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.
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.
npm install ts-runtime-typecheck- 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 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.
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 numberType 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.
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 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.
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} = ${value * value}`);
}
else {
// inside this block `value` is `unknown`
console.log('Invalid input', value);
}
}In addition all relevant Type Checks have an alternate variant that take the form isOpt{TYPE}. These variants return true if the value meets the given type or Nullish.
import { isOptNumber } from 'ts-runtime-typecheck';
export function printSq (input: unknown) {
if (isOptNumber(input)) {
// inside this block `input` is `number | undefined | null`
const value = input ?? 1; // use nullish coalescing operator to ensure value is number
console.log(`${value} * ${value} = ${value * value}`);
}
else {
// inside this block `input` is `unknown`
console.log('Invalid input', value);
}
}Type 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.
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 NumberType 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.
import { assertDefined } from 'ts-runtime-typecheck';
function main (meaningOfLife: Optional<number>) {
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 definedTypeAsserts 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.
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!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.
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.
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.
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);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.
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)
}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<string>:
import { isString, asArrayOf } from 'ts-runtime-typecheck';
function main (obj: unknown) {
const asStringArray = asArrayOf(isString);
const arr: string[] = asArrayOfString(obj);
}Or Array<Dictionary<number>>:
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 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.
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.
import { asStruct, isStruct, isString, isOptString, isNumber } from 'ts-runtime-typecheck';
import type { Optional } from 'ts-runtime-typecheck';
interface Declaration {
item: Item;
description: Optional<string>
}
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}`);
}When a value can be 2 or more types it is relatively easy to do Type Check.
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.
import {
isString,
isArray,
isUnion,
asArrayOf
} from 'ts-runtime-typecheck';
const check = asArrayOf(isUnion(isString, isArray));
const b = check(['hello', [0, 1, 2], 'world']);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.
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.
TypeScript supports value types for Primitives. For example
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.
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
}
}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.
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
);-
Cast
unknowntostring. -
Cast
unknowntonumber. -
Cast
unknowntoIndex. -
Cast
unknowntoPrimitive. -
Cast
unknowntoIndexable. -
Cast
unknowntoboolean. -
Cast
unknowntoArray<unknown>. -
Cast
unknowntoDictionary<unknown>. -
Cast
unknowntoUnknownFunction. -
Cast
Type | NullishtoType, whereTypeis a generic parameter. -
Cast
unknowntoJSONValue. This function recursively validates the value, and hence will fail if given a cyclic value. -
Cast
JSONValuetoJSONObject. UnlikeasJSONValuethis does not perform recursive validation, hence it only accepts aJSONValueso that the sub-elements are of a known type. -
Cast
JSONValuetoJSONArray. UnlikeasJSONValuethis does not perform recursive validation, hence it only accepts aJSONValueso that the sub-elements are of a known type. -
Takes a Type Cast function for
Typeand returns a new Type Cast function forArray<Type>where type is a generic parameter. Refer to Array/Object of Type Casts for examples. -
Takes a Type Cast function for
Typeand returns a new Type Cast function forDictionary<Type>where type is a generic parameter. Refer to Array/Object of Type Casts for examples. -
Takes an
InterfacePatternwhich is equivalent toTypeand returns a new Type Cast function forType, whereTypeis an interface defined by theTypeChecksspecified in the pattern. Refer to Validating interfaces for examples. -
Takes a class (not a instance of a class) and returns a new Type Cast for an instance of that class.
-
Takes a Primitive value and returns a new Type Cast for that specific value.
-
Takes a unknown value and a series of
TypeChecksand 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 firstTypeCheckis for the return value, and subsequent variadicTypeChecksare used for validating arguments.
