ts-runtime-typecheck

!100% coverage
!0 dependencies
!npm

Simple functions for validating complex data.

JavaScript is a very flexible language. Meaning it's easy to get tripped up by a value with an unexpected type. Even in TypeScript you occasionally have to deal with values which cannot be safely typed at compile time. This library provides a comprehensive selection of functions to simplify type checking code with clear and concise function calls. Ensuring strict type safety throughout your program, no matter the input.

Installation

Releases are available on the npm repository and our GitHub releases page. ESM and CJS formats are both included, as well as TypeScript type definition files. Both formats work without TypeScript if you prefer plain JS.

``bash
npm install ts-runtime-typecheck
`

---

Contents

- Installation
- Type Casts
- Type Checks
- Type Coerce
- Type Asserts
- JSON Types
- Ensuring an optional value is defined
- Array/Object of Type Casts
- Validating interfaces
- Union types
- Class instances
- Guarded functions
- Reference
- Changelog

Type Casts

Type Casts take an unknown value as an argument, and return a typed value as the result. These functions take the form as{TYPE}, for example asNumber. If the input value does not match the required type the function will throw. This does not perform any coercion on the value, passing a string of a number to asNumber will cause it to throw.

`typescript
import { asNumber } from 'ts-runtime-typecheck';

function square (input: unknown): number {
const value: number = asNumber(input);
return value * value;
}

square(10)
// 100
square()
// Error: Unable to cast undefined to number
square('10')
// Error: Unable to cast string to number
`

Type Casts are meant to primarily validate questionable values that are expected to be in a well defined structure. Such as network responses, interfacing with untyped JavaScript or reading data back from a file. If you are looking to validate a type, without throwing an error then take a look at Type Checks.

$3

In the situation you want to check a value meets an Optional type there exists an alternate function for each type cast. These take the form asOpt{TYPE}. Unlike the standard functions when a Nullish value is passed in they will emit undefined instead of throwing. If the input is not Nullish, then it behaves the same as the standard type casts. If the type condition is met then it emits the value, otherwise it will throw.

---

Type Checks

Type Checks take an unknown value as an argument, and return a boolean indicating if the given value matches the required type. These functions take the form is{TYPE}. In the correct situation TypeScript is capable of refining the type of a value through the use of these functions and flow analysis, like the below example.

`typescript
import { isNumber } from 'ts-runtime-typecheck';

export function printSq (value: unknown) {
if (isNumber(value)) {
// inside this block value is a number
console.log(${value} ${value} = ${value value});
}
else {
// inside this block value is unknown
console.log('Invalid input', value);
}
}
`

In addition all relevant Type Checks have an alternate variant that take the form isOpt{TYPE}. These variants return true if the value meets the given type or Nullish.

`typescript
import { isOptNumber } from 'ts-runtime-typecheck';

export function printSq (input: unknown) {
if (isOptNumber(input)) {
// inside this block input is number | undefined | null
const value = input ?? 1; // use nullish coalescing operator to ensure value is number
console.log(${value} ${value} = ${value value});
}
else {
// inside this block input is unknown
console.log('Invalid input', value);
}
}
`

---

Type Coerce

Type Coercion functions take an unknown value as an argument, and convert it into a specific type. These functions take the format make{TYPE}. Unlike the other functions this only works for small subset of types: number, string and boolean. They make a best effort to convert the type, but if the input is not suitable then they will throw an error. For instance passing a non-numeric string to makeNumber will cause it to throw, as will passing a string that is not "true" | "false" to makeBoolean. While these functions will take any input value, this is to allow the input of values that have not been validated. The only valid input types for all 3 functions are number | string | boolean. The intention here is to allow useful conversion, but prevent accidentally passing complex types.

There is an argument that makeString could support using the toString method of an object, but the default toString method returns the useless [object Object] string. It is possible to detect if an object has implemented it's own toString method, but is it correct to use it in this situation? That depends on the intention of the programmer. In the absence of a clear answer the line has been drawn at only accepting primitives.

`typescript
import { makeNumber } from 'ts-runtime-typecheck';

makeNumber('80') // 80
makeNumber(80) // 80
makeNumber(true) // 1
makeNumber(false) // 0
makeNumber('hello') // Error: Unable to cast string to Number
makeNumber({
toString () { return 'hello' }
}) // Error: Unable to cast object to Number

`

---

Type Asserts

Type Assert functions accept an unknown value and throw if the value does not meet the type requirement, they do not return a value. While this may seem very similar to Type Casts they are capable of providing a hint to the TypeScript compiler without needing to reassign the value. As such they are very helpful for validating function arguments before using them.

