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A terse, yet expressive assertion library

Is Assertive different from other assertion libraries?

Assertive aims to make the exact cause of breakage and intent of tests as fast and easy to spot as possible, with much attention paid to both the colour and alignment of expected and actual data, so you should be able to glean what you need immediately.

It also tries to pre-empt false negative tests from ever happening, by rigorously testing for correct assertion invocation and by avoiding to pick names for assertions with a track record of being misinterpreted, not just by people reading the code, but also by programmers writing them, which can make even 100%-test-coverage code fail on behalf of it testing for the wrong thing.

Semantic Versioning

Assertive uses semver version numbers, though we should point out that we may tighten assertion checks in minor version number updates, making code that previously silently passed, now fail.

Case in point: before v1.3.0, code using an assertion to verify that a string included the empty string, would do just that. In other words - nothing, since that assertion does not test anything. Now, such a test is flagged as a bug in your test suite that you should fix, as that is not asserting something about your code, but about strings in general.

In Assertive, breaking changes implying a major version bump, would be things like argument order changes. If you really do not want improved coverage against this type of error with a random minor version update you should pin a version you like in your package.json rather than a version range.


Each assertion lets you state a condition and an optional help message about what semantics your test asserts, which gets presented first, if the assertion fails. (This is generally much more useful than messages along the lines of "expected true to be false", especially when it may be hard to tell later what the intended purpose of a test really was.)

Besides failing when what each assertion guards against, they also all fail if you pass too few, too many or otherwise illegal parameters, as when a tired programmer expects "expect" to compare the two parameters he passed in some way and trip when they mismatch, though all it would ever test is that the first was truthy. To not get test suites full of almost-no-op tests like that, Assertive fails straight away like this:

Expected: true
Actually: 10

There have been test suites full of no-op tests similar to this, which have gone undetected for months or years, giving a false sense of what regressions you are guarded against.

You may pass any of the functions an item to be tested as a promise, and it will be tested after the promise is resolved. In this case, the test will return a promise which will be resolved or rejected as appropriate. A promise-aware test runner (e.g. Mocha version >= 1.18.0) is highly recommended.

These docs show a typical invocation, and what you see when it failed:


# fail if bool != true
expect '2 > 1', 2 > 1
Assertion failed: 2 > 1


Note: Using truthy in your tests is a code smell. More often than not there is another, more precise test. Only use truthy when there is no way of knowing what the actual value will be. If bool is the result of a boolean operation, use expect. If bool is an unknown value, use match or include to narrow it down.

assert.truthy(explanation, bool)
# fail if !bool
truthy 'something was populated in the email field',
Assertion failed: something was populated in the email field
expected undefined to be truthy


assert.equal(expected, actual)
assert.equal(explanation, expected, actual)
# fail unless actual === expected
Assertion failed: decode the Epoch to 0s after Jan 1st, 1970
Expected 86400000 to be
equal to 0


assert.deepEqual(expected, actual)
assert.deepEqual(explanation, expected, actual)
# fail unless _.isEqual(expected, actual)
Assertion failed: ensure that all methods we tested were handled, and in the right order
mismatch: {"methods":["GET"]} didn't
deepEqual {"methods":["GET","POST","PUT","DELETE"]}


assert.include(needle, haystack)
assert.include(explanation, needle, haystack)
# fail unless haystack has a substring needle, or _.include haystack, needle
Assertion failed: only accept supported, case-normalized method names
expected ["GET","POST","PUT","DELETE"]
to include "get"


assert.match(regexp, string)
assert.match(explanation, regexp, needle)
# fail unless regexp matches the given string, or regexp.test string
Assertion failed: only affirmative pirate answers accepted
Expected: /aye|yar+/
to match: "nay"


err = assert.throws(functionThatThrows)
err = assert.throws(explanation, functionThatThrows)
# fail unless the provided functionThatThrows() calls throw
# (on non-failures the return value is whatever was thrown)
Assertion failed: ensure that bad inputs throw an error
didn't throw an exception as expected to


assert.hasType(<type>, value);
assert.hasType(explanation, <type>, value);
assert.hasType(null, value)
assert.hasType(Date, value)
assert.hasType(Array, value)
assert.hasType(String, value)
assert.hasType(RegExp, value)
assert.hasType(Boolean, value)
assert.hasType(Function, value)
assert.hasType(Object, value)
assert.hasType(NaN, value)
assert.hasType(Number, value)
assert.hasType(undefined, value)
# fail unless _.isType(value) is true for given Type, or the
# same test for a more specific type (listed above) was true


promise = assert.resolves(promise)
promise = assert.resolves(explanation, promise)
# Wait for promise to resolve, then resolve if successful, reject otherwise
# Always returns a promise, unless called with non-promise (not allowed)
Assertion failed: should resolve to good stuff
Promise was rejected despite resolves assertion:
Timeout in 10000ms


promiseForErr = assert.rejects(promise)
promiseForErr = assert.rejects(explanation, promise)
# Wait for promise to reject, resolve with error if it does, reject otherwise
# Basically inverse of resolves(), but resolves with the error for more testing
# Always returns a promise, unless called with non-promise (not allowed)
Assertion failed: should reject after Timeout
Promise wasn't rejected as expected to

falsey, notEqual, notDeepEqual, notInclude, notMatch, notThrows, notHasType

Versions of the above functions taking the same arguments, but asserting the opposite outcome. The assertion failure messages are just as helpful.


BSD 3-Clause open source license