-
Cast
unknownvalue tostring | undefined. If value isNullishthen returnundefined. -
Cast
unknownvalue tonumber | undefined. If value isNullishthen returnundefined. -
Cast
unknownvalue toIndex | undefined. If value isNullishthen returnundefined. -
Cast
unknownvalue toPrimitive | undefined. If value isNullishthen returnundefined. -
Cast
unknownvalue toIndexable | undefined. If value isNullishthen returnundefined. -
Cast
unknownvalue toboolean | undefined. If value isNullishthen returnundefined. -
Cast
unknownvalue toArray<unknown> | undefined. If value isNullishthen returnundefined. -
Cast
unknownvalue toDictionary<unknown> | undefined. If value isNullishthen returnundefined. -
Cast
unknownvalue toUnknownFunction | undefined. If value isNullishthen returnundefined. -
Takes a variable number of type checks as parameters
<A>(...checks: TypeCheck<A>[])and returns a new type cast that composes them into type cast for the unionA. This allows creating a cast for a type union by composing any existing type checks. -
Identical to
asUnionbut it the resulting cast returnsA | null | undefined. -
Cast
unknownvalue toJSONValue | undefined. If value isNullishthen returnundefined. -
Cast
JSONValue | undefinedvalue toJSONObject | undefined. If value isNullishthen returnundefined. -
Cast
JSONValue | undefinedvalue toJSONArray | undefined. If value isNullishthen returnundefined. -
Takes a Type Cast function for
Typeand returns a new Type Cast function forArray<Type> | undefinedwhere type is a generic parameter. Refer to Array/Object of Type Casts for examples. -
Takes a Type Cast function for
Typeand returns a new Type Cast function forDictionary<Type> | undefinedwhere type is a generic parameter. Refer to Array/Object of Type Casts for examples. -
Takes an
InterfacePatternwhich is equivalent toTypeand returns a new Type Cast function forType | undefined, whereTypeis an interface defined by theTypeAssertsspecified in the pattern. Refer to Validating interfaces for examples. -
Takes a class (not a instance of a class) and returns a new Type Cast for a Optional instance of that class.
-
Takes a Primitive value and returns a new optional Type Cast for that specific value.
-
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeDictionary<unknown>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeUnknownFunction. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeboolean. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typestring. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typenumber. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeIndex. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typePrimitive. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeIndexable. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeArray<unknown>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeundefined. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeNullish. -
Takes an
unknownvalue and returns a boolean indicating if the value is not of the typeNullish. -
Takes a variable number of type checks as parameters
<A>(...checks: TypeCheck<A>[])and returns a new type check that composes them into union type checkTypeCheck<A>. 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 exampleif ( 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. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeJSONValue. -
Takes an
JSONValuevalue and returns a boolean indicating if the value is of the typeJSONArray. -
Takes an
JSONValuevalue and returns a boolean indicating if the value is of the typeJSONObject. -
Takes a Type Check function for
Typeand returns a new Type Check function forArray<Type>where Type is a generic parameter. -
Takes a Type Check function for
Typeand returns a new Type Check function forDictionary<Type>where Type is a generic parameter. -
Takes an
InterfacePatternwhich is equivalent toTypeand returns a newTypeAssertfunction forType, whereTypeis an interface defined by theTypeAssertsspecified in the pattern. Refer to Validating interfaces for examples. -
Takes a class (not a instance of a class) and returns a new Type Check for an instance of that class.
-
Takes a Primitive value and returns a new Type Check for that specific value.
-
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<Dictionary<unknown>>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<UnknownFunction>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<boolean>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<string>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<number>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<Primitive>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<Index>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<Indexable>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<Array<unknown>>. -
Identical to
isUnionbut it the resulting typecheck isTypeCheck<A | null | undefined>. -
Takes an
unknownvalue and returns a boolean indicating if the value is of the typeOptional<JSONValue>. -
Takes an
Optional<JSONValue>value and returns a boolean indicating if the value is of the typeOptional<JSONArray>. -
Takes an
Optional<JSONValue>value and returns a boolean indicating if the value is of the typeOptional<JSONObject>. -
Takes an
InterfacePatternwhich is equivalent toTypeand returns a newTypeAssertfunction forOptional<Type>, whereTypeis an interface defined by theTypeAssertsspecified in the pattern. Refer to Validating interfaces for examples. -
Takes a Type Check function for
Typeand returns a new Type Check function forOptional<Array<Type>>where Type is a generic parameter. -
Takes a Type Check function for
Typeand returns a new Type Check function forOptional<Dictionary<Type>>where Type is a generic parameter. -
Takes a class (not a instance of a class) and returns a new Type Check for a Optional instance of that class.
