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Runtime Type Checking


While mobx-keystone was built with first-class TypeScript support in mind, it is also possible to enforce runtime type checking. This feature is, however, completely optional. This is, if you are happy with the type safety that TypeScript offers at compilation time you are free to stick to it exclusively.

Type definitions

Type definitions are like the schemas for your data. They are usually associated with models like this:

class Todo extends Model({
text: tProp(types.string),
done: tProp(types.boolean, false),
}) {
// ...

// ModelData<Todo> = {
// text: string,
// done: boolean
// }

In this case whenever the model is created / changed it will be automatically type-checked in development mode and will throw an exception if the change results in a model that does not pass the checking.

If you want to enforce checks no matter if process.env.NODE_ENV is set to "production" or not you can do so like this:

modelAutoTypeChecking: ModelAutoTypeCheckingMode.AlwaysOn,

The possible values are:

  • ModelAutoTypeCheckingMode.DevModeOnly - Auto type-check models only in dev mode
  • ModelAutoTypeCheckingMode.AlwaysOn - Auto type-check models no matter the current environment
  • ModelAutoTypeCheckingMode.AlwaysOff - Do not auto type-check models no matter the current environment

It is also possible to trigger type checking manually:

const myTodo = new Todo({ text: "hi" })
const checkError = myTodo.typeCheck()
// or
const todoType = types.model(Todo)
const checkError = typeCheck(todoType, myTodo)

// also possible with non-models
const numberArrayType = types.array(types.number)
const checkError = typeCheck(numberArrayType, [1, 2, 3])

In all cases the returned value will be null if there are no errors or an instance of TypeCheckError, which will include:

  • path: Path - Sub-path where the type-check failed, or an empty array if the actual object/value failed the type-check.
  • expectedTypeName: string - String representation of the expected type.
  • actualValue: any - The actual value/sub-value that failed the type-check
  • throw(typeCheckedValue: any) - Throws the error as an exception.

While models are usually automatically type-checked, it is worth noting that other values (primitives, plain objects, arrays) are not until they become attached to some model. If you need to type-check those before they become attached to a model it is always possible to use typeCheck(type, value) as shown previously to trigger a manual validation.


These are the possible types:


A type that represents a certain value of a primitive (for example an exact number or string).

const hiType = types.literal("hi") // the string with value "hi"
const number5Type = types.literal(5) // the number with value 5

types.undefined / undefined

A type that represents the value undefined.

types.null / null

A type that represents the value null.

types.boolean / Boolean

A type that represents any boolean value.

types.number / Number

A type that represents any number value.


A type that represents any integer number value.

types.string / String

A type that represents any string value.


A type that represents any string value other than "".


An enum type, based on a TypeScript alike enum object.

enum Color {
Red = "red",
Green = "green",

const colorType = types.enum(Color)

types.or(...types) (AKA union)

A type that represents the union of several other types (a | b | c | ...).

const booleanOrNumberType = types.or(types.boolean, types.number)


A type that represents either a type or undefined.

const numberOrUndefinedType = types.maybe(types.number)


A type that represents either a type or null.

const numberOrNullType = types.maybeNull(types.number)

types.object(() => ({ ... })

A type that represents a plain object. Note that the parameter must be a function that returns an object. This is done so objects can support self / cross types.

// notice the `({ ... })`, not just `{ ... }`
const pointType = types.object(() => ({
x: types.number,
y: types.number,


A type that represents an array of values of a given type.

const numberArrayType = types.array(types.number)


A type that represents a tuple of values of a given type.

const stringNumberTupleType = types.tuple(types.string, types.number)


A type that represents an object-like map, an object with string keys and values all of a same given type.

// `{ [k: string]: number }`
const numberMapType = types.record(types.number)


A type that represents a model. The type referenced in the model decorator will be used for type checking. If you use recursive / cross referencing models and get TypeScript errors then consider using the lambda parameter instead.

const someModelType = types.model(SomeModel)
// or for recursive models
const someModelType = types.model<SomeModel>(() => SomeModel)

Note that most times just passing the model as type works (for example, tProp(SomeModel) and tProp(types.model(SomeModel)) are equivalent).


