. It is tightly integrated with TypeScript and is a good choice for projects that are already using TypeScript and decorators.

README

Yup

Yup is a JavaScript object schema validator and object parser. The API and style is stolen heavily inspired by Joi, which is an amazing library but is generally too large and difficult to package for use in a browser. Yup is a leaner in the same spirit without some of the fancy features. You can use it on the server as well, but in that case you might as well just use Joi.

Yup is also a a good bit less opinionated than joi, allowing for custom transformations and async validation. It also allows "stacking" conditions via when for properties that depend on more than one other sibling or child property. Yup separates the parsing and validating functions into separate steps so it can be used to parse json separate from validating it, via the cast method.

Table of Contents generated with DocToc

• Usage
• API
  • yup
    • .reach(Schema schema, String path, [Object value, Object context])
    • .addMethod(schemaType, name, method)
    • ValidationError(String|Array<String> errors, Any value, String path)
    • ref(String path, Object options)
  • mixed
    • mixed.clone()
    • mixed.label(String label)
    • mixed.concat(Schema schema)
    • mixed.validate(Any value, [Object options, Function callback])
    • mixed.isValid(Any value, [Object options, Function callback]) -> Promise
    • mixed.cast(value) -> Any
    • mixed.isType(Any value) -> Boolean
    • mixed.strict() (default: false)
    • mixed.withMutation(Function fn)
    • mixed.default(Any value)
    • mixed.default() -> Any
    • mixed.nullable(Bool isNullable = false)
    • mixed.required([String message])
    • mixed.typeError(String message)
    • mixed.oneOf(Array<Any> arrayOfValues, [String message]) Alias: equals
    • mixed.notOneOf(Array<Any> arrayOfValues, [String message])
    • mixed.when(String|Array<String> keys, Object options | Function func)
    • mixed.test(String name, String message, Function fn, [Bool callbackStyleAsync])
    • mixed.test(Object options)
    • mixed.transform(Function fn)
  • string
    • string.required([String message])
    • string.min(Number|Ref limit, [String message])
    • string.max(Number|Ref limit, [String message])
    • string.matches(Regex regex, [String message])
    • string.email([String message])
    • string.url([String message])
    • string.trim([String message])
    • string.lowercase([String message])
    • string.uppercase([String message])
  • number
    • number.min(Number|Ref limit, [String message])
    • number.max(Number|Ref limit, [String message])
    • number.positive([String message])
    • number.negative([String message])
    • number.integer([String message])
    • round(String type) - 'floor', 'ceil', 'round'
  • boolean
  • date
    • date.min(Date|String|Ref limit, [String message])
    • date.max(Date|String|Ref limit, [String message])
  • array
  • array.of(Schema type)
    • array.required([String message])
    • array.min(Number|Ref limit, [String message])
    • array.max(Number|Ref limit, [String message])
  • array.compact(Function rejector)
  • object
    • object.shape(Object schemaHash, [noSortEdges])
    • object.from(String fromKey, String toKey, Bool alias)
    • object.noUnknown([Bool onlyKnownKeys, String msg])
    • object.camelcase()
    • object.constantcase()
• Extending Schema Types

Usage

You define and create schema objects. Schema objects are immutable, so each call of a method returns a new schema object.

var yup = require('yup')

var schema = yup.object().shape({
  name:      yup.string().required(),
  age:       yup.number().required().positive().integer(),
  email:     yup.string().email(),
  website:   yup.string().url(),
  createdOn: yup.date().default(function() {
    return new Date
  })
})

//check validity
schema.isValid({
  name: 'jimmy',
  age: 24
})
.then(function(valid){
  valid // => true  
})


//you can try and type cast objects to the defined schema
schema.cast({
  name: 'jimmy',
  age: '24',
  createdOn: '2014-09-23T19:25:25Z'
})
// => { name: 'jimmy', age: 24, createdOn: Date }

API

yup

The module export.

var yup = require('yup')

yup.mixed
yup.string
yup.number
yup.boolean // also aliased as yup.bool
yup.date
yup.object
yup.array

yup.reach
yup.addMethod
yup.ValidationError

.reach(Schema schema, String path, [Object value, Object context])

For nested schema's yup.reach will retrieve a nested schema based on the provided path.

