It aims to be fully compatibe with the node.js assert module, same API and same behavior, just adding support for web browsers. The API and code may contain traces of the CommonJS Unit Testing 1.0 spec which they were based on, but both have evolved significantly since then.

A strict and a legacy mode exist, while it is recommended to only use strict mode.

Strict mode

When using the strict mode, any assert function will use the equality used in the strict function mode. So assert.deepEqual() will, for example, work the same as assert.deepStrictEqual().

It can be accessed using:

const assert = require('assert').strict;

Legacy mode

Deprecated: Use strict mode instead.

When accessing assert directly instead of using the strict property, the Abstract Equality Comparison will be used for any function without a “strict” in its name (e.g. assert.deepEqual()).

It can be accessed using:

const assert = require('assert');

It is recommended to use the strict mode instead as the Abstract Equality Comparison can often have surprising results. Especially in case of assert.deepEqual() as the used comparison rules there are very lax.

E.g.

// WARNING: This does not throw an AssertionError!
assert.deepEqual(/a/gi, new Date());

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.deepStrictEqual(actual, expected, [message])

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

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.