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;"}

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Readme

Source

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'

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Last updated on 05 Oct 2021

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