A type that represents the data of a data model. The type referenced in the model decorator will be used for type checking. If you use recursive / cross-referencing models and get TypeScript errors then consider using the lambda parameter instead.

const someModelDataType = types.dataModelData(SomeModel)
// or for recursive models
const someModelDataType = types.dataModelData<SomeModel>(() => SomeModel)


A type that represents a given value that won't be type-checked. This is basically a way to bail out of the runtime type checking system.

const uncheckedSomeModel = types.unchecked<SomeModel>()
const anyType = types.unchecked<any>()
const customUncheckedType = types.unchecked<(A & B) | C>()


A type that represents a reference to an object or model.

const refToSomeObject = types.ref(SomeObject)


A type that represents frozen data.

const frozenNumberType = types.frozen(types.number)
const frozenAnyType = types.frozen(types.unchecked<any>())
const frozenNumberArrayType = types.frozen(types.array(types.number))
const frozenUncheckedNumberArrayType = types.frozen(types.unchecked<number[]>())


A type that represents an object-like map ObjectMap.

// `ObjectMap<number>`
const numberMapType = types.objectMap(types.number)


A type that represents an array-backed set ArraySet.

// `ArraySet<number>`
const numberSetType = types.arraySet(types.number)

types.refinement(baseType, checkFn: (data) => boolean | TypeCheckError | null)

A refinement over a given type. This allows you to do extra checks over models, ensure numbers are integers, etc.

const integerType = types.refinement(
(n) => {
return Number.isInteger(n)

const sumModelType = types.refinement(types.model(Sum), (sum) => {
// imagine that for some reason `sum` includes a number `a`, a number `b`
// and the result `result`

const rightResult = sum.a + sum.b === sum.result

// simple mode that will just return that the whole model is incorrect
return rightResult

// this will return that the result field is wrong
return rightResult ? null : new TypeCheckError(["result"], "a+b", sum.result)

types.tag<T>(baseType, tag: T, typeName?: string)

Wraps a given type with tag information. This allows you to associate arbitrary metadata with the type of a prop that you can then use at runtime against instances.

const widthType = types.tag(
{ displayName: "Width in inches", required: true },

const heightType = types.tag(
{ displayName: "Height in inches", required: true },

class MyModel extends Model({
width: tProp(widthType, 10),
height: tProp(heightType, 10),
}) {}

const m = new MyModel({})
const type = types.model<typeof Model>(m.constructor)
const modelTypeInfo = getTypeInfo(type) as ModelTypeInfo
const propTypeInfo = modelTypeInfo.props.width.typeInfo as TagTypeInfo<{
displayName: string
const displayName = propTypeInfo.tag.displayName

Syntactic sugar for optional primitives with a default value

You can also do tProp(defaultValue: string | number | boolean), which is equivalent to tProp(types.string|number|boolean, defaultValue). In other words, if you use tProp(42), then the property will be a number and take the default value 42 when the value on the snapshot / model creation data is undefined.


It is also possible to get the type represented by a type via TypeToData:

const t = types.object(() => {
x: types.number,
y: types.number

// TypeToData<typeof t> =
// {
// x: number,
// y: number
// }

Reflection of runtime type info

Thanks to getTypeInfo(type: AnyType): TypeInfo it is possible to get the runtime info of a type. TypeInfo is the base class for the following classes:

  • LiteralTypeInfo (types.literal, types.undefined, types.null)
  • BooleanTypeInfo (types.boolean)
  • NumberTypeInfo (types.number)
  • StringTypeInfo (types.string)
  • OrTypeInfo (types.or, types.maybe, types.maybeNull, types.enum)
  • ArrayTypeInfo (types.array)
  • ModelTypeInfo (types.model)
  • ObjectTypeInfo (types.object)
  • RefinementTypeInfo (types.refinement, types.integer, types.nonEmptyString)
  • ObjectMapTypeInfo (types.objectMap)
  • ArraySetTypeInfo (types.arraySet)
  • RecordTypeInfo (types.record)
  • UncheckedTypeInfo (types.unchecked)
  • RefTypeInfo (types.ref)
  • FrozenTypeInfo (types.frozen)

Notes for mobx-state-tree users

  • Type checking in mobx-keystone is performed over instances once they have been created, not over snapshots, so the type definitions should be based on that fact.
  • There is no types.optional since setting default values is already covered by the tProp default values.
  • While models will automatically type-check themselves upon changes, other types will be only type-checked when they get attached to nodes. If for some reason you need to type-check them before then manually use the typeCheck method.