For nested schema that need to resolve dynamically, you can provide a value and optionally a context object.

var schema = object().shape({
      nested: object()
        .shape({
          arr: array().of(
            object().shape({ num: number().max(4) }))
      })
    })

reach(schema, 'nested.arr.num')
reach(schema, 'nested.arr[].num')  
reach(schema, 'nested.arr[1].num')  
reach(schema, 'nested["arr"][1].num')

.addMethod(schemaType, name, method)

Adds a new method to the core schema types. A friendlier convenience method for schemaType.prototype[name] = method.

  yup.addMethod(yup.date, 'format', function(formats, parseStrict) {

    return this.transform(function(value, originalValue){

      if ( this.isType(value) ) return value

      value = Moment(originalValue, formats, parseStrict)

      return date.isValid() ? date.toDate() : invalidDate
    })
  })

ValidationError(String|Array<String> errors, Any value, String path)

Thrown on failed validations, with the following properties

• name: ValidationError
• path: a string, indicating where there error was thrown. path is empty at the root level.
• errors: array of error messages
• inner: in the case of aggregate errors, inner is an array of ValidationErrors throw earlier in the validation chain. When the abortEarly option is false this is where you can inspect each error thrown, alternatively errors will have all the of the messages from each inner error.

ref(String path, Object options)

Creates a reference to another sibling or sibling descendant field. Ref's are resolved at run time and supported where specified. Ref's are evaluated in in the proper order so that the ref value is resolved before the field using the ref (be careful of circular dependencies!).

var schema = object({
  baz: ref('foo.bar'),
  foo: object({
    bar: string()
  })
  x: ref('$x')
})

inst.cast({ foo: { bar: 'boom' } }, { context: { x: 5 } })
// { baz: 'boom',  x: 5, { foo: { bar: 'boom' } }, }

mixed

Creates a schema that matches all types. All types inherit from this base type

var schema = yup.mixed();
schema.isValid(undefined, function(valid){
  valid //=> true
})

mixed.clone()

Creates a deep copy of the schema. Clone is used internally to return a new schema with every schema state change.

mixed.label(String label)

Overrides the key name which is used in error messages.

mixed.concat(Schema schema)

Creates a new instance of the schema by combining two schemas. Only schemas of the same type can be concatenated.

mixed.validate(Any value, [Object options, Function callback])

Returns the value (a cast value if isStrict is false) if the value is valid, and returns the errors otherwise. This method is asynchronous and returns a Promise object, that is fulfilled with the value, or rejected with a ValidationError. If you are more comfortable with Node style callbacks, then you can provide one to be called when the validation is complete (called with the Error as the first argument, and value as the second).

The options argument is an object hash containing any schema options you may want to override (or specify for the first time).

• strict -> boolean: default false, only validate the input, and skip and coercion or transformation
• abortEarly -> boolean: default true, return from validation methods on the first error rather than after all validations run.
• stripUnknown -> boolean: default false remove unspecified keys from objects.
• recursive -> boolean: default true when false validations will not descend into sub schemas (relavant for objects or arrays).
• context -> an object containing any context for validating schema conditions (see: when())

schema.validate({ name: 'jimmy',age: 24 })
  .then(function(value){
    value // => { name: 'jimmy',age: 24 }  
  })

schema.validate({ name: 'jimmy', age: 'hi' })
  .catch(function(err){
    err.name   // 'ValidationError'
    err.errors // => ['age must be a number']
  })

//or with callbacks

schema.validate({ name: 'jimmy',age: 24 }, function(err, value){
  err === null // true
  value        // => { name: 'jimmy',age: 24 }  
})

schema.validate({ name: 'jimmy', age: 'hi' }, function(err, value){
  err.name   // 'ValidationError'
  err.errors // => ['age must be a number']
  value === undefined // true
})

mixed.isValid(Any value, [Object options, Function callback]) -> Promise

Returns true when the passed in value matches the schema. if false then the schema also has a .errors field which is an array of validation error messages (strings), thrown by the schema. isValid is asynchronous and returns a Promise object. If you are more comfortable with Node style callbacks, providing a function as the last argument will opt into that interface.

