# Assert lets you test your code

## Prerequisites

• Node.js installed
• No prior knowledge

## Skill Level

• Basic/Beginner

## What?

Assert - verb - state a fact or belief confidently and forcefully. (dictionary definition)

When writing code we write tests to automatically check our code is working as we expect it to.

Assert is the most rudimentary way of writing tests. It provides no feedback when running your tests unless one fails.

The assert module has 11 methods but you will only (regularly) use a few of them: assert.equal, assert.deepEqual and assert.throws. Each are explained with examples below.

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

The first method (alphabetically), but the least useful for practical purposes (unless you expect a method/test to always fail).

Throws an exception that displays the values for actual and expected separated by the provided operator.

Example:

assert.fail(21, 42, 'Test Failed', '###')


Output:

  throw new assert.AssertionError({
^
AssertionError: Faild
at Object.<anonymous> (assert.js:2:8)
at Module._compile (module.js:456:26)
at Object.Module._extensions..js (module.js:474:10)
at Function.Module.runMain (module.js:497:10)
at startup (node.js:119:16)
at node.js:906:3


Usefulness: virtually none. I haven't found a practical use for this. http://nodejs.org/api/assert.html#assert_assert_fail_actual_expected_message_operator

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

Tests if value is "truthy", it is equivalent to:

assert.equal(true, value, message);


The simplest assertion.

Example:

var assert = require('assert');

return a + b;
}

assert( expected === 3, 'one plus two is three');


This will not have any output. If you want to see output, you need to make the test fail:

var assert = require('assert');

return a + b;
}

assert( expected === 4, 'one plus two is three');


Output:

assert.js:92
throw new assert.AssertionError({
^
AssertionError: one plus two is NOT four
at Object.<anonymous> (/Users/n/code/node-js-by-example/core/assert/assert.js:8:1)
at Module._compile (module.js:456:26)


assert.ok(value, [message]) is essentially the same as assert(value, message);

var assert = require('assert');

return a + b;
}

assert.ok( expected === 3, 'one plus two is three');


Again, no output because the test passes. To see some feedback, make the test fail.

Usefulness: universal. assert can be used to test any code.

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

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

#### Why would you use assert.equal() instead of assert() ?

If you want to make your test clearer use assert.equal otherwise there is no benefit to the additional verbosity.

Example:

var assert = require('assert');

return a + b;
}

// these three assertions are equivalent:
assert(expected == 3, 'one plus two is three');
assert.ok(expected == 3, 'one plus two is three');
assert.equal(expected, 3, 'one plus two is three');


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

Tests shallow, coercive non-equality with the not equal comparison operator ( != ).

Example:

var assert = require('assert');

return a + b;
}

// these three assertions are equivalent:
assert(expected != 4, 'one plus two is three');
assert.ok(expected != 4, 'one plus two is three');
assert.notEqual(expected, 4, 'one plus two is three (NOT Four!)');


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

Tests for deep equality.

assert.deepEqual is the second of the useful methods. We use it for comparing two objects (or arrays for equality).

Example:

var assert = require('assert');

var list1 = [1, 2, 3, 4, 5];
var list2 = [1, 2, 3, 4, 5];

assert.deepEqual(list1, list2, 'deepEqual checks the elements in the arrays are identical');

var person1 = { "name":"john", "age":"21" };
var person2 = { "name":"john", "age":"21" };

// deepEqual checks the elements in the objects are identical
assert.deepEqual(person1, person2, 'these two objects are the same');


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

Tests for any deep inequality. Useful when confirming two objects or arrays are not equal.

Example:

var assert = require('assert');

var list1 = [1, 2, ,3, 4, 5];
var list2 = [1, 2, 3, 4, 5];

assert.deepEqual(list1, list2, 'deepEqual checks the elements in the arrays are identical');

var person1 = { "name":"john", "age":"21" };
var person2 = { "name":"jane", "age":"19" };

// deepEqual checks the elements in the objects are identical
assert.notDeepEqual(person1, person2, 'these two objects are NOT the same');


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

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

Similar to assert.equal but "strict" (type coersion).

Example:

var assert = require('assert');

assert.strictEqual(1, '1', 'not the same'); // this will fail


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

Tests strict non-equality, as determined by the strict not equal operator ( !== )

The opposite of the strictEqual.

Example:

var assert = require('assert');

assert.notStrictEqual(1, true, 'not the same (strictly)');


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

Expects block to throw an error. error can be constructor, RegExp or validation function.

The assert.throws lets you check for specific errors in your functions.

Validate instanceof using constructor:

Example:

assert.throws(
function() {
throw new Error("Wrong value");
},
Error
);


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

Expects block not to throw an error, see assert.throws for details.

Example:

assert.doesNotThrow(
function() {
console.log("Nothing to see here");
},
Error
);


Not particularly useful method because its too vague. Its good to know your method did not throw an error under normal circumstances.

### 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.

Example:

// define a simple function with callback(err, value)
function sayHello(name, callback) {
var error = false;
var str   = "Hello "+name;
callback(error, str);
}

// use the function
sayHello('World', function(err, value){
assert.ifError(err);
assert.equal(value, "Hello World");
})


## Try it!

As with all code, you can't expect to learn without trying. Open the assert.js file in your editor and try a few examples. Remember you won't see any output unless your test fails. Run it with the command:

node assert.js


## In Practice (The "Real World")

In the "real world" people rarely use the node assert module by its' self. Instead a test runner is used.

Examples of Node.js test runners include:

Each of these has their merits and will be discussed in a later chapter on testing.