-
Takes a Primitive value and returns a new optional Type Check for that specific value.
-
Takes an
unknownvalue and converts it to it's textual representation. A value that cannot be cleanly converted will trigger an error. -
Takes an
unknownvalue and converts it to it's numerical representation. A value that cannot be cleanly converted will trigger an error. -
Takes an
unknownvalue and converts it to it's boolean representation. A value that cannot be cleanly converted will trigger an error. -
Takes a value of the generic type
Tand returns a copy of the object excluding any members that wereNullish. The returned object meets the typeStrictPartial<T>.
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.
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.
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.
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.
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.
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.
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[].
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.
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.
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.
Assert value of type unknown is Optional<number>. Accepts an optional name for the value that is included in the error if the value is not a Optional<number>.
Assert value of type unknown is Optional<string>. Accepts an optional name for the value that is included in the error if the value is not a Optional<string>.
Assert value of type unknown is Optional<boolean>. Accepts an optional name for the value that is included in the error if the value is not a Optional<boolean>.
Assert value of type unknown is Optional<Index>. Accepts an optional name for the value that is included in the error if the value is not a Optional<Index>.
Assert value of type unknown is Optional<Primitive>. Accepts an optional name for the value that is included in the error if the value is not a Optional<Primitive>.
Assert value of type unknown is Optional<unknown[]>. Accepts an optional name for the value that is included in the error if the value is not a Optional<unknown[]>.
Assert value of type unknown is Optional<Dictionary>. Accepts an optional name for the value that is included in the error if the value is not a Optional<Dictionary>.
Assert value of type unknown is Optional<Indexable>. Accepts an optional name for the value that is included in the error if the value is not a Optional<Indexable>.
Assert value of type unknown is Optional<UnknownFunction>. Accepts an optional name for the value that is included in the error if the value is not a Optional<UnknownFunction>.
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.
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.
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.
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.
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.
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.
-
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
TypeAssertionis thrown with the given message. If the condition is the result of aTypeCheckthen the type predicate is enforced by the invariant.import { invariant, isString } from 'ts-runtime-typecheck'; function main (username: unknown) { invariant(isString(username), 'Expected username to be a string'); username // string }
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.
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-
A union of all the JSON compatible types:
JSONArray,JSONObject,number,string,boolean,null. -
An alias to
Dictionary<JSONValue>which can represent any JSONObjectvalue. -
An alias to
Array<JSONValue>which can represent any JSONArrayvalue. -
An alias to
Record<string, Type>whereTypeis a generic parameter that default tounknown. This type can be used to represent a typical key-value map constructed from a JSObject. Where possible useMapinstead, as it is specifically designed for this purpose and has better protection against null errors in TS. -
A union of the
numberandstringtypes that represent a value that could be used to index an element within a JSObject. -
A union of the
number,stringandbooleantypes that are the key primitive values in JS. -
An alias to
Record<Index, Type>whereTypeis a generic parameter that default tounknown. This type can be used to represent an unknown key-value object that can be indexed using anumberorstring. It is intended to be used to ease the transition of JS project to TS. Where possible useDictionaryor preferablyMapinstead, as it is specifically designed for this purpose and has better protection against null errors in TS. -
A union of
undefinedandnull. Generally preferable to eithernullorundefinedon non-validated input. However, be aware of varying behavior between these 2 types in JS around optional members, default parameters and equality. -
A union of
TypeandNullishwhereTypeis a generic parameter. -
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. -
Identical to
UnknownFunctionin all ways but 1, it returnsPromise<unknown>instead. -
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. -
An alias for a
DictionaryofTypeAssertfunctions. When used in conjunction withisStructorasStructthey can validate anobjectagainst the equivalent interface to the pattern. -
A variant of the inbuilt
Required<T>, which is the opposite ofOptional<T>in that it subtracts the typeundefinedfrom each member of the typeT. StrictRequired varies in that it also subtracts the typenullfrom each member. Ensuring that all members meet the constraintNonNullable. -
A variant of the
Partial<T>inbuilt, and closely related toFuzzyPartial.Partialmakes no guarantees about the members of the typeT, as such they can be unions ofnull. This can introduce inconsistency for users of the type; expecting that members can be specified using eithernullorundefined, where only some can also usenull.StrictPartialresolves this by specifying that no members of the typeTcan benull, ensuring a consistent interface. -
A variant of the
Partial<T>inbuilt, and closely related toStrictPartial.Partialmakes no guarantees about the members of the typeT, as such they can be unions ofnull. This can introduce inconsistency for users of the type; expecting that members can be specified using eithernullorundefined, where only some can also usenull.FuzzyPartialresolves this by specifying that all members of the typeTcan ALSO benull, ensuring a consistent interface. -
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.
