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2.0.3 • Public • Published


This library is a simple JSON-data typechecker for TypeScript. It will be useful when you have some arbitrary JSON-data (for example, API response) and you need to check them and cast them to the right type.


npm install ts-json-check


yarn add ts-json-check


import {
  // ...
} from "ts-json-check";

// ...

// You can't be sure that the data from the server is correct, so
// apiResponse has an 'any' type.
const apiResponse = getSomeData();

// Describe the desired shape of data using ts-json-check
// guard functions.
const isResponse = isObject({
  id: isAnyOf(isNumber, isString),
  title: isString,
  archived: isBoolean,

// To extract the resulting type from the guard use the GuardedType utility
// type. The APIResponse here is:
// { id: number|string, title: string, archived: boolean }
type APIResponse = GuardedType<typeof isResponse>;

if (isResponse(apiResponse)) {
  // The apiResponse have an APIResponse type here
} else {
  // apiResponse have a wrong type


This library is intended only for JSON processing, so it does not attempt to simulate all the types available in TypeScript.

Primitive guards

The following guards are corresponds to the primitive JSON types:

  • isNull
  • isNumber
  • isString
  • isBoolean

Constant guard

There is one guard that checks that the argument is a constant value (or any of constant values) of primitive JSON type: isConst.

Use it as: isConst(42) or as isConst(42, 43)

It is useful when your data can have different shape depending on value of some field (the discriminant in TS terms).

The multi-argument form of this guard (isConst(42, 43)) is equivalent to the following isAnyOf form: isAnyOf(isConst(42), isConst(43)).

'Any' guard

There is isAny guard that is always returns true and keeps it argument as any. It is useful when you don't know the exact type of your data yet and want to keep some fields untyped.

Composite guards

The composite JSON types are expressed by the following functions:


JSON object: { "foo": 42, "bar": "baz" }

Use it as: isObject({ foo: isNumber, bar: isString}). You can use any guards as the values of the argument object. The input data object can have additional keys, it is not an error.


An array of values of the same type: [1, 2, 3, 4]

Use it as: isArray(isNumber)


An array of values of different types: [42, "baz"]. In TypeScript this corresponds to tuples.

Use it as: isTuple(isNumber, isString)

The length of the input data array must be equal to the count of the isTuple arguments. The argument list may not be empty.

Utility checkers


Checks that value have one of the given types.

Use it as: isAnyOf(isNumber, isString)

The resulting type will be number | string. The isAnyOf accepts two or more checkers as arguments.


Marks object field as optional.

Use it as: isObject({ id: isNumber, title: isOptional(isString) })

The resulting type will be { id: number, title?: string | undefined }.

Custom guards

Although this is not usually necessary, you can create your own guard functions. The type is simple: Guard<T> = (v: any) => v is T. Guard function should check argument and return true if it has the correct type of false otherwise (see the TypeScript docs).

For example let's create guard for positive numbers:

import { isNumber } from "ts-json-check";

function isPositiveNumber(v: any): v is number {
  return isNumber(v) && v > 0;

Note that the guarded type of isPositiveNumber is still a number. TypeScript hasn't a special type for the positive numbers.

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  • davidmz