Each type assert takes an optional second argument that is a label for the passed value, this will be included in the thrown TypeAssertion error if the value does not meet the type requirement, making it easier to isolate the type violation.

`typescript
import { assertDefined } from 'ts-runtime-typecheck';

function main (meaningOfLife: Optional) {
meaningOfLife // number | null | undefined
assertDefined(meaningOfLife, 'Meaning of Life');
meaningOfLife // number
return 'but what is the question?';
}

main(42); // 'but what is the question?'
main(); // TypeAssertion: Meaning of Life is not defined
`

TypeAsserts cannot be based on generic types, due to limits in the TypeScript type system. Hence there is no analogous function to isStruct and similar TypeCheck. However, there is an alternative. It's possible to utilise an invariant ( or assert) function with a TypeCheck to get the same effect, and an implementation of invariant is provided for this purpose.

`typescript
import { invariant, isLiteral } from 'ts-runtime-typecheck';

function main (meaningOfLife: unknown) {
meaningOfLife // unknown
invariant(isLiteral(42)(meaningOfLife), "Universe is broken, meaning of life isn't 42!");
meaningOfLife // 42
return 'but what is the question?';
}

main(42); // 'but what is the question?'
main(); // TypeAssertion: Universe is broken, meaning of life isn't 42!
`

---

JSON Types

Dealing with validating JSON values can often be frustrating, so to make it a little easier JSON specific types and checks are provided. Using the JSONValue type in your code will ensure that TS statically analyses any literal values as serializable to JSON.

`typescript
import type { JSONArray, JSONObject, JSONValue } from 'ts-runtime-typecheck';

// JSONArray is an Array of JSONValues
const a: JSONArray = [12, 'hello'];
// JSONObject is a Dictionary of JSONValues
const b: JSONObject = {
num: 12,
str: 'hello'
};
// JSONValue can be any of the following: JSONObject, JSONArray, string, number, boolean or null
const c: JSONValue = 12;

const d: JSONValue = new Error('hi'); // Type 'Error' is not assignable to type 'JSONValue'
`

For dynamic data isJSONValue and asJSONValue provide recursive type validation on a value.

Type Check and Type Casts are provided for JSONArrays and JSONObjects, with the caveat that they only accept JSONValues. This is to avoid needing to recursively validate values which have already been validated.

`typescript
import { asJSONValue, isJSONObject, isJSONArray } from 'ts-runtime-typecheck';
import type { JSONValue } from 'ts-runtime-typecheck';

function main (a: unknown) {
const obj: JSONValue = asJSONValue(a);
// obj: JSONValue
if (isJSONArray(obj)) {
// obj: JSONArray
}
else if (isJSONObject(obj)) {
// obj: JSONObject
}
else {
// obj: number | string | boolean | null
}
}
`

One other caveat of JSONValue is that it does not guarantee that the value is not cyclic. It is not possible to serialize cyclic object with JSON.stringify, but they are otherwise valid. Using isJSONValue or asJSONValue on a cyclic object will fail.

`typescript
import { asJSONValue } from 'ts-runtime-typecheck';
import type { Dictionary } from 'ts-runtime-typecheck';

const almost_right: Dictionary = {};
almost_right.self = almost_right;

// BANG! this will fail, it recurses endlessly
const obj = asJSONValue(almost_right);
`

---

Ensuring an optional value is defined

A common situation is that you have an Optional value, with a well defined type. At a specific time it should be defined, but the type system is not aware of this. TypeScript will allow you to cast the value to a non-optional type using !, but this is often discouraged in style guides. A safer alternative is to use asDefined and friends, which can be used to subtract the optionality from the type.

`typescript
import { asDefined, assertDefined, isDefined } from 'ts-runtime-typecheck';

function doThing (value: number) {
// [...]
}

function branching (value?: number) {
// used for conditions, allows for an alternative to throwing or custom error behavior
if (isDefined(value)) {
doThing(value)
} else {
console.log('bad things')
}
}

function inline (value?: number) {
// used inline, throws if the value is Nullish otherwise returns the value
doThing(asDefined(value))
}

function assertion (value?: number) {
// used to ensure execution doesn't progress if the value isn't defined, throws if the value is Nullish
assertDefined(value)
doThing(value)
}
`

