2.2.0

changelog.md

@@ -1,5 +1,36 @@
 # Changelog
 
+## 2025-04-29 - 2.2.0 - feat(generics)
+Improve assertion and matcher type definitions by adding execution mode generics for better async/sync support
+
+- Updated ts/index.ts to import and use TExecutionType in the expect function
+- Modified Assertion class to use a generic execution mode (M) for improved type inference
+- Revised all matcher namespaces (array, boolean, date, function, number, object, string, type) to accept the new generic parameter
+- Enhanced async/sync distinction for assertion methods like resolves and rejects
+
+## 2025-04-29 - 2.1.2 - fix(ts/index.ts)
+Remove deprecated expectAsync function and advise using .resolves/.rejects on expect for async assertions
+
+- Deleted the redundant expectAsync export in ts/index.ts
+- Users should now call expect(...).resolves or expect(...).rejects for asynchronous assertions
+
+## 2025-04-29 - 2.1.1 - fix(Assertion)
+Improve chainability by fixing return types in assertion methods
+
+- Update runCheck method to explicitly return the correct chainable type for both async and sync assertions
+- Ensure customAssertion propagates the chainable Assertion instance
+- Refactor internal promise handling for clarity and consistency
+
+## 2025-04-28 - 2.1.0 - feat(core)
+Add new matchers and improve negation messaging
+
+- Added expect.any() and expect.anything() matchers for enhanced object pattern matching
+- Introduced new number matchers: toBeNaN(), toBeFinite(), and toBeWithinRange()
+- Implemented alias toBeEmpty() for both string and array matchers
+- Enhanced function matchers with toThrowErrorMatching() and toThrowErrorWithMessage()
+- Improved negation messaging to provide clearer failure messages (e.g. 'Expected 5 not to be greater than 3')
+- Enhanced object assertions with a toHaveOwnProperty() shorthand that outputs unified diff-style messages
+
 ## 2025-04-28 - 2.0.1 - fix(assertion-matchers)
 Refactor matcher implementations to consistently use customAssertion for improved consistency and clarity.
 

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@push.rocks/smartexpect",
-  "version": "2.0.1",
+  "version": "2.2.0",
   "private": false,
   "description": "A testing library to manage expectations in code, offering both synchronous and asynchronous assertion methods.",
   "main": "dist_ts/index.js",

readme.plan.md

@@ -21,17 +21,35 @@ This document captures the roadmap for evolving the `expect` / `expectAsync` API
 - [ ] Group matchers under `.string`, `.array`, `.number`, etc. for discoverability.
 
 ## Phase 6: Jest-Style Convenience
-- [ ] Add `.toMatchObject()`, `.toMatchSnapshot()`, `expect.any()`, `expect.anything()`, etc.
+- [x] Add `expect.any()` and `expect.anything()` matchers for use in `.toMatchObject()` patterns
+  (Snapshot matchers still TBD)
 
-## Phase 7: Error Messages & Diffs
-- [ ] Integrate a diffing library for clear failure output with colorized diffs.
+The next items to tackle:
 
-## Phase 8: Nested Access Chaining
-- [ ] Provide `.at(path)` or lens-based API for deep property assertions in one go.
+3. Improve negation (`.not`) messaging
+   - Today `.not` simply flips pass/fail, but the failure message isn’t very descriptive. We should capture positive/negative message templates so e.g.
+     > expect(5).not.toBeGreaterThan(3)
+     emits:
+     "Expected 5 not to be greater than 3"
 
-## Phase 9: Pluggable Reporters
-- [ ] Allow consumers to swap output format: JSON, TAP, HTML, etc.
+4. Richer error output for objects/arrays
+   - Integrate a diff library (or extend `fast-deep-equal`) to show unified diffs between expected and actual values
 
