assert

What is assert?

The assert npm package is a module that provides a set of assertion functions for verifying invariants. It is primarily used for writing tests to ensure that code behaves as expected. The module includes functions to test strict equality, deep equality, and various other conditions.

What are assert's main functionalities?

Strict Equality Assertion

This feature is used to test if two values are strictly equal using the === operator.

const assert = require('assert');
assert.strictEqual(1, 1, '1 is strictly equal to 1');

Deep Equality Assertion

This feature is used to test if two objects or arrays are equal by comparing their properties or elements.

const assert = require('assert');
assert.deepStrictEqual({ a: 1 }, { a: 1 }, 'Objects are deeply equal');

Assertion with a Predicate Function

This feature is used to test if a value passes a specified predicate function.

const assert = require('assert');
assert.ok(value => value > 10, 'Value is greater than 10');

Throws Assertion

This feature is used to test if a function throws an error as expected.

const assert = require('assert');
assert.throws(
  () => { throw new Error('Wrong value'); },
  Error,
  'Function should throw an Error'
);

Other packages similar to assert

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 plugins and a more flexible API compared to assert.

expect

Expect is a minimalistic assertion library that provides a set of assertion functions and is often used with Jest. It offers a more fluent and readable syntax compared to assert.

should

Should.js is an expressive, readable, framework-agnostic assertion library. It extends the Object prototype with a single non-enumerable getter that allows for a more fluent and chainable API, providing a different style of coding compared to assert.

README

assert

This module is used for writing unit tests for your applications, you can access it with require('assert').

The API is derived from the commonjs 1.0 unit testing spec and the node.js assert module

assert.fail(actual, expected, message, operator)

Throws an exception that displays the values for actual and expected separated by the provided operator.

assert(value, message), assert.ok(value, [message])

Tests if value is truthy, it is equivalent to assert.equal(true, !!value, message);

assert.equal(actual, expected, [message])

Tests shallow, coercive equality with the equal comparison operator ( == ).

assert.notEqual(actual, expected, [message])

Tests shallow, coercive non-equality with the not equal comparison operator ( != ).

assert.deepEqual(actual, expected, [message])

Tests for deep equality.

assert.notDeepEqual(actual, expected, [message])

Tests for any deep inequality.

assert.strictEqual(actual, expected, [message])

Tests strict equality, as determined by the strict equality operator ( === )

assert.notStrictEqual(actual, expected, [message])

Tests strict non-equality, as determined by the strict not equal operator ( !== )

assert.throws(block, [error], [message])

Expects block to throw an error. error can be constructor, regexp or validation function.

Validate instanceof using constructor:

assert.throws(function() { throw new Error("Wrong value"); }, Error);

Validate error message using RegExp:

assert.throws(function() { throw new Error("Wrong value"); }, /value/);

Custom error validation:

assert.throws(function() {
    throw new Error("Wrong value");
}, function(err) {
    if ( (err instanceof Error) && /value/.test(err) ) {
        return true;
    }
}, "unexpected error");

assert.doesNotThrow(block, [message])

Expects block not to throw an error, see assert.throws for details.

assert.ifError(value)

Tests if value is not a false value, throws if it is a true value. Useful when testing the first argument, error in callbacks.