Takes the same options as validate().

mixed.cast(value) -> Any

Attempts to coerce the passed in value to a value that matches the schema. For example: '5' will cast to 5 when using the number() type. Failed casts generally return null, but may also return results like NaN and unexpected strings.

mixed.isType(Any value) -> Boolean

Runs a type check against the passed in value. It returns true if it matches, it does not cast the value. When nullable() is set null is considered a valid value of the type. You should use isType for all Schema type checks.

mixed.strict() (default: false)

Sets the strict option to true. Strict schemas skip coercion and transformation attempts, validating the value "as is".

mixed.withMutation(Function fn)

First the legally required Rich Hickey quote:

If a tree falls in the woods, does it make a sound?

If a pure function mutates some local data in order to produce an immutable return value, is that ok?

withMutation allows you to mutate the schema in place, instead of the default behavior which clones before each change. Generally this isn't necessary since the vast majority of schema changes happen during the initial declaration, and only happen once over the lifetime of the schema, so performance isn't an issue. However certain mutations do occur at cast/validation time, (such as conditional schema using when()), or when instantiating a schema object.

object()
  .shape({ key: string() })
  .withMutation(schema => {
    return arrayOfObjectTests.forEach(test => {
      schema.test(test)
    })
  })

mixed.default(Any value)

Sets a default value to use when the value is undefined (or null when the schema is not nullable). Defaults are created after transformations are executed, but before validations, to help ensure that safe defaults are specified. The default value will be cloned on each use, which can incur performance penalty for objects and arrays. To avoid this overhead you can also pass a function that returns an new default.

  yup.string.default('nothing');

  yup.object.default({ number: 5}); // object will be cloned every time a default is needed

  yup.object.default(() => ({ number: 5})); // this is cheaper

  yup.date.default(() => new Date()); //also helpful for defaults that change over time

mixed.default() -> Any

Calling default with no arguments will return the current default value

mixed.nullable(Bool isNullable = false)

Indicates that null is a valid value for the schema. Without nullable() null is treated as a different type and will fail isType() checks.

mixed.required([String message])

Mark the schema as required. All field values apart from undefined meet this requirement.

mixed.typeError(String message)

Define an error message for failed type checks. The ${value} and ${type} interpolation can be used in the message argument.

mixed.oneOf(Array<Any> arrayOfValues, [String message]) Alias: equals

Whitelist a set of values. Values added are automatically removed from any blacklist if they are in it. The ${values} interpolation can be used in the message argument.

var schema = yup.mixed().oneOf(['jimmy', 42]);
schema.isValid(42)       //=> true
schema.isValid('jimmy')  //=> true
schema.isValid(new Date) //=> false

mixed.notOneOf(Array<Any> arrayOfValues, [String message])

Blacklist a set of values. Values added are automatically removed from any whitelist if they are in it. The ${values} interpolation can be used in the message argument.

var schema = yup.mixed().notOneOf(['jimmy', 42]);
schema.isValid(42)       //=> false
schema.isValid(new Date) //=> true

mixed.when(String|Array<String> keys, Object options | Function func)

Adjust the schema based on a sibling or sibling children fields. You can provide an object literal where the key is is value or a matcher function, then provides the true schema and/or otherwise for the failure condition.

is conditions are strictly compared (===) if you want to use a different form of equality you can provide a function like: is: (value) => value == true.

Like joi you can also prefix properties with $ to specify a property that is dependent on context passed in by validate() or isValid. when conditions are additive.

var inst = yup.object({
      isBig: yup.boolean(),
      count: yup.number()
        .when('isBig', {
          is: true,  // alternatively: (val) => val == true
          then:      yup.number().min(5),
          otherwise: yup.number().min(0)
        })
        .when('$other', (other, schema) => other === 4
          ? schema.max(6)
          : schema)
    })

inst.validate(value, { context: { other: 4 }})

You can also specify more than one dependent key, in which case each value will be spread as an argument.

var inst = yup.object({
      isSpecial: yup.bool()
      isBig: yup.bool(),
      count: yup.number()
        .when(['isBig', 'isSpecial'], {
          is: true,  // alternatively: (isBig, isSpecial) => isBig && isSpecial
          then:      yup.number().min(5),
          otherwise: yup.number().min(0)
        })
    })

inst.validate({
  isBig: true,
  isSpecial: true,
  count: 10
})