-## Phase 10: API Cleanup
-- [ ] Audit and remove legacy aliases and redundant methods.
-- [ ] Finalize deprecations and bump to a major version.
+5. More built-in matchers
+   - toBeNaN(), toBeFinite()
+   - toBeWithinRange(min, max)
+   - toHaveKeys(...), toHaveOwnKeys(...)
+   - toThrowErrorMatching(/regex/), toThrowErrorWithMessage('…')
+   - string/array: toBeEmpty() alias
+   - object: toHaveOwnProperty() shorthand
+
+6. TypeScript-friendliness
+   - Enhance `.d.ts` so editors autocomplete namespace methods (e.g. `expect(x).string.`)
+   - Statically type matcher arguments to catch wrong types at compile time
+
+7. Async assertions and timeouts improvements
+   - Support `.not.resolves`, `.rejects.toThrow()`
+   - Provide clearer timeout errors (e.g. "Expected promise to resolve within …")
+
+8. Plugin/extension API
+   - Formalize `Assertion.extend()` plugin API for shipping matcher bundles

test/test.diffOutput.ts

@@ -0,0 +1,32 @@
+import { tap, expect as tExpect } from '@push.rocks/tapbundle';
+import * as smartexpect from '../dist_ts/index.js';
+
+tap.test('diff-like output for object.toEqual mismatch', async () => {
+  const a = { x: 1, y: 2 };
+  const b = { x: 1, y: 3 };
+  try {
+    smartexpect.expect(a).object.toEqual(b);
+    throw new Error('Assertion did not throw');
+  } catch (err: any) {
+    const msg: string = err.message;
+    tExpect(msg.includes('Expected objects to be deeply equal')).toBeTrue();
+    tExpect(msg.includes('Received:')).toBeTrue();
+    tExpect(msg.includes('"y": 2')).toBeTrue();
+  }
+});
+
+tap.test('diff-like output for array.toContainEqual mismatch', async () => {
+  const arr = [{ id: 1 }, { id: 2 }];
+  const item = { id: 3 };
+  try {
+    smartexpect.expect(arr).array.toContainEqual(item);
+    throw new Error('Assertion did not throw');
+  } catch (err: any) {
+    const msg: string = err.message;
+    tExpect(msg.includes('Expected array to contain equal to')).toBeTrue();
+    tExpect(msg.includes('Received:')).toBeTrue();
+    tExpect(msg.includes('"id": 1')).toBeTrue();
+  }
+});
+
+export default tap.start();

test/test.expectAny.ts

@@ -0,0 +1,38 @@
+import { tap } from '@push.rocks/tapbundle';
+import * as smartexpect from '../dist_ts/index.js';
+
+tap.test('expect.any and expect.anything basic usage', async () => {
+  const obj = { a: 1, b: 'two', d: new Date() };
+  // Using expect.any to match types
+  smartexpect.expect(obj).object.toMatchObject({
+    a: smartexpect.expect.any(Number),
+    b: smartexpect.expect.any(String),
+    d: smartexpect.expect.any(Date),
+  });
+  // Using expect.anything to match any defined value
+  smartexpect.expect(obj).object.toMatchObject({
+    a: smartexpect.expect.anything(),
+    b: smartexpect.expect.anything(),
+    d: smartexpect.expect.anything(),
+  });
+});
+
+tap.test('expect.any mismatch and anything null/undefined rejection', async () => {
+  const obj = { a: 1, b: null };
+  // Mismatch for expect.any
+  try {
+    smartexpect.expect(obj).object.toMatchObject({ a: smartexpect.expect.any(String) });
+    throw new Error('Expected mismatch for expect.any did not throw');
+  } catch (err) {
+    // success: thrown on mismatch
+  }
+  // anything should reject null or undefined
+  try {
+    smartexpect.expect(obj).object.toMatchObject({ b: smartexpect.expect.anything() });
+    throw new Error('Expected anything() to reject null or undefined');
+  } catch (err) {
+    // success: thrown on null
+  }
+});
+
+export default tap.start();

