11 Assertion API

11.1 Assertions in software development

In software development, assertions state facts about values or pieces of code that must be true. If they aren’t, an exception is thrown. Node.js supports assertions via its built-in module assert – for example:

import assert from 'node:assert/strict';
assert.equal(3 + 5, 8);

This assertion states that the expected result of 3 plus 5 is 8. The import statement uses the recommended strict version of assert.

11.2 How assertions are used in this book

In this book, assertions are used in two ways: to document results in code examples and to implement test-driven exercises.

11.2.1 Documenting results in code examples via assertions

In code examples, assertions express expected results. Take, for example, the following function:

function id(x) {
  return x;
}

id() returns its parameter. We can show it in action via an assertion:

assert.equal(id('abc'), 'abc');

In the examples, I usually omit the statement for importing assert.

The motivation behind using assertions is:

• We can specify precisely what is expected.
• Code examples can be tested automatically, which ensures that they really work.

11.2.2 Implementing test-driven exercises via assertions

The exercises for this book are test-driven, via the test framework Mocha. Checks inside the tests are made via methods of assert.

The following is an example of such a test:

// For the exercise, we must implement the function hello().
// The test checks if we have done it properly.
test('First exercise', () => {
  assert.equal(hello('world'), 'Hello world!');
  assert.equal(hello('Jane'), 'Hello Jane!');
  assert.equal(hello('John'), 'Hello John!');
  assert.equal(hello(''), 'Hello !');
});

For more information, see “Getting started with exercises” (§12).

11.3 Normal comparison vs. deep comparison

The strict equal() uses === to compare values. Therefore, an object is only equal to itself – even if another object has the same content (because === does not compare the contents of objects, only their identities):

assert.notEqual({foo: 1}, {foo: 1});

deepEqual() is a better choice for comparing objects:

assert.deepEqual({foo: 1}, {foo: 1});

This method works for Arrays, too:

assert.notEqual(['a', 'b', 'c'], ['a', 'b', 'c']);
assert.deepEqual(['a', 'b', 'c'], ['a', 'b', 'c']);

11.4 Quick reference: module assert

For the full documentation, see the Node.js docs.

11.4.1 Normal equality: assert.equal()

• assert.equal(actual, expected, message?)

  actual === expected must be true. If not, an AssertionError is thrown.

  assert.equal(3+3, 6);
  

• assert.notEqual(actual, expected, message?)

  actual !== expected must be true. If not, an AssertionError is thrown.

  assert.notEqual(3+3, 22);
  

The optional last parameter message can be used to explain what is asserted. If the assertion fails, the message is used to set up the AssertionError that is thrown.

let e;
try {
  const x = 3;
  assert.equal(x, 8, 'x must be 8')
} catch (err) {
  assert.equal(
    String(err),
    'AssertionError [ERR_ASSERTION]: x must be 8\n\n3 !== 8\n');
}

11.4.2 Deep equality: assert.deepEqual()

• assert.deepEqual(actual, expected, message?)

  actual must be deeply equal to expected. If not, an AssertionError is thrown.

  assert.deepEqual([1,2,3], [1,2,3]);
  assert.deepEqual([], []);
  
  // To .equal(), an object is only equal to itself:
  assert.notEqual([], []);
  

• assert.notDeepEqual(actual, expected, message?)

  actual must not be deeply equal to expected. If it is, an AssertionError is thrown.

  assert.notDeepEqual([1,2,3], [1,2]);
  

11.4.3 Expecting exceptions: assert.throws()

If we want to (or expect to) receive an exception, we need assert.throws(): This function calls its first parameter, the function callback, and only succeeds if it throws an exception. Additional parameters can be used to specify what that exception must look like.

• assert.throws(callback, message?): void

  assert.throws(
    () => {
      null.prop;
    }
  );
  

• assert.throws(callback, errorClass, message?): void

  assert.throws(
    () => {
      null.prop;
    },
    TypeError
  );
  

• assert.throws(callback, errorRegExp, message?): void

  assert.throws(
    () => {
      null.prop;
    },
    /^TypeError: Cannot read properties of null \(reading 'prop'\)$/
  );
  

• assert.throws(callback, errorObject, message?): void

  assert.throws(
    () => {
      null.prop;
    },
    {
      name: 'TypeError',
      message: "Cannot read properties of null (reading 'prop')",
    }
  );
  

11.4.4 Always fail: assert.fail()

• assert.fail(messageOrError?)

  By default, it throws an AssertionError when it is called. That is occasionally useful for unit testing. messageOrError can be:

  • A string. That enables to override the default error message.
  • An instance of Error (or a subclass). That enables us to throw a different value.

  try {
    functionThatShouldThrow();
    assert.fail();
  } catch (_) {
    // Success
  }