Alternatively you can provide a function the returns a schema (called with the value of the key and the current schema).

var inst = yup.object({
      isBig: yup.boolean(),
      count: yup.number()
        .when('isBig', (isBig, schema) => {
          return isBig ? schema.min(5) : schema.min(0)
        })
    })

inst.validate({ isBig: false, count: 4 })

mixed.test(String name, String message, Function fn, [Bool callbackStyleAsync])

Adds a test function to the validation chain. Tests are run after any object is cast. Many types have some tests built in, but you can create custom ones easily. In order to allow asynchronous custom validations all tests are run asynchronously. A consequence of this is that test execution order cannot be guaranteed.

All tests must provide a name, an error message and a validation function that must return true or false or a ValidationError. To make a test async return a promise that resolves true or false or a ValidationError. If you prefer the Node callback style, you can pass true for callbackStyleAsync and the validation function will pass in an additional done function as the last parameter to be called with the validity.

for the message argument you can provide a string which is will interpolate certain values if specified using the ${param} syntax. By default all test messages are passed a path value which is valuable in nested schemas.

the test function is called with the current value, along with path and context if they exist. For more advanced validations you can use the alternate signature to provide more options (see below):

var jimmySchema = yup.string()
  .test('is-jimmy', '${path} is not Jimmy', value => value === 'jimmy');

// or make it async by returning a promise
var asyncJimmySchema = yup.string()
  .test('is-jimmy', '${path} is not Jimmy', function (value){
    return fetch('/is-jimmy/' + value)
      .then(response => response.responseText === 'true')
  });

// or callback style for asynchrony
var asynCallbackJimmySchema = yup.string()
  .test('is-jimmy', '${path} is not Jimmy', test, true);

function test(value, done){
  // error argument is for exceptions, not an failed tests
  done(null, value === 'jimmy')
}

schema.isValid('jimmy').then(...) //=> true

schema.isValid('john').then(...) //=> false
schema.errors // => [ 'this is not Jimmy!']

test functions are called with a special context, or this value, that exposes some useful metadata and functions.

• this.path: the string path of the current validation
• this.schema: the resolved schema object that the test is running against.
• this.options: the options object that validate() or isValid() was called with
• this.parent: in the case of nested schema, this is the value of the parent object
• this.createError(Object: { path: String, message: String }): create and return a validation error. Useful for dynamically setting the path, or more likely, the error message. If either option is omitted it will use the current path, or default message.

mixed.test(Object options)

Alternative test(..) signature. options is an object containing some of the following options:

• name: string, all validations must have a name.
• test: function(value), the validator run against the value, should return true or false or a promise that resolves to true or false
• message: string, validation error message
• params: object, passed to message for interpolation
• exclusive: boolean (default false), when true, there can only be one active test of the same name on a schema, validations of the same name will replace previous ones. when false the validations will stack. e.g. max is an exclusive validation, whereas the string matches is not. This is helpful for "toggling" validations on and off.
• useCallback: boolean (default false), use the callback interface for asynchrony instead of promises

In the case of mixing exclusive and non-exclusive tests the following logic is used. If a non-exclusive test is added to a schema with an exclusive test of the same name the exclusive test is removed and further tests of the same name will be stacked.

If an exclusive test is added to a schema with non-exclusive tests of the same name the previous tests are removed and further tests of the same name will replace each other.

var schema = yup.mixed().test({
      name: 'max',
      exclusive: true,
      params: { max },
      message: '${path} must be less than ${max} characters',
      test: value => value == null || value.length <= max
    });

mixed.transform(Function fn)

Adds a transformation to the transform chain. Transformations are central to the casting process, default transforms for each type coerce values to the specific type (as verified by isType()). transforms are run before validations and only applied when strict is true. Some types have built in transformations.