test/test.negation.ts

@@ -0,0 +1,22 @@
+import { tap, expect as tExpect } from '@push.rocks/tapbundle';
+import * as smartexpect from '../dist_ts/index.js';
+
+tap.test('negation message for numeric matcher', async () => {
+  try {
+    smartexpect.expect(5).not.toBeGreaterThan(3);
+    throw new Error('Assertion did not throw');
+  } catch (err: any) {
+    tExpect(err.message).toEqual('Expected number not to be greater than 3');
+  }
+});
+
+tap.test('negation message for string matcher', async () => {
+  try {
+    smartexpect.expect('hello').not.string.toInclude('he');
+    throw new Error('Assertion did not throw');
+  } catch (err: any) {
+    tExpect(err.message).toEqual('Expected string not to include "he"');
+  }
+});
+
+export default tap.start();

ts/00_commitinfo_data.ts

@@ -3,6 +3,6 @@
  */
 export const commitinfo = {
   name: '@push.rocks/smartexpect',
-  version: '2.0.1',
+  version: '2.2.0',
   description: 'A testing library to manage expectations in code, offering both synchronous and asynchronous assertion methods.'
 }

ts/index.ts

@@ -1,4 +1,5 @@
-import { Assertion } from './smartexpect.classes.assertion.js';
+import { Assertion, AnyMatcher, AnythingMatcher } from './smartexpect.classes.assertion.js';
+import type { TExecutionType } from './types.js';
 // import type { TMatcher } from './smartexpect.classes.assertion.js'; // unused
 
 /**
@@ -12,32 +13,29 @@ import { Assertion } from './smartexpect.classes.assertion.js';
  * Entry point for assertions. 
  * Automatically detects Promises to support async assertions.
  */
-export function expect<T>(value: Promise<T>): Assertion<T>;
-export function expect<T>(value: T): Assertion<T>;
-export function expect<T>(value: any): Assertion<T> {
+export function expect<T>(value: Promise<T>): Assertion<T, 'async'>;
+export function expect<T>(value: T): Assertion<T, 'sync'>;
+export function expect<T>(value: any): Assertion<T, TExecutionType> {
   const isThenable = value != null && typeof (value as any).then === 'function';
   const mode: 'sync' | 'async' = isThenable ? 'async' : 'sync';
-  return new Assertion<T>(value, mode);
+  return new Assertion<T, TExecutionType>(value, mode);
 }
 /**
  * Register custom matchers.
  */
 export namespace expect {
   export const extend = Assertion.extend;
+  /**
+   * Matcher for a specific constructor. Passes if value is instance of given constructor.
+   */
+  export function any(constructor: any) {
+    return new AnyMatcher(constructor);
+  }
+  /**
+   * Matcher for any defined value (not null or undefined).
+   */
+  export function anything() {
+    return new AnythingMatcher();
+  }
 }
 
-/**
- * @deprecated Use `expect(...)` with `.resolves` or `.rejects` instead.
- */
-/**
- * @deprecated Use `expect(...)` with `.resolves` or `.rejects` instead.
- */
-/**
- * @deprecated Use `expect(...)` with `.resolves` or `.rejects` instead.
- */
-export const expectAsync = (baseArg: any) => {
-  // eslint-disable-next-line no-console
-  console.warn('[DEPRECATED] expectAsync() is deprecated. Use expect(...).resolves / .rejects');
-  return new Assertion<any>(baseArg, 'async');
-};
-

ts/namespaces/array.ts

@@ -1,11 +1,12 @@
 import { Assertion } from '../smartexpect.classes.assertion.js';
 import * as plugins from '../plugins.js';
+import type { TExecutionType } from '../types.js';
 