- Initial release
- Documentation update.
- Fix: asDefined was returning
unknown. - Breaking change: rename ObjectDict to Dictionary.
- Add: Nullish type (
null | undefined). - Change: Dictionary no longer contains
T | undefinedunion. - Change: Optional type now also includes
nullin the type union.
- Change: return type of
asOpt{TYPE}is nowTYPE | undefinedinstead ofOptional<TYPE>( removes null from union ) - Documentation corrections.
- Add: Introduce
isStructandasStructthat 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:
asDefinedcan longer acceptnullas a fallback parameter. - Change:
asIndexablenow accepts arrays. - Add:
isIndexabletype check. - Change: Expose the
TypeAsserttype publicly. - Add:
InterfacePatterntype. - Change: modify the type names in errors to be closer to the TypeScript names.
- Add:
isUnionandisOptUnionto allow checking if a value matches any type of a type union. - Add:
asUnionandasOptUnionto allow casting a value to a type union. - Add:
isArrayOfandisOptArrayOfto allow checking if a value is an array of a given type. - Add:
isDictionaryOfandisOptDictionaryOfto 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)toasArraryOf(isOptString)or (similar) the output array may containnullas the elements are no longer transformed by an inner cast ( optional cast methods normalize output toundefined). - Add:
isInstanceandisOptInstanceto allow checking if a value is an instance of a given class. - Add:
asInstanceandasOptInstanceto allow casting a value to an instance of a given class. - Breaking Change:
asRecord,asOptRecord,asRecordRecursiveandasOptRecordRecursivehave been renamed toasDictionary,asOptDictionary,asDictionaryOfandasOptDictionaryOfrespectively. - Breaking Change:
asArrayRecursiveandasOptArrayRecursivehave been renamed toasArrayOfandasOptArrayOfrespectively. - Breaking Change: rename
TypeAsserttoTypeCheck.
- Add:
makeStrictPartialfor convertingPartial<T>toStrictPartial. - Add: types
StrictPartialandFuzzyPartial, variants of the inbuiltPartialtype. - Add: type
StrictRequired, variant ofRequired.
- Fix: incorrect constraint on
makeStrictPartialprevented passing in non-indexable instances.
- Add:
assertDefinedthrows if the passed value isNullish. - Add:
TypeAssertionerror class thrown by TypeAsserts.
- Fix: update sub-dependency to resolve npm advisory 1654
- Fix:
asInstance,asOptInstance,isInstanceandisOptInstancewere not exported from the package.
- Change: build target to ES2018 instead of ES3.
- Add:
invariantfunction 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/asStructexamples
- Add:
inspectTypefunction to describe the type of a value - Fix:
makeNumberno longer returns a number strings prefixed with a number - Change: Type error messages now use the more descriptive
inspectTypeinstead oftypeoffor erroneous values
- Add:
asGuardedFunctionto 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