Transformations are useful for arbitrarily altering how the object is cast, however, you should take care not to mutate the passed in value. Transforms are run sequentially so each value represents the current state of the cast, you can use the orignalValue param if you need to work on the raw initial value.

var schema = yup.string().transform(function(currentValue, originalvalue){
  return this.isType(value) && value !== null
    ? value.toUpperCase()
    : value
});

schema.cast('jimmy') //=> 'JIMMY'

Each types will handle basic coercion of values to the proper type for you, but occasionally you may want to adjust or refine the default behavior. For example, if you wanted to use a different date parsing strategy than the default one you could do that with a transform.

yup.date().transform(function(formats = 'MMM dd, yyyy'){
  //check to see if the previous transform already parsed the date
  if ( this.isType(value) ) return value

  //the default coercion failed so lets try it with Moment.js instead
  value = Moment(originalValue, formats)

  //if its valid return the date object, otherwise return an `InvalidDate`
  return date.isValid() ? date.toDate() : new Date('')
})

string

Define a string schema. Supports all the same methods as mixed.

var schema = yup.string();
schema.isValid('hello') //=> true

string.required([String message])

The same as the mixed() schema required, except that empty strings are also considered 'missing' values. To allow empty strings but fail on undefined values use: string().required().min(0)

string.min(Number|Ref limit, [String message])

Set an minimum length limit for the string value. The ${min} interpolation can be used in the message argument

string.max(Number|Ref limit, [String message])

Set an maximum length limit for the string value. The ${max} interpolation can be used in the message argument

string.matches(Regex regex, [String message])

Provide an arbitrary regex to match the value against.

var v = string().matches(/(hi|bye)/);
v.isValid('hi').should.eventually.equal(true)
v.isValid('nope').should.eventually.equal(false)

string.email([String message])

Validates the value as an email address via a regex.

string.url([String message])

Validates the value as a valid URL via a regex.

string.trim([String message])

Transforms string values by removing leading and trailing whitespace. If strict() is set it will only validate that the value is trimmed.

string.lowercase([String message])

Transforms the string value to lowercase. If strict() is set it will only validate that the value is lowercase.

string.uppercase([String message])

Transforms the string value to uppercase. If strict() is set it will only validate that the value is uppercase.

number

Define a number schema. Supports all the same methods as mixed.

var schema = yup.number();
schema.isValid(10) //=> true

number.min(Number|Ref limit, [String message])

Set the minimum value allowed. The ${min} interpolation can be used in the message argument.

number.max(Number|Ref limit, [String message])

Set the maximum value allowed. The ${max} interpolation can be used in the message argument.

number.positive([String message])

Value must be a positive number.

number.negative([String message])

Value must be a negative number.

number.integer([String message])

Transformation that coerces the value into an integer via truncation value | 0. If strict() is set it will only validate that the value is an integer.

round(String type) - 'floor', 'ceil', 'round'

Rounds the value by the specified method (defaults to 'round').

boolean

Define a boolean schema. Supports all the same methods as mixed.

var schema = yup.boolean();
schema.isValid(true) //=> true

date

Define a Date schema. By default ISO date strings will parse correctly, for more robust parsing options see the extending schema types at the end of the readme. Supports all the same methods as mixed.

var schema = yup.date();
schema.isValid(new Date) //=> true

date.min(Date|String|Ref limit, [String message])

Set the minimum date allowed.

date.max(Date|String|Ref limit, [String message])

Set the maximum date allowed.

array

Define an array schema. Arrays can be typed or not, When specifying the element type, cast and isValid will apply to the elements as well. Options passed into isValid are passed also passed to child schemas. Supports all the same methods as mixed.

var schema = yup.array().of(number().min(2));
schema.isValid([2, 3])   //=> true
schema.isValid([1, -24]) //=> false

schema.cast(['2', '3'])  //=> [2, 3]

array.of(Schema type)

Specify the schema of array elements. of() is optional and when omitted the array schema will not validate its contents.

array.required([String message])

The same as the mixed() schema required, except that empty arrays are also considered 'missing' values. To allow empty arrays but fail on undefined values use: array().required().min(0)

array.min(Number|Ref limit, [String message])

Set an minimum length limit for the array. The ${min} interpolation can be used in the message argument.

array.max(Number|Ref limit, [String message])

Set an maximum length limit for the array. The ${max} interpolation can be used in the message argument.

array.compact(Function rejector)

Removes falsey values from the array. Providing a rejecter function lets you specify the rejection criteria yourself.

