JavaScript Crash Course

Here is a small JavaScript crash course which i wrote while going thru one of JavaScript's online open courses. This PDf has been very helpful to quick refresh my Javascript fundamentals whenever needed.

Hope this will be helpful to you all too. Please click here to download this PDF file.

Angular Directive VS Service usage

Angular is built from ground up keeping separation of concern, modularity and testing in mind. One of the frequently asked question in AngularJS is where to use Directives and Services.

Directives 
Directives are a way to teach HTML new tricks. During DOM compilation directives are matched against the HTML and executed. This allows directives to register behavior, or transform the DOM - (angularjs.org/guide/directive)

As mentioned in the definition, Directives are strictly for the DOM manipulation only. Directives are genius way to make DOM more intelligent e.g. we can create a directive which creates new element with node name as "tab" which angular at run time which resolve as HTML. This gives HTML a new meaning by creating new reusable tags and also gives us one code place to explain what tag "tab" are and easy to maintain. 

Services
A service is any functionality which can be abstracted and then called by different functions e.g. $http, $routeProvider, etc. These services are needed to be injected in the controllers or other services using them to avoid direct reference to the functions. On injecting, these services are initialized and then can be used in the code without be directly object of the function. This is good code practice as we can mock the services for the test.

Ideally, directives should be strictly used for any UI view manipulation and services are abstracted functionalities which can be mocked for unit tests. Service should not be used for any DOM manipulation or event binding.

Project Euler - Problem 2 in F#

Even Fibonacci numbers

Problem 2

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

Here is my solution

    // Create a mutable list, so that we can add elements 
    // (this corresponds to standard .NET 'List<T>' type)
    let l = new ResizeArray<_>([1;2])
 
    let rec fabonacci a b upperLimit = 
        match a, b with
            | _, _ when a + b <= upperLimit -> 
                let c = a + b
                l.Add(c)
                fabonacci b c upperLimit
            | _ -> l
 
    let sumOfEven =
        (fabonacci 1 2 4000000)
            |> List.ofSeq
            |> Seq.filter (fun x -> x % 2 = 0) 
            |> Seq.sum 

Since its dynamic seq we are creating which depends on the last two numbers, we need a mutable list which we can populate according to our need and logic. For Fibonacci, we need to have access to previous two numbers in the list to generate new one.

This solution is flexible too with the upper limit, so this logic can be used anywhere to create the Fibonacci sequence with any upper bound. First step is to create Fibonacci sequence with the upper bound and initial number to kick start the sequence.Once seq is created up to desired upper bound, we are left with filtering of the sequence for even numbers only and get their sum.

Please leave your feedback and happy coding.