Array/Object of Type Casts

Validating that a value is an array or dictionary is easy enough, but how about the type of the contents? asArrayOf and asDictionaryOf allow you to cast the elements of a collection using a user defined Type Check. For example, to cast to Array:

`typescript
import { isString, asArrayOf } from 'ts-runtime-typecheck';

function main (obj: unknown) {
const asStringArray = asArrayOf(isString);

const arr: string[] = asArrayOfString(obj);
}
`

Or Array>:

`typescript
import { isNumber, isDictionaryOf, asArrayOf } from 'ts-runtime-typecheck';

function main () {
const isDictionaryOfNumber = isDictionaryOf(isNumber);
const asArrayOfDictionaryOfNumber = asArrayOf(isDictionaryOfNumber);

const arr = asArrayOfDictionaryOfNumber([
{
a: 12,
b: 42
},
{
n: 90
}
]);
}

`

Validating interfaces

Validating the shape of an object using a combination of asDictionary and other Type Casts specific to property types can be a bit verbose. To simplify this scenario you can use asStruct. This function takes an InterfacePattern that defines a specific structure and returns a new function that will cast an unknown value to that structure. An InterfacePattern is a fancy name for a Dictionary of Type Checks.

`typescript
import { asStruct, isString, isNumber } from 'ts-runtime-typecheck';

interface Item {
name: string;
value: number;
}

const asItem = asStruct({ name: isString, value: isNumber })

function main (obj: unknown) {
const item: Item = asItem(obj);

console.log(${item.name} = ${item.value});
}
`

There is also a Type Check variant of the this function called isStruct which works in a very similar way. As an InterfacePattern is composed of Type Check functions it's possible to compose nested interface Type Checks.

`typescript
import { asStruct, isStruct, isString, isOptString, isNumber } from 'ts-runtime-typecheck';
import type { Optional } from 'ts-runtime-typecheck';

interface Declaration {
item: Item;
description: Optional
}

interface Item {
name: string;
value: number;
}

const isItem = isStruct({ name: isString, value: isNumber });
const asDeclaration = asStruct({ item: isItem, description: isOptString });

function main (obj: unknown) {
const { item, description } = asDeclaration(obj);

const comment: string = description ? // ${description} : '';
console.log(${item.name} = ${item.value} ${comment});
}
`

---

Union types

When a value can be 2 or more types it is relatively easy to do Type Check.

`typescript
import { isString, isArray } from 'ts-runtime-typecheck';

if (isString(a) || isArray(a)) {
// a: string | unknown[]
}
`

But you can't cast to that type, or pass it into a function like asArrayOf or isStruct which require a Type Check for their input. To do this you can use isUnion or asUnion. These functions take a variable number of Type Checks and produce a union of them.

`typescript
import {
isString,
isArray,
isUnion,
asArrayOf
} from 'ts-runtime-typecheck';

const check = asArrayOf(isUnion(isString, isArray));
const b = check(['hello', [0, 1, 2], 'world']);
`

---

Class instances

Under most scenarios you will know if a value is an instance of a given class. However, there are scenarios where this is not the case. For these situations you can use isInstance or asInstance to ensure you have the correct type.

`typescript
import { isInstance } from 'ts-runtime-typecheck';

function print_error (err) {
if (isInstance(Error)(err)) {
print_string(err.message);
} else {
print_unknown(err)
}
}
`

When validating a value matches an interface it may be desirable to instead use isInstance instead of isStruct. While it doesn't provide the same guarantees it will often be significantly faster, as it does not perform a Type Check on each member to see that they exist and contain the right type of value.

---

Literal types

TypeScript supports value types for Primitives. For example

`typescript
let a: 'hello world' = 'hello world';

a = 'goodbye world'; // Type '"goodbye world"' is not assignable to type '"hello world"'
`

You can use asLiteral and isLiteral to construct TypeCasts and TypeChecks respectively for these value types. These checks can be particularly useful for discriminated unions.

`typescript
interface A {
type: 'a';
value: number;
}

interface B {
type: 'b';
value: number;
}

const isA = isStruct({
type: isLiteral('a'),
value: isNumber,
});

function main (n: A | B) {
n // A | B
if (isA(n)) {
n // A
} else {
n // B
}
}
`

---

Guarded Functions

TypeScript provides excellent compile time protections against misusing functions. The number of parameters, the types of parameters and the return type of the function are all statically type checked. But it provides no runtime protections. In a pure TS project this isn't normally required, but sometimes you need to pass around abstract functions and have them behave reliably. Alternatively you could be authoring a library and want to ensure that developers using your library without TS use your functions correctly. In both these scenarios you can wrap the function using asGuardedFunction, specifying the return and parameter types. This will return a new function that when called will validate the incoming arguments, and the outgoing return value at runtime. If the types don't match up it will throw an error. The resulting function will have accurate parameter and return types, irrespective of the original function.

