Assertions and Validations for TypeScript

The assertic library provides a set of utilities to build type-safe and compile-time verified assertions in TypeScript.

The library has no external dependencies and is designed to be a lightweight drop-in for any existing project.

Terminology

The library provides checks, assertions, and assertion factories.

Checks

A check is a function that accepts some unknown value and returns true or false as the result of the check: <T>(value: unknown) => value is T;

Examples of check functions: isString, isEmail, isUuid.

Assertions

An assertion checks that its parameter satisfies some rules and throws an error if the check fails. Examples of assertions: assertTruthy, assertString, assertEmail.

Assertion Factories

An assertion factory is used to build a new custom assertion function.

Examples of assertion factories: objectAssertion, arrayAssertion, stringAssertion.

Usage:

import {assertTruthy} from 'assertic';

// Asserts that *value* is truthy.
const value: string | undefined = getStringOrUndefined();
assertTruthy(value, 'Expected value is not set!');
// Now *value* is 'string'. The assertion removed the 'undefined' type.

import {truthy} from 'assertic';

// Asserts that *value* is truthy.
const value: string = truthy(getStringOrUndefined());
// Now *value* is 'string'. The assertion removed the 'undefined' type.

Asserting (validating) unknown JSON:

import {assertNumber, assertObject, assertString} from 'assertic';

interface ExpectedType {
    name: string;
    count: number;
}

const someObject: unknown = {}; // Some data with unknown type. For example, passed as JSON input to an HTTP endpoint.
assertObject<ExpectedType>(someObject, {name: assertString, count: assertNumber});
// *someObject* has ExpectedType here.
const {name, count} = someObject;

And for arrays too:

import {assertArray, assertNumber} from 'assertic';

type ExpectedElementType = number;
const someArray: unknown = []; // Some data with unknown type. For example, passed as JSON input to an HTTP endpoint.

assertArray<ExpectedElementType>(someObject, assertNumber, {minLength: 1});
// *someArray* has Array<ExpectedElementType> here.
const value: number = someArray[0];

Check unit tests for more examples.