What is Testable Code?

Every Piece of Software is Testable.
Somehow.

What you might be able to test:

• return value;
• output;
• other side effects;
• doesn’t crash;
• ...

Testable Code
in the Context of
Unit Testing.

Unit Testing

Also component testing, or module testing.

Unit Testing

• Dynamic testing of individual units. In isolation.
• Find defects in software components (e.g., functions, classes).
• Done by developers.

Dynamic Testing

• Execution of code.
• Primarily verification.
• Find failures.
• Different testing methods:
  • black-box testing;
  • white-box testing;
  • ...

Verification

The process of evaluating a system or component to determine whether the system of a given development phase satisfies the conditions imposed at the start of that phase. IEEE Std 610.12-1990

“Are we building the product right?„

White-box Testing

• Structure-based testing.
• Close examination of procedural level of detail.
• Knowledge of internals required.
• Different white-box testing techniques:
  • statement testing;
  • decision testing;
  • (multiple) condition testing;
  • ...

Unit Test Examples

Example 1: Description

• A class with a stupid simple method.
• No external dependencies.
• Thus use a unit testing framework only:
  • PHP:
    • atoum;
    • PHPUnit;
    • SimpleTest;
    • ...
  • JavaScript:
    • Jasmine;
    • Mocha;
    • QUnit;
    • tape;
    • ...

Example 1: Subject under Test (SUT)

class Oracle {

    public function get_answer() {

        return '42';
    }
}

Example 1: Test

class OracleTest extends PHPUnit_Framework_TestCase {

    public function test_get_answer() {

        $testee = new Oracle();

        $this->assertSame( '42', $testee->get_answer() );
    }
}

Example 1: JavaScript Version

var test = require( 'tape' );

var Oracle = require( './path/to/oracle' );

test( function( assert ) {
    var testee = new Oracle();

    assert.equal( testee.getAnswer(), '42' );

    assert.end();
} );

Example 2: Description

• A class with a conditional method.
• An external dependency.
• Thus (maybe) use a mocking framework:
  • PHP:
    • Mockery;
    • Prophecy;
    • ...
  • JavaScript:
    • Sinon.JS;
    • ...

Example 2: SUT

class FortuneTeller {

    private $oracle;

    public function __construct( Oracle $oracle ) {

        $this->oracle = $oracle;
    }

    public function get_answer( $money = 0 ) {

        if ( (int) $money < 5 ) {
            return '';
        }

        return $this->oracle->get_answer();
    }
}

Example 2: Test

class FortuneTellerTest extends PHPUnit_Framework_TestCase {

    public function test_get_answer() {

        $oracle = $this->getMockBuilder( 'Oracle' )
            ->getMock();

        $oracle->method( 'get_answer' )
            ->willReturn( 'answer' );

        $testee = new FortuneTeller( $oracle );

        $this->assertSame( '', $testee->get_answer( 0 ) );
        $this->assertSame( 'answer', $testee->get_answer( 5 ) );
        // ...use a data provider for this test.
    }
}

Example 2: Mockery Version

class FortuneTellerMockeryTest extends PHPUnit_Framework_TestCase {

    public function test_get_answer() {

        $oracle = Mockery::mock( 'Oracle', [
            'get_answer' => 'answer',
        ] );

        $testee = new FortuneTeller( $oracle );

        $this->assertSame( '', $testee->get_answer( 0 ) );
        $this->assertSame( 'answer', $testee->get_answer( 5 ) );
        // ...use a data provider for this test.
    }
}

Example 2: JavaScript Version

var test = require( 'tape' );
var sinon = require( 'sinon' );

var FortuneTeller = require( './path/to/fortune-teller' );

test( function( assert ) {
    var testee = new FortuneTeller( {
        getAnswer: sinon.stub().returns( 'answer' )
    } );

    assert.equal( testee.getAnswer( 0 ), '' );
    assert.equal( testee.getAnswer( 5 ), 'answer' );
    // ...

    assert.end();
} );

Example 3: Description

• Similar to example 1.
• Fires a WordPress action hook.
• Provides a WordPress filter hook.
• Thus use a WordPress mocking framework (PHP):
  • Brain Monkey;
  • WP_Mock;
  • ...

Example 3: SUT

class HookOracle {

    public function get_answer() {

        do_action( 'give_answer' );

        return (string) apply_filters( 'the_answer', '42' );
    }
}

Example 3: Test

class HookOracleBrainMonkeyTest extends PHPUnit_Framework_TestCase {

    // Set up and tear down Brain Monkey...

    public function test_get_answer() {

        $testee = new HookOracle();

        $this->assertSame( '42', $testee->get_answer() );
        $this->assertSame( 1, did_action( 'give_answer' ) );
        $this->assertTrue( Brain\Monkey::filters()->applied( 'the_answer' ) > 0 );
    }
}

Example 3: WP_Mock Version

class HookOracleWP_MockTest extends PHPUnit_Framework_TestCase {

    // Set up and tear down WP_Mock...

    public function test_get_answer() {

        $testee = new HookOracle();

        WP_Mock::expectAction( 'give_answer' );
        WP_Mock::onFilter( 'the_answer' )
            ->with( '42' )
            ->reply( '4815162342' );
        $this->assertSame( '4815162342', $testee->get_answer() );
    }
}

Bad Practices
and How to
Make Them Good

Issue: No Feedback

function maybeDoSomething() {
    if ( ! checkSomething() ) {
        return;
    }

    doSomething();
}

Solution: Give Feedback

function maybeDoSomething() {
    if ( ! checkSomething() ) {
        return false;
    }

    doSomething();

    return true;
}

But consider potential side effects (e.g., JavaScript event listeners).