 /**
  * Namespace for array-specific matchers
  */
-export class ArrayMatchers<T> {
-  constructor(private assertion: Assertion<T[]>) {}
+export class ArrayMatchers<T, M extends TExecutionType> {
+  constructor(private assertion: Assertion<T[], M>) {}
 
   toBeArray() {
     return this.assertion.customAssertion(
@@ -31,7 +32,9 @@ export class ArrayMatchers<T> {
   toContainEqual(item: T) {
     return this.assertion.customAssertion(
       (value) => (value as T[]).some((e) => plugins.fastDeepEqual(e, item)),
-      `Expected array to contain equal to ${JSON.stringify(item)}`
+      (value) =>
+        `Expected array to contain equal to ${JSON.stringify(item)}` +
+        `\nReceived: ${JSON.stringify(value, null, 2)}`
     );
   }
 
@@ -55,6 +58,12 @@ export class ArrayMatchers<T> {
       `Expected array to be empty`
     );
   }
+  /**
+   * Alias for empty array check
+   */
+  toBeEmpty() {
+    return this.toBeEmptyArray();
+  }
 
   toHaveLengthGreaterThan(length: number) {
     return this.assertion.customAssertion(

ts/namespaces/boolean.ts

@@ -1,10 +1,11 @@
 import { Assertion } from '../smartexpect.classes.assertion.js';
+import type { TExecutionType } from '../types.js';
 
 /**
  * Namespace for boolean-specific matchers
  */
-export class BooleanMatchers {
-  constructor(private assertion: Assertion<boolean>) {}
+export class BooleanMatchers<M extends TExecutionType> {
+  constructor(private assertion: Assertion<boolean, M>) {}
 
   toBeTrue() {
     return this.assertion.customAssertion(

ts/namespaces/date.ts

@@ -1,10 +1,11 @@
 import { Assertion } from '../smartexpect.classes.assertion.js';
+import type { TExecutionType } from '../types.js';
 
 /**
  * Namespace for date-specific matchers
  */
-export class DateMatchers {
-  constructor(private assertion: Assertion<Date>) {}
+export class DateMatchers<M extends TExecutionType> {
+  constructor(private assertion: Assertion<Date, M>) {}
 
   toBeDate() {
     return this.assertion.customAssertion(

ts/namespaces/function.ts

@@ -1,10 +1,11 @@
 import { Assertion } from '../smartexpect.classes.assertion.js';
+import type { TExecutionType } from '../types.js';
 
 /**
  * Namespace for function-specific matchers
  */
-export class FunctionMatchers {
-  constructor(private assertion: Assertion<Function>) {}
+export class FunctionMatchers<M extends TExecutionType> {
+  constructor(private assertion: Assertion<Function, M>) {}
 
   toThrow(expectedError?: any) {
     return this.assertion.customAssertion(
@@ -26,4 +27,36 @@ export class FunctionMatchers {
       `Expected function to throw${expectedError ? ` ${expectedError}` : ''}`
     );
   }
+  /**
+   * Assert thrown error message matches the given regex
+   */
+  toThrowErrorMatching(regex: RegExp) {
+    return this.assertion.customAssertion(
+      (value) => {
+        try {
+          (value as Function)();
+        } catch (e: any) {
+          return regex.test(e && e.message);
+        }
+        return false;
+      },
+      `Expected function to throw error matching ${regex}`
+    );
+  }
+  /**
+   * Assert thrown error message equals the given string
+   */
+  toThrowErrorWithMessage(expectedMessage: string) {
+    return this.assertion.customAssertion(
+      (value) => {
+        try {
+          (value as Function)();
+        } catch (e: any) {
+          return e && e.message === expectedMessage;
+        }
+        return false;
+      },
+      `Expected function to throw error with message "${expectedMessage}"`
+    );
+  }
 }

ts/namespaces/number.ts

@@ -1,10 +1,11 @@
 import { Assertion } from '../smartexpect.classes.assertion.js';
+import type { TExecutionType } from '../types.js';
 
