Assert | Node.js v7.10.1 Documentation (original) (raw)

Assert#

Stability: 2 - Stable

The assert module provides a simple set of assertion tests that can be used to test invariants.

assert(value[, message])#

Added in: v0.5.9

• value
• message

An alias of assert.ok().

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

• actual
• expected
• message

Tests for deep equality between the actual and expected parameters. Primitive values are compared with the equal comparison operator ( == ).

Only enumerable "own" properties are considered. The deepEqual()implementation does not test object prototypes, attached symbols, or non-enumerable properties. This can lead to some potentially surprising results. For example, the following example does not throw an AssertionErrorbecause the properties on the Error object are non-enumerable:

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

"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:

const assert = require('assert');

const obj1 = {
  a : {
    b : 1
  }
};
const obj2 = {
  a : {
    b : 2
  }
};
const obj3 = {
  a : {
    b : 1
  }
};
const obj4 = Object.create(obj1);

assert.deepEqual(obj1, obj1);
// OK, object is equal to itself

assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
// values of b are different

assert.deepEqual(obj1, obj3);
// OK, objects are equal

assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
// Prototypes are ignored

If the values are not equal, an AssertionError is thrown with a messageproperty set equal to the value of the message parameter. If the messageparameter is undefined, a default error message is assigned.

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

• actual
• expected
• message

Generally identical to assert.deepEqual() with two exceptions. First, primitive values are compared using the strict equality operator ( === ). Second, object comparisons include a strict equality check of their prototypes.

const assert = require('assert');

assert.deepEqual({a:1}, {a:'1'});
// OK, because 1 == '1'

assert.deepStrictEqual({a:1}, {a:'1'});
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using strict equality

If the values are not equal, an AssertionError is thrown with a messageproperty set equal to the value of the message parameter. If the messageparameter is undefined, a default error message is assigned.

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

• block
• error |
• message

Asserts that the function block does not throw an error. Seeassert.throws() for more details.

When assert.doesNotThrow() is called, it will immediately call the blockfunction.

If an error is thrown and it is the same type as that specified by the errorparameter, then an AssertionError is thrown. If the error is of a different type, or if the error parameter is undefined, the error is propagated back to the caller.

The following, for instance, will throw the TypeError because there is no matching error type in the assertion:

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

However, the following will result in an AssertionError with the message 'Got unwanted exception (TypeError)..':

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

If an AssertionError is thrown and a value is provided for the messageparameter, the value of message will be appended to the AssertionErrormessage:

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError,
  'Whoops'
);
// Throws: AssertionError: Got unwanted exception (TypeError). Whoops

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

Added in: v0.1.21

• actual
• expected
• message

Tests shallow, coercive equality between the actual and expected parameters using the equal comparison operator ( == ).

const assert = require('assert');

assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'

assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({a: {b: 1}}, {a: {b: 1}});
//AssertionError: { a: { b: 1 } } == { a: { b: 1 } }

If the values are not equal, an AssertionError is thrown with a messageproperty set equal to the value of the message parameter. If the messageparameter is undefined, a default error message is assigned.

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

Added in: v0.1.21

• actual
• expected
• message
• operator

Throws an AssertionError. If message is falsy, the error message is set as the values of actual and expected separated by the provided operator. Otherwise, the error message is the value of message.

const assert = require('assert');

assert.fail(1, 2, undefined, '>');
// AssertionError: 1 > 2

assert.fail(1, 2, 'whoops', '>');
// AssertionError: whoops

assert.ifError(value)#

Added in: v0.1.97

• value

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

const assert = require('assert');

assert.ifError(0);
// OK
assert.ifError(1);
// Throws 1
assert.ifError('error');
// Throws 'error'
assert.ifError(new Error());
// Throws Error

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

Added in: v0.1.21

• actual
• expected
• message

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

const assert = require('assert');

const obj1 = {
  a : {
    b : 1
  }
};
const obj2 = {
  a : {
    b : 2
  }
};
const obj3 = {
  a : {
    b : 1
  }
};
const obj4 = Object.create(obj1);

assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj2);
// OK, obj1 and obj2 are not deeply equal

assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj4);
// OK, obj1 and obj2 are not deeply equal

If the values are deeply equal, an AssertionError is thrown with a messageproperty set equal to the value of the message parameter. If the messageparameter is undefined, a default error message is assigned.

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

Added in: v1.2.0

• actual
• expected
• message

Tests for deep strict inequality. Opposite of assert.deepStrictEqual().

const assert = require('assert');

assert.notDeepEqual({a:1}, {a:'1'});
// AssertionError: { a: 1 } notDeepEqual { a: '1' }

assert.notDeepStrictEqual({a:1}, {a:'1'});
// OK

If the values are deeply and strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

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

Added in: v0.1.21

• actual
• expected
• message

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

const assert = require('assert');

assert.notEqual(1, 2);
// OK

assert.notEqual(1, 1);
// AssertionError: 1 != 1

assert.notEqual(1, '1');
// AssertionError: 1 != '1'

If the values are equal, an AssertionError is thrown with a messageproperty set equal to the value of the message parameter. If the messageparameter is undefined, a default error message is assigned.

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

Added in: v0.1.21

• actual
• expected
• message

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

const assert = require('assert');

assert.notStrictEqual(1, 2);
// OK

assert.notStrictEqual(1, 1);
// AssertionError: 1 !== 1

assert.notStrictEqual(1, '1');
// OK

If the values are strictly equal, an AssertionError is thrown with amessage property set equal to the value of the message parameter. If themessage parameter is undefined, a default error message is assigned.

assert.ok(value[, message])#

Added in: v0.1.21

• value
• message

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

If value is not truthy, an AssertionError is thrown with a messageproperty set equal to the value of the message parameter. If the messageparameter is undefined, a default error message is assigned.

const assert = require('assert');

assert.ok(true);
// OK
assert.ok(1);
// OK
assert.ok(false);
// throws "AssertionError: false == true"
assert.ok(0);
// throws "AssertionError: 0 == true"
assert.ok(false, 'it\'s false');
// throws "AssertionError: it's false"

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

Added in: v0.1.21

• actual
• expected
• message

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

const assert = require('assert');

assert.strictEqual(1, 2);
// AssertionError: 1 === 2

assert.strictEqual(1, 1);
// OK

assert.strictEqual(1, '1');
// AssertionError: 1 === '1'

If the values are not strictly equal, an AssertionError is thrown with amessage property set equal to the value of the message parameter. If themessage parameter is undefined, a default error message is assigned.

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

• block
• error |
• message

Expects the function block to throw an error.

If specified, error can be a constructor, RegExp, or validation function.

If specified, message will be the message provided by the AssertionError if the block fails to throw.

Validate instanceof using constructor:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  Error
);

Validate error message using RegExp:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  /value/
);

Custom error validation:

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

Note that error can not be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used formessage instead. This can lead to easy-to-miss mistakes:

// THIS IS A MISTAKE! DO NOT DO THIS!
assert.throws(myFunction, 'missing foo', 'did not throw with expected message');

// Do this instead.
assert.throws(myFunction, /missing foo/, 'did not throw with expected message');