`typescript

import { asGuardedFunction } from 'ts-runtime-typecheck';

export const publicFunction = asGuardedFunction(
(a: number, b: number) => ${(a / b) * 100}%, // function to wrap
isString, // return value
isNumber, isNumber // parameters
);
`

---

Reference

$3

- ### asString

Cast unknown to string.

- ### asNumber

Cast unknown to number.

- ### asIndex

Cast unknown to Index.

- ### asPrimitive

Cast unknown to Primitive.

- ### asIndexable

Cast unknown to Indexable.

- ### asBoolean

Cast unknown to boolean.

- ### asArray

Cast unknown to Array.

- ### asDictionary

Cast unknown to Dictionary.

- ### asFunction

Cast unknown to UnknownFunction.

- ### asDefined

Cast Type | Nullish to Type, where Type is a generic parameter.

- ### asJSONValue

Cast unknown to JSONValue. This function recursively validates the value, and hence will fail if given a cyclic value.

- ### asJSONObject

Cast JSONValue to JSONObject. Unlike asJSONValue this does not perform recursive validation, hence it only accepts a JSONValue so that the sub-elements are of a known type.

- ### asJSONArray

Cast JSONValue to JSONArray. Unlike asJSONValue this does not perform recursive validation, hence it only accepts a JSONValue so that the sub-elements are of a known type.

- ### asArrayOf

Takes a Type Cast function for Type and returns a new Type Cast function for Array where type is a generic parameter. Refer to Array/Object of Type Casts for examples.

- ### asDictionaryOf

Takes a Type Cast function for Type and returns a new Type Cast function for Dictionary where type is a generic parameter. Refer to Array/Object of Type Casts for examples.

- ### asStruct

Takes an InterfacePattern which is equivalent to Type and returns a new Type Cast function for Type, where Type is an interface defined by the TypeChecks specified in the pattern. Refer to Validating interfaces for examples.

- ### asInstance

Takes a class (not a instance of a class) and returns a new Type Cast for an instance of that class.

- ### asLiteral

Takes a Primitive value and returns a new Type Cast for that specific value.

- ### asGuardedFunction

Takes a unknown value and a series of TypeChecks and returns a strongly typed function. It will be a new function that wraps the original with runtime type validation, ensuring that the arguments and return value are the expected types. The first TypeCheck is for the return value, and subsequent variadic TypeChecks are used for validating arguments.

$3

- ### asOptString

Cast unknown value to string | undefined. If value is Nullish then return undefined.

- ### asOptNumber

Cast unknown value to number | undefined. If value is Nullish then return undefined.

- ### asOptIndex

Cast unknown value to Index | undefined. If value is Nullish then return undefined.

- ### asOptPrimitive

Cast unknown value to Primitive | undefined. If value is Nullish then return undefined.

- ### asOptIndexable

Cast unknown value to Indexable | undefined. If value is Nullish then return undefined.

- ### asOptBoolean

Cast unknown value to boolean | undefined. If value is Nullish then return undefined.

- ### asOptArray

Cast unknown value to Array | undefined. If value is Nullish then return undefined.

- ### asOptDictionary

Cast unknown value to Dictionary | undefined. If value is Nullish then return undefined.

- ### asOptFunction

Cast unknown value to UnknownFunction | undefined. If value is Nullish then return undefined.

- ### asUnion

Takes a variable number of type checks as parameters (...checks: TypeCheck[]) and returns a new type cast that composes them into type cast for the union A. This allows creating a cast for a type union by composing any existing type checks.

- ### asOptUnion

Identical to asUnion but it the resulting cast returns A | null | undefined.

- ### asOptJSONValue

Cast unknown value to JSONValue | undefined. If value is Nullish then return undefined.

- ### asOptJSONObject

Cast JSONValue | undefined value to JSONObject | undefined. If value is Nullish then return undefined.

- ### asOptJSONArray

Cast JSONValue | undefined value to JSONArray | undefined. If value is Nullish then return undefined.

- ### asOptArrayOf

Takes a Type Cast function for Type and returns a new Type Cast function for Array | undefined where type is a generic parameter. Refer to Array/Object of Type Casts for examples.