 /**
  * Namespace for number-specific matchers
  */
-export class NumberMatchers {
-  constructor(private assertion: Assertion<number>) {}
+export class NumberMatchers<M extends TExecutionType> {
+  constructor(private assertion: Assertion<number, M>) {}
 
   toBeGreaterThan(value: number) {
     return this.assertion.customAssertion(
@@ -53,4 +54,31 @@ export class NumberMatchers {
       `Expected number to equal ${value}`
     );
   }
+  /**
+   * Checks for NaN
+   */
+  toBeNaN() {
+    return this.assertion.customAssertion(
+      (v) => Number.isNaN(v as number),
+      `Expected number to be NaN`
+    );
+  }
+  /**
+   * Checks for finite number
+   */
+  toBeFinite() {
+    return this.assertion.customAssertion(
+      (v) => Number.isFinite(v as number),
+      `Expected number to be finite`
+    );
+  }
+  /**
+   * Checks if number is within inclusive range
+   */
+  toBeWithinRange(min: number, max: number) {
+    return this.assertion.customAssertion(
+      (v) => (v as number) >= min && (v as number) <= max,
+      `Expected number to be within range ${min} - ${max}`
+    );
+  }
 }

ts/namespaces/object.ts

@@ -1,30 +1,53 @@
-import { Assertion } from '../smartexpect.classes.assertion.js';
+import { Assertion, AnyMatcher, AnythingMatcher } from '../smartexpect.classes.assertion.js';
+import type { TExecutionType } from '../types.js';
 import * as plugins from '../plugins.js';
 
 /**
  * Namespace for object-specific matchers
  */
-export class ObjectMatchers<T extends object> {
-  constructor(private assertion: Assertion<T>) {}
+export class ObjectMatchers<T extends object, M extends TExecutionType> {
+  constructor(private assertion: Assertion<T, M>) {}
 
   toEqual(expected: any) {
     return this.assertion.customAssertion(
       (v) => plugins.fastDeepEqual(v, expected),
-      `Expected objects to be deeply equal to ${JSON.stringify(expected)}`
+      (v) =>
+        `Expected objects to be deeply equal to ${JSON.stringify(expected, null, 2)}` +
+        `\nReceived: ${JSON.stringify(v, null, 2)}`
     );
   }
 
   toMatchObject(expected: object) {
     return this.assertion.customAssertion(
       (v) => {
+        const obj = v as any;
         for (const key of Object.keys(expected)) {
-          if (!plugins.fastDeepEqual((v as any)[key], (expected as any)[key])) {
+          const expectedVal = (expected as any)[key];
+          const actualVal = obj[key];
+          if (expectedVal instanceof AnyMatcher) {
+            const ctor = expectedVal.expectedConstructor;
+            if (ctor === Number) {
+              if (typeof actualVal !== 'number') return false;
+            } else if (ctor === String) {
+              if (typeof actualVal !== 'string') return false;
+            } else if (ctor === Boolean) {
+              if (typeof actualVal !== 'boolean') return false;
+            } else {
+              if (!(actualVal instanceof ctor)) return false;
+            }
+          } else if (expectedVal instanceof AnythingMatcher) {
+            if (actualVal === null || actualVal === undefined) {
+              return false;
+            }
+          } else if (!plugins.fastDeepEqual(actualVal, expectedVal)) {
             return false;
           }
         }
         return true;
       },
-      `Expected object to match properties ${JSON.stringify(expected)}`
+      (v) =>
+        `Expected object to match properties ${JSON.stringify(expected, null, 2)}` +
+        `\nReceived: ${JSON.stringify(v, null, 2)}`
     );
   }
 
@@ -86,4 +109,25 @@ export class ObjectMatchers<T extends object> {
       `Expected value to be null or undefined`
     );
   }
+  /**
+   * Checks own property only (not inherited)
+   */
+  toHaveOwnProperty(property: string, value?: any) {
+    return this.assertion.customAssertion(
+      (v) => {
+        const obj = v as any;
+        if (!Object.prototype.hasOwnProperty.call(obj, property)) {
+          return false;
+        }
+        if (arguments.length === 2) {
+          return plugins.fastDeepEqual(obj[property], value);
+        }
+        return true;
+      },
+      (v) =>
+        `Expected object to have own property ${property}` +
+        (value !== undefined ? ` with value ${JSON.stringify(value)}` : ``) +
+        `\nReceived: ${JSON.stringify(v, null, 2)}`
+    );
+  }
 }

ts/namespaces/string.ts

@@ -1,10 +1,11 @@
 import { Assertion } from '../smartexpect.classes.assertion.js';
+import type { TExecutionType } from '../types.js';
 