Issue: Terminating the Execution

class Checker {

    public function check_data( $data ) {

        if ( ! $data ) {
            exit();
        }

        // ...
    }
}

Solution: Delegation

class Checker {

    public function check_data( $data ) {

        if ( ! $data ) {
            $this->call_exit();
        }

        // ...
    }

    protected function call_exit() {

        exit();
    }
}

Issue: Leaving the Context

function processData( data ) {
    data = prepareData( data );
    sendData( data );

    window.location.href = 'http://example.com';
}

Solution: Delegation

function setLocation( url ) {
    window.location.href = url;
}

function processData( data ) {
    data = prepareData( data );
    sendData( data );

    setLocation( 'http://example.com' );
}

Issue: Creating (Real) Objects

class Renderer {

    public function render_formatted_data( $data ) {

        $formatter = new Formatter();
        echo $formatter->format( $data );
    }
}

Solution: Dependency Injection

Here, constructor injection:

class Renderer {

    private $formatter;

    public function __construct( Formatter $formatter ) {

        $this->formatter = $formatter;
    }

    public function render_formatted_data( $data ) {

        echo $this->formatter->format( $data );
    }
}

“What if I cannot work with an injected object,
but need to create it dynamically?„

Solution: Use a Factory

class Renderer {

    private $formatter_factory;

    public function __construct( FormatterFactory $formatter_factory ) {

        $this->formatter_factory = $formatter_factory;
    }

    public function render_formatted_data( $data, $options ) {

        $formatter = $this->formatter_factory->create( $options );
        echo $formatter->format( $data );
    }
}

Issue: Global State

function someFunction() {
    if ( 42 === globalFoo ) {
        // ...
    }

    // ...
}

Solution: Inject What You Need

function someFunction( localFoo ) {
    if ( 42 === localFoo ) {
        // ...
    }

    // ...
}

Issue: Using Unresettable Singletons

class Singleton {

    private static $instance;

    // protected/private __construct() and __clone(), ...

    // ...and other code, which maybe manipulates the object's state...

    public static function get_instance() {

        if ( ! self::$instance ) {
            self::$instance = new self();
        }

        return self::$instance;
    }
}

Solution: Add a reset() Method

class Singleton {

    // ...

    public static function get_instance() {

        if ( ! self::$instance ) {
            self::$instance = new self();
        }

        return self::$instance;
    }

    public static function reset() {

        self::$instance = null;
    }
}

Issue: Breaking the Law of Demeter

function isIdEven( someObject ) {
    var id = someObject.getOtherObject().getId();

    return ! ( id % 2 );
}

Excursus: Law of Demeter

A method m of an object o should invoke only the methods of:

• o itself;
• any objects passed to m;
• any objects created/instantiated in m;
• any properties/members of o.

Issue: Breaking the Law of Demeter

function isIdEven( someObject ) {
    var id = someObject.getOtherObject().getId();

    return ! ( id % 2 );
}

function filterByEvenId( someObject ) {
    if ( isIdEven( someObject ) ) {
        return someObject;
    }

    return undefined;
}

Solution: Don’t Talk to Strangers

function isIdEven( otherObject ) {
    var id = otherObject().getId();

    return ! ( id % 2 );
}

function filterByEvenId( someObject ) {
    if ( isIdEven( someObject.getOtherObject() ) ) {
        return someObject;
    }

    return undefined;
}

Issue: Calling Static Methods

class Renderer {

    public function render_if_okay( $data ) {

        if ( ThirdPartyChecker::is_okay( $data ) ) {
            echo $data;
        }
    }
}

Solution: Wrap Static Method

class Checker {

    public function is_okay( $data ) {

        return ThirdPartyChecker::is_okay( $data );
    }
}

Solution: Wrap Static Method

class Renderer {

    private $checker;

    public function __construct( Checker $checker ) {

        $this->checker = $checker;
    }

    public function render_if_okay( $data ) {

        if ( $this->checker->is_okay( $data ) ) {
            echo $data;
        }
    }
}

Issue: Incomplete Initialization

function Renderer() {
    var formatter;

    this.setFormatter = function( f ) {
        formatter = f;
    };

    this.getFormattedData = function( data ) { // <-- SUT
        return formatter.format( data );
    };
}

Solution: Use Constructor Injection

function Renderer( formatter ) {
    this.getFormattedData = function( data ) {
        return formatter.format( data );
    };
}

Issue: Creating Intermediary Objects

function render_post_title_with_id( $post_id ) {

    $post = get_post( $post_id );
    if ( $post ) {
        echo $post->post_title . '(' . $post_id . ')';
    }
}

Solution: Inject What You Need

function render_post_title_with_id( WP_Post $post ) {

    echo $post->post_title . '(' . $post->ID . ')';
}

Issue: eval Expressions

function badlyDesignedFunction( data ) {
    return eval( '42 === data' );
}

Solution: Write Regular Code

function okayDesignedFunction( data ) {
    return 42 === data;
}

Issue: Insane Complexity

class Processor {

    public function process( $data ) {

        // 16 lines full of different checks, a lot of if—else, ...

        // 23 lines full of data preparing, switch—case all over, ...

        // 42 lines full of data processing...

        // 8 lines full of post-processing, if—elseif—else again...
    }
}

Solution: Write Modular Code

class Processor {

    public function process( $data ) {

        if ( ! $this->checker->check( $data ) ) {
            return false;
        }

        $data = $this->prepare( $data );

        // 42 lines full of data processing...

        $this->post_processor->process( $data );

        return true;
    }
}