- ### asOptDictionaryOf

Takes a Type Cast function for Type and returns a new Type Cast function for Dictionary | undefined where type is a generic parameter. Refer to Array/Object of Type Casts for examples.

- ### asOptStruct

Takes an InterfacePattern which is equivalent to Type and returns a new Type Cast function for Type | undefined, where Type is an interface defined by the TypeAsserts specified in the pattern. Refer to Validating interfaces for examples.

- ### asOptInstance

Takes a class (not a instance of a class) and returns a new Type Cast for a Optional instance of that class.

- ### asOptLiteral

Takes a Primitive value and returns a new optional Type Cast for that specific value.

$3

- ### isDictionary

Takes an unknown value and returns a boolean indicating if the value is of the type Dictionary.

- ### isFunction

Takes an unknown value and returns a boolean indicating if the value is of the type UnknownFunction.

- ### isBoolean

Takes an unknown value and returns a boolean indicating if the value is of the type boolean.

- ### isString

Takes an unknown value and returns a boolean indicating if the value is of the type string.

- ### isNumber

Takes an unknown value and returns a boolean indicating if the value is of the type number.

- ### isIndex

Takes an unknown value and returns a boolean indicating if the value is of the type Index.

- ### isPrimitive

Takes an unknown value and returns a boolean indicating if the value is of the type Primitive.

- ### isIndexable

Takes an unknown value and returns a boolean indicating if the value is of the type Indexable.

- ### isArray

Takes an unknown value and returns a boolean indicating if the value is of the type Array.

- ### isUndefined

Takes an unknown value and returns a boolean indicating if the value is of the type undefined.

- ### isNullish

Takes an unknown value and returns a boolean indicating if the value is of the type Nullish.

- ### isDefined

Takes an unknown value and returns a boolean indicating if the value is not of the type Nullish.

- ### isUnion

Takes a variable number of type checks as parameters (...checks: TypeCheck[]) and returns a new type check that composes them into union type check TypeCheck. This allows creating a test for a type union by composing any existing type checks. For inline code it will generally make sense to use logical OR operators instead of this, for example if ( isNumber(n) || isArray(n) ) {}. This particular function is intended for when you wish to compose a type check or cast that contains a union, or create a library type check for a common union type.

- ### isJSONValue

Takes an unknown value and returns a boolean indicating if the value is of the type JSONValue.

- ### isJSONArray

Takes an JSONValue value and returns a boolean indicating if the value is of the type JSONArray.

- ### isJSONObject

Takes an JSONValue value and returns a boolean indicating if the value is of the type JSONObject.

- ### isArrayOf

Takes a Type Check function for Type and returns a new Type Check function for Array where Type is a generic parameter.

- ### isDictionaryOf

Takes a Type Check function for Type and returns a new Type Check function for Dictionary where Type is a generic parameter.

- ### isStruct

Takes an InterfacePattern which is equivalent to Type and returns a new TypeAssert function for Type, where Type is an interface defined by the TypeAsserts specified in the pattern. Refer to Validating interfaces for examples.

- ### isInstance

Takes a class (not a instance of a class) and returns a new Type Check for an instance of that class.

- ### isLiteral

Takes a Primitive value and returns a new Type Check for that specific value.

$3

- ### isOptDictionary

Takes an unknown value and returns a boolean indicating if the value is of the type Optional>.

- ### isOptFunction

Takes an unknown value and returns a boolean indicating if the value is of the type Optional.

- ### isOptBoolean

Takes an unknown value and returns a boolean indicating if the value is of the type Optional.

- ### isOptString

Takes an unknown value and returns a boolean indicating if the value is of the type Optional.

- ### isOptNumber

Takes an unknown value and returns a boolean indicating if the value is of the type Optional.

- ### isOptPrimitive

Takes an unknown value and returns a boolean indicating if the value is of the type Optional.

- ### isOptIndex

Takes an unknown value and returns a boolean indicating if the value is of the type Optional.

- ### isOptIndexable

Takes an unknown value and returns a boolean indicating if the value is of the type Optional.

- ### isOptArray

Takes an unknown value and returns a boolean indicating if the value is of the type Optional>.

- ### isOptUnion

Identical to isUnion but it the resulting typecheck is TypeCheck.

- ### isOptJSONValue

Takes an unknown value and returns a boolean indicating if the value is of the type Optional.

- ### isOptJSONArray

Takes an Optional value and returns a boolean indicating if the value is of the type Optional.