 /**
  * Namespace for string-specific matchers
  */
-export class StringMatchers {
-  constructor(private assertion: Assertion<string>) {}
+export class StringMatchers<M extends TExecutionType> {
+  constructor(private assertion: Assertion<string, M>) {}
 
   toStartWith(prefix: string) {
     return this.assertion.customAssertion(
@@ -47,4 +48,13 @@ export class StringMatchers {
       `Expected string to have length ${length}`
     );
   }
+  /**
+   * Alias for empty string check
+   */
+  toBeEmpty() {
+    return this.assertion.customAssertion(
+      (value) => (value as string).length === 0,
+      `Expected string to be empty`
+    );
+  }
 }

ts/namespaces/type.ts

@@ -1,10 +1,11 @@
 import { Assertion } from '../smartexpect.classes.assertion.js';
+import type { TExecutionType } from '../types.js';
 
 /**
  * Namespace for type-based matchers
  */
-export class TypeMatchers {
-  constructor(private assertion: Assertion<any>) {}
+export class TypeMatchers<M extends TExecutionType> {
+  constructor(private assertion: Assertion<any, M>) {}
 
   toBeTypeofString() {
     return this.assertion.customAssertion(

ts/smartexpect.classes.assertion.ts

@@ -18,8 +18,16 @@ import type { TMatcher, TExecutionType } from './types.js';
 /**
  * Core assertion class. Generic over the current value type T.
  */
-export class Assertion<T = unknown> {
-  executionMode: TExecutionType;
+/**
+ * Internal matcher classes for expect.any and expect.anything
+ */
+export class AnyMatcher {
+  constructor(public expectedConstructor: any) {}
+}
+export class AnythingMatcher {}
+
+export class Assertion<T = unknown, M extends TExecutionType = 'sync'> {
+  executionMode: M;
   baseReference: any;
   propertyDrillDown: Array<string | number> = [];
 
@@ -33,8 +41,10 @@ export class Assertion<T = unknown> {
   private isResolves = false;
   private failMessage: string;
   private successMessage: string;
+  /** Computed negation failure message for the current assertion */
+  private negativeMessage: string;
 
-  constructor(baseReferenceArg: any, executionModeArg: TExecutionType) {
+  constructor(baseReferenceArg: any, executionModeArg: M) {
     this.baseReference = baseReferenceArg;
     this.executionMode = executionModeArg;
   }
@@ -131,6 +141,16 @@ export class Assertion<T = unknown> {
       .replace('{value}', formattedValue)
       .replace('{path}', drillDown || '');
   }
+  /**
+   * Compute a negated failure message by inserting 'not' into the positive message.
+   */
+  private computeNegationMessage(message: string): string {
+    const idx = message.indexOf(' to ');
+    if (idx !== -1) {
+      return message.slice(0, idx) + ' not' + message.slice(idx);
+    }
+    return 'Negated: ' + message;
+  }
 
   public get not() {
     this.notSetting = true;
@@ -139,20 +159,23 @@ export class Assertion<T = unknown> {
   /**
    * Assert that a Promise resolves.
    */
-  public get resolves(): this {
-    this.isResolves = true;
-    this.isRejects = false;
-    this.executionMode = 'async';
-    return this;
+  /**
+   * Switch to async (resolve) mode. Subsequent matchers return Promises.
+   */
+  public get resolves(): Assertion<T, 'async'> {
+    return new Assertion<T, 'async'>(this.baseReference, 'async');
   }
   /**
    * Assert that a Promise rejects.
    */
-  public get rejects(): this {
-    this.isRejects = true;
-    this.isResolves = false;
-    this.executionMode = 'async';
-    return this;
+  /**
+   * Switch to async (reject) mode. Subsequent matchers return Promises.
+   */
+  public get rejects(): Assertion<T, 'async'> {
+    const a = new Assertion<T, 'async'>(this.baseReference, 'async');
+    // mark to expect rejection
+    (a as any).isRejects = true;
+    return a;
   }
 
