TypeScript Assertion Library

This library and its API is designed to provide a good developer experience in TypeScript environment, so it is recommended only for TypeScript developers.

Features

• TypeScript-friendly API
• Nicely formatted error messages in a rtext format
• Side-effect free assertions
• Using lodash deep equality and matching functions
• Snapshot testing in a browser

Namespaces for different Types

When expect(obj) function is invoked, it will return different Assertion depending on its type.

In order for compiler to get correct type when expect(obj) is invoked, we are using function overloading.

function expect(obj: null): NullAssertion;
function expect(obj: undefined): UndefinedAssertion;
function expect(obj: void): UndefinedAssertion;
function expect(obj: number): NumberAssertion;
function expect(obj: boolean): BooleanAssertion;
function expect(obj: string): StringAssertion;
function expect(obj: symbol): SymbolAssertion;
function expect(obj: Function): FunctionAssertion;
function expect<T>(obj: Array<T>): ArrayAssertion<T>;
function expect(obj: Date): DateAssertion;
function expect(obj: RegExp): RegExpAssertion;
function expect<K, V>(obj: Map<K, V>): MapAssertion<K, V>;
function expect<K extends object, V>(obj: WeakMap<K, V>): WeakMapAssertion<K, V>;
function expect<V>(obj: Set<V>): SetAssertion<V>;
function expect<V>(obj: WeakSet<V>): WeakSetAssertion<V>;
function expect<E extends Error>(obj: E): ErrorAssertion<E>;
function expect<T extends object>(obj: T): ObjectAssertion<T>;
function expect(obj: any): Assertion<any>;

Adding your own type

import { Assertion, addAssertionType } from "iko";
import { MyType } from "./mylib";

declare module "iko" {
  function expect(obj: MyType): MyTypeAssertion;
}

export class MyTypeAssertion extends ObjectAssertion<MyType> {
  constructor(obj: MyType) {
    super(obj, "MyType");
  }

  // ...
}

addAssertionType((obj: any) => {
  if (typeof obj === "object" && obj instanceof MyType) {
    return new MyTypeAssertion(obj);
  }
  return undefined;
}):

Extending existing namespaces

import { Assertion, errMsg, r } from "iko";
import { richText } from "rtext-writer";

declare module "iko" {
  interface Assertion<T> {
    toBeMyType(): MyTypeAssertion;
  }
}

Assertion.prototype.toBeMyType = function(
  message?: ErrorMessage<string, MyType>,
): MyTypeAssertion {
  const received = this.obj;
  const pass = typeof received === "object" && obj instanceof MyType;
  if (!pass) {
    const message = errMsg()
      .matcherHint("toBeMyType", "received", "")
      .info("Expected Object to have MyType type:\n")
      .info("  ", r(received), "\n");

    throw new AssertionError(message.compose(), received, "MyType", this.toBeMyType);
  }

  return this as any as MyTypeAssertion;
}

API

class Assertion<T> {
  readonly obj: T;

  constructor(obj: T);

  toSnapshot(): string | { lang?: string, code: string };

  assert<E>(
    expr: (() => boolean) | boolean,
    message: (expected: E, actual?: T) => RichText,
    expected: E,
    actual?: T,
  ): this;

  toBeFalsy(): this;
  toBeTruthy(): this;
  toBe(value: T): this;
  toBeNull(): NullAssertion;
  toBeUndefined(): UndefinedAssertion;
  toBeInstanceOf(type: Function): this;

  notToBe(value: T): this;
  notToBeNull(): this;
  notToBeUndefined(): this;
  notToBeInstanceOf(type: Function): this;

  toBeObject<O extends object>(): ObjectAssertion<O>;
  toBeArray<U>(): ArrayAssertion<U>;
  toBeBoolean(): BooleanAssertion;
  toBeNumber(): NumberAssertion;
  toBeString(): StringAssertion;
  toBeFunction(): FunctionAssertion;
  toBeSymbol(): SymbolAssertion;
  toBeDate(): DateAssertion;
  toBeRegExp(): RegExpAssertion;
  toBeError<E extends Error>(): ErrorAssertion<E>;
  toBeMap<K extends object = any, V = any>(): MapAssertion<K, V>;
  toBeWeakMap<K extends object = any, V = any>(): WeakMapAssertion<K, V>;
  toBeSet<V = any>(): SetAssertion<V>;
  toBeWeakSet<V = any>(): WeakSetAssertion<V>;
}