- ### isOptJSONObject

Takes an Optional value and returns a boolean indicating if the value is of the type Optional.

- ### isOptStruct

Takes an InterfacePattern which is equivalent to Type and returns a new TypeAssert function for Optional, where Type is an interface defined by the TypeAsserts specified in the pattern. Refer to Validating interfaces for examples.

- ### isOptArrayOf

Takes a Type Check function for Type and returns a new Type Check function for Optional> where Type is a generic parameter.

- ### isOptDictionaryOf

Takes a Type Check function for Type and returns a new Type Check function for Optional> where Type is a generic parameter.

- ### isOptInstance

Takes a class (not a instance of a class) and returns a new Type Check for a Optional instance of that class.

- ### isOptLiteral

Takes a Primitive value and returns a new optional Type Check for that specific value.

$3

- ### makeString

Takes an unknown value and converts it to it's textual representation. A value that cannot be cleanly converted will trigger an error.

- ### makeNumber

Takes an unknown value and converts it to it's numerical representation. A value that cannot be cleanly converted will trigger an error.

- ### makeBoolean

Takes an unknown value and converts it to it's boolean representation. A value that cannot be cleanly converted will trigger an error.

- ### makeStrictPartial

Takes a value of the generic type T and returns a copy of the object excluding any members that were Nullish. The returned object meets the type StrictPartial.

$3

- ### assertDefined

Assert value of type Type | Nullish is Type, where Type is a generic parameter. Accepts an optional name for the value that is included in the error if the value is nullish.

- ### assertNumber

Assert value of type unknown is number. Accepts an optional name for the value that is included in the error if the value is not a number.

- ### assertString

Assert value of type unknown is string. Accepts an optional name for the value that is included in the error if the value is not a string.

- ### assertBoolean

Assert value of type unknown is boolean. Accepts an optional name for the value that is included in the error if the value is not a boolean.

- ### assertIndex

Assert value of type unknown is Index. Accepts an optional name for the value that is included in the error if the value is not a Index.

- ### assertPrimitive

Assert value of type unknown is Primitive. Accepts an optional name for the value that is included in the error if the value is not a Primitive.

- ### assertArray

Assert value of type unknown is unknown[]. Accepts an optional name for the value that is included in the error if the value is not a unknown[].

- ### assertDictionary

Assert value of type unknown is Dictionary. Accepts an optional name for the value that is included in the error if the value is not a Dictionary.

- ### assertIndexable

Assert value of type unknown is Indexable. Accepts an optional name for the value that is included in the error if the value is not a Indexable.

- ### assertFunction

Assert value of type unknown is UnknownFunction. Accepts an optional name for the value that is included in the error if the value is not a UnknownFunction.

- ### assertOptNumber

Assert value of type unknown is Optional. Accepts an optional name for the value that is included in the error if the value is not a Optional.

- ### assertOptString

Assert value of type unknown is Optional. Accepts an optional name for the value that is included in the error if the value is not a Optional.

- ### assertOptBoolean

Assert value of type unknown is Optional. Accepts an optional name for the value that is included in the error if the value is not a Optional.

- ### assertOptIndex

Assert value of type unknown is Optional. Accepts an optional name for the value that is included in the error if the value is not a Optional.

- ### assertOptPrimitive

Assert value of type unknown is Optional. Accepts an optional name for the value that is included in the error if the value is not a Optional.

- ### assertOptArray

Assert value of type unknown is Optional. Accepts an optional name for the value that is included in the error if the value is not a Optional.

- ### assertOptDictionary

Assert value of type unknown is Optional. Accepts an optional name for the value that is included in the error if the value is not a Optional.

- ### assertOptIndexable

Assert value of type unknown is Optional. Accepts an optional name for the value that is included in the error if the value is not a Optional.

- ### assertOptFunction

Assert value of type unknown is Optional. Accepts an optional name for the value that is included in the error if the value is not a Optional.

- ### assertJSONValue

Assert value of type unknown is JSONValue. Accepts an optional name for the value that is included in the error if the value is not a JSONValue.

- ### assertJSONArray

Assert value of type JSONValue is JSONArray. Accepts an optional name for the value that is included in the error if the value is not a JSONArray.

- ### assertJSONObject

Assert value of type JSONValue is JSONObject. Accepts an optional name for the value that is included in the error if the value is not a JSONObject.