   /**
@@ -183,25 +206,29 @@ export class Assertion<T = unknown> {
     return this;
   }
 
-  private runCheck(checkFunction: () => any) {
+  // Internal check runner: returns Promise in async mode, else sync Assertion
+  // Internal check runner; returns Promise or this at runtime, but typed via customAssertion
+  private runCheck(checkFunction: () => any): any {
     const runDirectOrNegated = (checkFunction: () => any) => {
       if (!this.notSetting) {
         return checkFunction();
       } else {
         let isOk = false;
         try {
-          runDirectOrNegated(checkFunction());
+        // attempt positive assertion and expect it to throw
+        checkFunction();
         } catch (e) {
           isOk = true;
         }
         if (!isOk) {
-          throw new Error(this.failMessage || 'Negated assertion failed');
+          const msg = this.failMessage || this.negativeMessage || 'Negated assertion failed';
+          throw new Error(msg);
         }
       }
     };
 
     if (this.executionMode === 'async') {
-      const done = plugins.smartpromise.defer();
+      const done = plugins.smartpromise.defer<Assertion<T, M>>();
       const isThenable = this.baseReference && typeof (this.baseReference as any).then === 'function';
       if (!isThenable) {
         done.reject(new Error(`Expected a Promise but received: ${this.formatValue(this.baseReference)}`));
@@ -219,47 +246,59 @@ export class Assertion<T = unknown> {
           (res: any) => {
             done.reject(new Error(`Expected Promise to reject but it resolved with ${this.formatValue(res)}`));
           },
-          (err: any) => {
-            this.baseReference = err;
-            try {
-              const ret = runDirectOrNegated(checkFunction);
-              done.resolve(ret);
-            } catch (e: any) {
-              done.reject(e);
+            (err: any) => {
+              this.baseReference = err;
+              try {
+                runDirectOrNegated(checkFunction);
+                done.resolve(this);
+              } catch (e: any) {
+                done.reject(e);
+              }
             }
-          }
         );
       } else {
         (this.baseReference as Promise<any>).then(
-          (res: any) => {
-            this.baseReference = res;
-            try {
-              const ret = runDirectOrNegated(checkFunction);
-              done.resolve(ret);
-            } catch (e: any) {
-              done.reject(e);
-            }
-          },
+            (res: any) => {
+              this.baseReference = res;
+              try {
+                runDirectOrNegated(checkFunction);
+                done.resolve(this);
+              } catch (e: any) {
+                done.reject(e);
+              }
+            },
           (err: any) => {
             done.reject(err);
           }
         );
       }
-      return done.promise;
+      // return a promise resolving to this for chaining
+      return done.promise.then(() => this) as any;
     }
-    return runDirectOrNegated(checkFunction);
+    // sync: run and return this for chaining
+    runDirectOrNegated(checkFunction);
+    return this as any;
   }
 
+  /**
+   * Execute a custom assertion. Returns a Promise in async mode, else returns this.
+   */
   public customAssertion(
     assertionFunction: (value: any) => boolean,
-    errorMessage: string
-  ) {
+    errorMessage: string | ((value: any) => string)
+  ): M extends 'async' ? Promise<Assertion<T, M>> : Assertion<T, M> {
+    // Prepare negation message based on the positive error template, if static
+    if (typeof errorMessage === 'string') {
+      this.negativeMessage = this.computeNegationMessage(errorMessage);
+    }
     return this.runCheck(() => {
       const value = this.getObjectToTestReference();
       if (!assertionFunction(value)) {
-        throw new Error(this.failMessage || errorMessage);
+        const msg = this.failMessage
+          || (typeof errorMessage === 'function' ? errorMessage(value) : errorMessage);
+        throw new Error(msg);
       }
-    });
+    }) as any;
   }
 