class NullAssertion extends Assertion<null> { }

class UndefinedAssertion extends Assertion<undefined> { }

class BooleanAssertion extends Assertion<boolean> { }

class FunctionAssertion extends Assertion<Function> {
  toHaveArgumentsLength(length: number): this;
  toThrow<E extends Error | ErrorConstructor>(expected?: string | E): this;

  notToHaveArgumentsLength(length: number): this;
  notToThrow<E extends Error | ErrorConstructor>(expected?: string | E): this;
}

class NumberAssertion extends Assertion<number> {
  toBeApproximatelyEqual(number: number, epsilon = Number.EPSILON): this;
  toBeEssentiallyEqual(number: number, epsilon = Number.EPSILON): this;
  toBeDefinetelyGreaterThan(number: number, epsilon = Number.EPSILON): this;
  toBeDefinetelyLessThan(number: number, epsilon = Number.EPSILON): this;
  toBeGreaterThan(number: number): this;
  toBeGreaterThanOrEqual(number: number): this;
  toBeLessThan(number: number): this;
  toBeLessThanOrEqual(number: number): this;
  toBeNaN(): this;

  notToBeApproximatelyEqual(number: number, epsilon = Number.EPSILON): this;
  notToBeEssentiallyEqual(number: number, epsilon = Number.EPSILON): this;
  notToBeDefinetelyGreaterThan(number: number, epsilon = Number.EPSILON): this;
  notToBeDefinetelyLessThan(number: number, epsilon = Number.EPSILON): this;
  notToBeGreaterThan(number: number): this;
  notToBeGreaterThanOrEqual(number: number): this;
  notToBeLessThan(number: number): this;
  notToBeLessThanOrEqual(number: number): this;
  notToBeNaN(): this;
}

class StringAssertion extends Assertion<string> {
  toHaveLength(length: number): this;
  toInclude(text: string): this;
  toMatch(text: string | RegExp): this;

  notToHaveLength(length: number): this;
  notToInclude(text: string): this;
  notToMatch(text: string | RegExp): this;
}

class SymbolAssertion extends Assertion<Symbol> { }

class ObjectAssertion<T extends object> extends Assertion<T> {
  readonly type: string;

  constructor(obj: T, type = "object");

  toBeEqual(expected: T): this;
  toBeEqualWith(expected: T, customizer: isMatchCustomizer): this;
  toMatch(expected: any): this;
  toMatchWith(expected: any, customizer: isMatchCustomizer): this;

  notToBeEqual(expected: T): this;
  notToBeEqualWith(expected: T, customizer: isMatchCustomizer): this;
  notToMatch(expected: any): this;
  notToMatchWith(expected: any, customizer: isMatchCustomizer): this;
}

class ArrayAssertion<T> extends ObjectAssertion<T[]> {
  toHaveLength(length: number): this;
  notToHaveLength(length: number): this;
  toContain(value: T): this;
  notToContain(value: T): this;
}

class DateAssertion extends ObjectAssertion<Date> { }

class ErrorAssertion<E extends Error> extends ObjectAssertion<E> { }

class RegExpAssertion extends ObjectAssertion<RegExp> {
  toTest(text: string): this;

  notToTest(text: string): this;
}

class MapAssertion<K, V> extends ObjectAssertion<Map<K, V>> {
  toHaveSize(size: number): this;
  toHave(key: K): this;

  notToHaveSize(size: number): this;
  notToHave(key: K): this;
}

class SetAssertion<V> extends ObjectAssertion<Set<V>> {
  toHaveSize(size: number): this;
  toHave(value: V): this;

  notToHaveSize(size: number): this;
  notToHave(value: V): this;
}

class WeakMapAssertion<K extends object, V> extends ObjectAssertion<WeakMap<K, V>> {
  toHave(key: K): this;

  notToHave(key: K): this;
}

class WeakSetAssertion<V> extends ObjectAssertion<WeakSet<V>> {
  toHave(value: V): this;

  notToHave(value: V): this;
}

License

MIT