- ### assertOptJSONValue

Assert value of type unknown is JSONValue | undefined. Accepts an optional name for the value that is included in the error if the value is not a JSONValue | undefined.

- ### assertOptJSONArray

Assert value of type JSONValue | undefined is JSONArray | undefined. Accepts an optional name for the value that is included in the error if the value is not a JSONArray | undefined.

- ### assertOptJSONObject

Assert value of type JSONValue | undefined is JSONObject | undefined. Accepts an optional name for the value that is included in the error if the value is not a JSONObject | undefined.

$3

- ### invariant

An invariant is a logical declaration about a condition which the programmer knows to be true, but the compiler cannot. Many of the patterns in ts-runtime-typecheck are based around this concept, albeit to encourage a stricter and safer environment. This helper accepts a logical condition and a message. If the condition is true nothing happens, if it's false then a TypeAssertion is thrown with the given message. If the condition is the result of a TypeCheck then the type predicate is enforced by the invariant.

`typescript
import { invariant, isString } from 'ts-runtime-typecheck';

function main (username: unknown) {
invariant(isString(username), 'Expected username to be a string');

username // string
}
`

- ### inspectType

Inspects the type of a given value and returns a description of it as a string. Useful for constructing debug messages from unknown values. Instances of classes are described using the name of their class. Abstract objects are traversed recursively so that their elements can be described as well. Recursion uses a depth limit to prevent overlarge descriptions. Once the limit has been reached abstract objects will be described as Dictionary.

An optional second argument can be passed to modify the default behaviour. For example you can change the maxDepth limit from it's default of 3; a value of 0 would prevent recursion whereas Infinity would remove the limit.

Arrays currently do not feature recursive type description, but this might be introduced in future.

Internally this is used to describe values in type error messages.

`typescript
import { inspectType } from 'ts-runtime-typecheck';

class Example {}

inspectType('hello world'); // string
inspectType(null) // null
inspectType([]) // Array
inspectType(new Example) // Example
inspectType({}) // {}
inspectType({
foo: 'hello',
bar: 'world'
}) // { foo: string, bar: string }
inspectType({ foo: 'bar' }, { maxDepth: 0 }); // Dictionary
`

$3

- ### JSONValue

A union of all the JSON compatible types: JSONArray, JSONObject, number, string, boolean, null.

- ### JSONObject

An alias to Dictionary which can represent any JSON Object value.

- ### JSONArray

An alias to Array which can represent any JSON Array value.

- ### Dictionary

An alias to Record where Type is a generic parameter that default to unknown. This type can be used to represent a typical key-value map constructed from a JS Object. Where possible use Map instead, as it is specifically designed for this purpose and has better protection against null errors in TS.

- ### Index

A union of the number and string types that represent a value that could be used to index an element within a JS Object.

- ### Primitive

A union of the number, string and boolean types that are the key primitive values in JS.

- ### Indexable

An alias to Record where Type is a generic parameter that default to unknown. This type can be used to represent an unknown key-value object that can be indexed using a number or string. It is intended to be used to ease the transition of JS project to TS. Where possible use Dictionary or preferably Map instead, as it is specifically designed for this purpose and has better protection against null errors in TS.

- ### Nullish

A union of undefined and null. Generally preferable to either null or undefined on non-validated input. However, be aware of varying behavior between these 2 types in JS around optional members, default parameters and equality.

- ### Optional

A union of Type and Nullish where Type is a generic parameter.

- ### UnknownFunction

A stricter alternative to the type Function. It accepts any number of unknown parameters, and returns an unknown value. Allowing you to reference an untyped function in a slightly safer manner. This does not provide any arity or type checks for the parameters.

- ### UnknownAsyncFunction

Identical to UnknownFunction in all ways but 1, it returns Promise instead.

- ### TypeCheck

An alias for a function that meets the requirements of TypeScript Type Guards. They take the format (value: unknown) => value is TYPE. With the exception of specialist JSON checks all TypeChecks conform to this type.

- ### InterfacePattern

An alias for a Dictionary of TypeAssert functions. When used in conjunction with isStruct or asStruct they can validate an object against the equivalent interface to the pattern.

- ### StrictRequired

A variant of the inbuilt Required, which is the opposite of Optional in that it subtracts the type undefined from each member of the type T. StrictRequired varies in that it also subtracts the type null from each member. Ensuring that all members meet the constraint NonNullable.