array()
  .compact()
  .cast(['', 1, 0, 4, false, null]) // => [1,4]

array()
  .compact(function(v){
    return v == null
  })
  .cast(['', 1, 0, 4, false, null]) // => ['',1, 0, 4, false]

object

Define an object schema. Options passed into isValid are also passed to child schemas. Supports all the same methods as mixed.

yup.object().shape({
  name:      string().required(),
  age:       number().required().positive().integer(),
  email:     string().email(),
  website    string().url(),
})

object.shape(Object schemaHash, [noSortEdges])

Define the keys of the object and the schemas for said keys.

object.from(String fromKey, String toKey, Bool alias)

Transforms the specified key to a new key. If alias is true then the old key will be left.

var schema = object()
      .shape({
        myProp: mixed(),
        Other: mixed(),
      })
      .from('prop', 'myProp')
      .from('other', 'Other', true)

inst.cast({ prop: 5, other: 6}) // => { myProp: 5, other: 6, Other: 6 }

object.noUnknown([Bool onlyKnownKeys, String msg])

Validate that the object value only contains keys specified in shape, pass false as the first argument to disable the check. Restricting keys to known, also enables stripUnknown option, when not in strict mode.

object.camelcase()

Transforms all object keys to camelCase

object.constantcase()

Transforms all object keys to CONSTANT_CASE.

Extending Schema Types

The simplest way to extend an existing type is just to cache a configured schema and use that through your application.

  var yup = require('yup');
  var parseFormats = ['MMM dd, yyy']
  var invalidDate = new Date('');

  module.exports = yup.date()
    .transform(function(value, originalValue){
        if ( this.isType(value) ) return value
        //the default coercion transform failed so lets try it with Moment instead
        value = Moment(originalValue, parseFormats)
        return date.isValid() ? date.toDate() : invalidDate
    })

Alternatively, each schema is a normal JavaScript constructor function that you can mutate or delegate to using the normal patterns. Generally you should not inherit from mixed unless you know what you are doing, better to think of it as an abstract class. The other types are fair game though.

You should keep in mind some basic guidelines when extending schemas

• never mutate an existing schema, always clone() and then mutate the new one before returning it. Built-in methods like test and transform take care of this for you, so you can safely use them (see below) without worrying
• transforms should never mutate the value passed in, and should return an invalid object when one exists (NaN, InvalidDate, etc) instead of null for bad values.
• by the time validations run the value is guaranteed to be the correct type, however if nullable is set then null is a valid value for that type, so don't assume that a property or method exists on the value.

Adjust core Types

var invalidDate = new Date('');

function parseDateFromFormats(formats, parseStrict) {

  return this.transform(function(value, originalValue){
    if (this.isType(value))
      return value

    value = Moment(originalValue, formats, parseStrict)

    return date.isValid() ? date.toDate() : invalidDate
  })
}

// `addMethod` doesn't do anything special it's
// equivalent to: yup.date.protoype.format = parseDateFromFormats
yup.addMethod(yup.date, 'format', parseDateFromFormats)

Creating new Types

Yup schema use the common constructor pattern for modeling inheritance. You can use any utility or pattern that works with that pattern. The below demonstrates using the es6 class syntax since its less verbose, but you absolutely aren't required to use it.

var DateSchema = yup.date
var invalidDate = new Date(''); // our failed to coerce value

class MomentDateSchemaType extends DateSchema {
  constructor() {
    super();
    this._validFormats = [];

    this.withMutation(() => {
      this.transform(function (value, originalvalue) {
        if (this.isType(value)) // we have a valid value
          return value
        return Moment(originalValue, this._validFormats, true)
      })
    })
  }

  _typeCheck(value) {
    return super._typeCheck(value)
        || (moment.isMoment(value) && value.isValid())
  }

  format(formats) {
    if (!formats)
      throw new Error('must enter a valid format')
    let next = this.clone()
    next._validFormats = {}.concat(formats);
  }
}

var schema = new MomentDateSchemaType()

schema
  .format('YYYY-MM-DD')
  .cast('It is 2012-05-25') // Fri May 25 2012 00:00:00 GMT-0400 (Eastern Daylight Time)