A javascript unit testing framework in 50 lines – plus some assertions

Surprised by a TDD session at XP-Man in which we rolled our own unit test framework from scratch - using the growing framework itself to test itself as it grew - I added some basic assertions to my 50 line javascript test runner.

I suggest that assertions should be thought of as separate to a test runner. Assertions are at the heart of - and should grow into - your domain specific testing language. So you should think of assretions as owned by your application, not the testing framework. As your application grows, so domain specific assertions should grow. A testing framework should probably provide minimal assertions for the language it runs on: the 'primitive' true/false/equal/notEqual assertions which will be used to define more domain specific assertions; and doing the legwork of tedious but common cases, such as testing equality by value of complex objects and collections.

In the case of javascript, the below might do. The tricky thing for assertions is to come up with something that is fluent and not verbose. C# extension methods are perfect for this. In javascript one could add methods to Object.prototype, but that doesn't have the same scoping restriction as C# so feels too much like trampling all over everyone else's garden: it's poor namespacing and a failure of good manners.

/* Assertions for a test framework. http://www.cafe-encounter.net/p756/ */

function Assert(actual, assertionGender){
    this.actual=actual;
    this.notted=!assertionGender;
    this.Expected=  assertionGender ? "Expected" : "Did not expect";
}

function assert(actual){return new Assert(actual, true);}
function assertNot(actual){return new Assert(actual, false);}

Assert.prototype.IsTrue = function(msg){
    if( this.actual == this.notted ){ throw msg||this.Expected + " true.";}
};

Assert.prototype.Equals = function(expected,msg){
    if( (this.actual==expected) == this.notted ){
        throw msg|| this.Expected + " " + this.actual + " to equal " + expected ;
    }
};

Assert.prototype.EqualsByPropertyValue = function(expected,msg){
    var actualValue =  JSON.stringify( this.actual );
    var expectedValue = JSON.stringify( expected );
    if( (actualValue == expectedValue) == this.notted ){
        throw msg|| this.Expected + " " + actualValue + " to equal " + expectedValue + " by value";
    }
};

$(function(){
    new Specification("SpecificationsForPrimitiveAssertionPasses",{
        isTrueShouldBeTrueForTrue : function(){
            assert(true).IsTrue();
        },
        equalsShouldPassForEqualArguments : function(){
            assert( 1+1 ).Equals( 2 );
        },
        equalsByPropertyValueShouldPassForValueEqualArguments : function(){
            assert( { a: "a", b : function(){ return "b";}} )
                .EqualsByPropertyValue( { a: "a", b : function(){ return "b";}} );
        },
        assertNotShouldReverseTheOutcomeOfAnAssertion : function(){
            assertNot(false).IsTrue();
            assertNot(1+1).Equals(3);
            assertNot({ a: "a"}).EqualsByPropertyValue( { a: "b"});
        }
    }).runTests();

    new Specification("SpecificationsForPrimitiveAssertion_Failures",{
        isTrueShouldNotBeTrueForFalse : function(){
            assert(false).IsTrue();
        },
        equalsShouldFailForNonEqualArguments : function(){
            assert(1).Equals(2);
        },
        equalsByPropertyValueShouldFailForDifferingPropertyValues : function(){
            assert( { a: "a" } ).EqualsByPropertyValue( { a: "A" } );
        },
        equalsByPropertyValueShouldFailForDifferingProperties : function(){
            assert( { a: "a" } ).EqualsByPropertyValue( { aa: "a" } );
        },
        assertNotShouldReverseTheOutcomeOfIsTrue : function(){
            assertNot(true).IsTrue();
            assertNot({ a: "a"}).EqualsByPropertyValue( { a: "a"});
        },
        assertNotShouldReverseTheOutcomeOfEquals : function(){
            assertNot(1+1).Equals(2);
        },
        assertNotShouldReverseTheOutcomeOfEqualsByValue : function(){
            assertNot({ a: "a"}).EqualsByPropertyValue( { a: "a"});
        },
        assertTrueFailureWithCustomerMessageShouldOutputCustomMessage : function(){
            assert(false).IsTrue("This assertion failure should output this custom message");
        },
        assertEqualsFailureWithCustomerMessageShouldOutputCustomMessage : function(){
            assert(1).Equals(2, "This assertion failure should output this custom message");
        },
        assertEqualsByValueFailureWithCustomerMessageShouldOutputCustomMessage : function(){
            assert({a:"a"}).EqualsByPropertyValue( 1, "This assertion failure should output this custom message");
        }

    }).runTests();
});