- ### StrictPartial

A variant of the Partial inbuilt, and closely related to FuzzyPartial. Partial makes no guarantees about the members of the type T, as such they can be unions of null. This can introduce inconsistency for users of the type; expecting that members can be specified using either null or undefined, where only some can also use null. StrictPartial resolves this by specifying that no members of the type T can be null, ensuring a consistent interface.

- ### FuzzyPartial

A variant of the Partial inbuilt, and closely related to StrictPartial. Partial makes no guarantees about the members of the type T, as such they can be unions of null. This can introduce inconsistency for users of the type; expecting that members can be specified using either null or undefined, where only some can also use null. FuzzyPartial resolves this by specifying that all members of the type T can ALSO be null, ensuring a consistent interface.

- ### TypeAssertion

A custom error with the name TypeAssertion. This type is exported as a value so that Errors of this type can be isolated from other errors using instance checks. It is possible to use the constructor to create and throw your own Errors if you wish, but this may change in future.

Changelog

$3

- Initial release

$3

- Documentation update.
- Fix: asDefined was returning unknown.
- Breaking change: rename ObjectDict to Dictionary.
- Add: Nullish type ( null | undefined ).
- Change: Dictionary no longer contains T | undefined union.
- Change: Optional type now also includes null in the type union.

$3

- Change: return type of asOpt{TYPE} is now TYPE | undefined instead of Optional ( removes null from union )
- Documentation corrections.

$3

- Add: Introduce isStruct and asStruct that allow the inspection of a object to see if it meets a specific interface.
- Add: Optional variants of Type Checks with form isOpt{TYPE}.
- Breaking Change: asDefined can longer accept null as a fallback parameter.
- Change: asIndexable now accepts arrays.
- Add: isIndexable type check.
- Change: Expose the TypeAssert type publicly.
- Add: InterfacePattern type.
- Change: modify the type names in errors to be closer to the TypeScript names.

$3

- Add: isUnion and isOptUnion to allow checking if a value matches any type of a type union.
- Add: asUnion and asOptUnion to allow casting a value to a type union.
- Add: isArrayOf and isOptArrayOf to allow checking if a value is an array of a given type.
- Add: isDictionaryOf and isOptDictionaryOf to allow checking if a value is a Dictionary of a given type.
- Breaking Change: Recursive Type Casts now take a Type Check as an argument instead of a Type Cast, and no longer emit a copy of the input. As a side effect if you are upgrading from asArrayRecursive(asOptString) to asArraryOf(isOptString) or (similar) the output array may contain null as the elements are no longer transformed by an inner cast ( optional cast methods normalize output to undefined ).
- Add: isInstance and isOptInstance to allow checking if a value is an instance of a given class.
- Add: asInstance and asOptInstance to allow casting a value to an instance of a given class.
- Breaking Change: asRecord, asOptRecord, asRecordRecursive and asOptRecordRecursive have been renamed to asDictionary, asOptDictionary, asDictionaryOf and asOptDictionaryOf respectively.
- Breaking Change: asArrayRecursive and asOptArrayRecursive have been renamed to asArrayOf and asOptArrayOf respectively.
- Breaking Change: rename TypeAssert to TypeCheck.

$3

- Add: makeStrictPartial for converting Partial to StrictPartial.
- Add: types StrictPartial and FuzzyPartial, variants of the inbuilt Partial type.
- Add: type StrictRequired, variant of Required.

$3

- Fix: incorrect constraint on makeStrictPartial prevented passing in non-indexable instances.

$3

- Add: assertDefined throws if the passed value is Nullish.
- Add: TypeAssertion error class thrown by TypeAsserts.

$3

- Fix: update sub-dependency to resolve npm advisory 1654

$3

- Fix: asInstance, asOptInstance, isInstance and isOptInstance were not exported from the package.

$3

- Change: build target to ES2018 instead of ES3.

$3

- Add: invariant function to assist type assertion
- Add: JSON assertion functions
- Add: Basic type assertion functions
- Add: Literal type casts and checks
- Add: Primitive type, casts and checks
- Documentation: Correct some typos in the isStruct/asStruct examples

$3

- Add: inspectType function to describe the type of a value
- Fix: makeNumber no longer returns a number strings prefixed with a number
- Change: Type error messages now use the more descriptive inspectType instead of typeof for erroneous values

$3

- Add: asGuardedFunction` to wrap a function with parameter/return value type checks
- Change: Depreciate the fallback parameter for all TypeCasts
- Documentation: Simplify the contents into primary headings only
- Change: Instead of being a custom subclass of Error TypeAssertion is now an alias to TypeError