assertion-error

What is assertion-error?

The assertion-error package is used to create error objects that are specifically meant for testing assertions. It is commonly used in conjunction with assertion libraries to provide a more descriptive error message when an assertion fails. The error object created by assertion-error includes a message and can also include properties that describe the assertion that failed.

What are assertion-error's main functionalities?

Creating an AssertionError instance

This feature allows you to create an instance of an AssertionError with a custom message. This is useful when you want to throw a specific error in your tests when an assertion fails.

{"const AssertionError = require('assertion-error');
const error = new AssertionError('expected value to be true');
throw error;"}

Other packages similar to assertion-error

chai

Chai is a BDD / TDD assertion library for node and the browser that can be delightfully paired with any javascript testing framework. It offers more comprehensive features for assertions compared to assertion-error, including a range of assertion styles (expect, should, assert) and numerous plugins.

should

Should.js is an expressive, readable, framework-agnostic assertion library. The key difference is that should.js provides a rich set of assertions and chainable methods to construct assertions, whereas assertion-error is focused on creating error objects for failed assertions.

expect.js

Expect.js is a minimalistic BDD-style assertions library that can be used in node.js or in the browser. It provides a similar set of assertion checks as assertion-error but with a different API and additional features like chainable assertions.

README

AssertionError and AssertionResult classes.

What is AssertionError?

Assertion Error is a module that contains two classes: AssertionError, which is an instance of an Error, and AssertionResult which is not an instance of Error.

These can be useful for returning from a function - if the function "succeeds" return an AssertionResult and if the function fails return (or throw) an AssertionError.

Both AssertionError and AssertionResult implement the Result interface:

interface Result {
  name: "AssertionError" | "AssertionResult";
  ok: boolean;
  toJSON(...args: unknown[]): Record<string, unknown>;
}

So if a function returns AssertionResult | AssertionError it is easy to check which one is returned by checking either .name or .ok, or check instanceof Error.

Installation

Node.js

assertion-error is available on npm.

$ npm install --save assertion-error

Deno

assertion_error is available on Deno.land

import {
  AssertionError,
  AssertionResult,
} from "https://deno.land/x/assertion_error@2.0.0/mod.ts";