   /**
@@ -267,9 +306,9 @@ export class Assertion<T = unknown> {
    * @param propertyName Name of the property to navigate into.
    * @returns Assertion of the property type.
    */
-  public property<K extends keyof NonNullable<T>>(propertyName: K): Assertion<NonNullable<T>[K]> {
+  public property<K extends keyof NonNullable<T>>(propertyName: K): Assertion<NonNullable<T>[K], M> {
     this.propertyDrillDown.push(propertyName as string);
-    return this as unknown as Assertion<NonNullable<T>[K]>;
+    return this as unknown as Assertion<NonNullable<T>[K], M>;
   }
 
   /**
@@ -277,9 +316,9 @@ export class Assertion<T = unknown> {
    * @param index Index of the array item.
    * @returns Assertion of the element type.
    */
-  public arrayItem(index: number): Assertion<T extends Array<infer U> ? U : unknown> {
+  public arrayItem(index: number): Assertion<T extends Array<infer U> ? U : unknown, M> {
     this.propertyDrillDown.push(index);
-    return this as unknown as Assertion<T extends Array<infer U> ? U : unknown>;
+    return this as unknown as Assertion<T extends Array<infer U> ? U : unknown, M>;
   }
 
   public log() {
@@ -331,37 +370,38 @@ export class Assertion<T = unknown> {
   public toBeTypeofBoolean() { return this.type.toBeTypeofBoolean(); }
   public toBeTypeOf(typeName: string) { return this.type.toBeTypeOf(typeName); }
   public toBeDefined() { return this.type.toBeDefined(); }
+  
   // Namespaced matcher accessors
   /** String-specific matchers */
-  public get string() {
-    return new StringMatchers(this as Assertion<string>);
+  public get string(): StringMatchers<M> {
+    return new StringMatchers<M>(this as Assertion<string, M>);
   }
   /** Array-specific matchers */
-  public get array() {
-    return new ArrayMatchers<any>(this as Assertion<any[]>);
+  public get array(): ArrayMatchers<any, M> {
+    return new ArrayMatchers<any, M>(this as Assertion<any[], M>);
   }
   /** Number-specific matchers */
-  public get number() {
-    return new NumberMatchers(this as Assertion<number>);
+  public get number(): NumberMatchers<M> {
+    return new NumberMatchers<M>(this as Assertion<number, M>);
   }
   /** Boolean-specific matchers */
-  public get boolean() {
-    return new BooleanMatchers(this as Assertion<boolean>);
+  public get boolean(): BooleanMatchers<M> {
+    return new BooleanMatchers<M>(this as Assertion<boolean, M>);
   }
   /** Object-specific matchers */
-  public get object() {
-    return new ObjectMatchers<any>(this as Assertion<object>);
+  public get object(): ObjectMatchers<any, M> {
+    return new ObjectMatchers<any, M>(this as Assertion<object, M>);
   }
   /** Function-specific matchers */
-  public get function() {
-    return new FunctionMatchers(this as Assertion<Function>);
+  public get function(): FunctionMatchers<M> {
+    return new FunctionMatchers<M>(this as Assertion<Function, M>);
   }
   /** Date-specific matchers */
-  public get date() {
-    return new DateMatchers(this as Assertion<Date>);
+  public get date(): DateMatchers<M> {
+    return new DateMatchers<M>(this as Assertion<Date, M>);
   }
   /** Type-based matchers */
-  public get type() {
-    return new TypeMatchers(this as Assertion<any>);
+  public get type(): TypeMatchers<M> {
+    return new TypeMatchers<M>(this as Assertion<any, M>);
   }
 }