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Assert

Stability: 3 - Locked

This module is used so that Node.js can test itself. It can be accessed with require('assert'). However, it is recommended that a userland assertion library be used instead.

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

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

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

Tests for deep equality. Primitive values are compared with the equal comparison operator ( == ).

This only considers enumerable properties. It does not test object prototypes, attached symbols, or non-enumerable properties. This can lead to some potentially surprising results. For example, this does not throw an AssertionError because the properties on the Error object are non-enumerable:

// WARNING: This does not throw an AssertionError!
assert.deepEqual(Error('a'), Error('b'));

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

Tests for deep equality. Primitive values are compared with the strict equality operator ( === ).

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

Expects block not to throw an error. See assert.throws() for more details.

If block throws an error and if it is of a different type from error, the thrown error will get propagated back to the caller. The following call will throw the TypeError, since we're not matching the error types in the assertion.

assert.doesNotThrow(
  function() {
    throw new TypeError('Wrong value');
  },
  SyntaxError
);

In case error matches with the error thrown by block, an AssertionError is thrown instead.

assert.doesNotThrow(
  function() {
    throw new TypeError('Wrong value');
  },
  TypeError
);

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

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

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

Throws an AssertionError. If message is falsy, it displays the values for actual and expected separated by the provided operator. Otherwise, it displays message (and does not use actual, expected, and operator).

assert.ifError(value)

Throws value if value is truthy. This is useful when testing the error argument in callbacks.

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

Tests for any deep inequality. Opposite of assert.deepEqual.

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

Tests for deep inequality. Opposite of assert.deepStrictEqual.

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

Tests shallow, coercive inequality with the not equal comparison operator ( != ).

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

Tests strict inequality as determined by the strict not equal operator ( !== ).

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

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

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

Expects block to throw an error. error can be a 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'
);

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