Learning Outcome

What I learnt today

I learnt a bit of typescript as I paired with my Mentor on refactoring a Retirement calculator Kata I had previously written in purely Javascript to use some typescript code.

So what is Typescript?

By definition, “TypeScript is JavaScript for application-scale development.” TypeScript is a strongly typed, object oriented, compiled language. It was designed by Anders Hejlsberg at Microsoft. TypeScript is both a language and a set of tools. TypeScript is a typed superset of JavaScript compiled to JavaScript. In other words, TypeScript is JavaScript plus some additional features. (Excerpt Tutorial Point)

1. Typescript performs static type validation.

   So then: Do I need to validate everything? Simply, No 

   Checking all the variables of my application is time-consuming from a development and performance perspective.

   Example:

Here is how my validation test looked like when I writing pure JavaScript

You would notice I am testing by throwing exception for wrong input type ‘string’.

checkout out by blog where I wrote about Don't use exceptions for non-exceptional conditions.

In my constructor function, I wrote a conditional statement that checks if either of currentAge, desiredRetirementAge, or currentYear is of type string, throw an exception with a custom message “Invalid, all inputs must be an Integer“, using the javascript Number.isInteger method

Migrating my logic from Javascript to Typescript, I setup a config file in the root director of my project named tsconfig.json, this is for managing my project’s options, such as which files I want to include, and what sorts of checking I want to perform.

What my tsconfig.json file.

On Line 7, I am telling typescript to read in any files it understands in the lib directory (with include).

I modified my file extension to .ts from .js. At this point, I ran tsc at the root of my project, I could see output files in the built directory. The layout of files in built looks identical to the layout of lib. I now have TypeScript working with my project.

This is how my constructor logic now looks like after refactoring

First defined the variables and assigned them a type, before passing them as arguments into the constructor function.

Next, I took out the conditional statement, because it is not needed anymore since typescript is already handling validation by ensuring the values for currentAge, desiredRetirementAge and currentAge must all be integers.

I also deleted my test cases, because they really aren’t needed anymore

You can see that giving a string as an argument in my calculator instance, will automatically prompt an error, indicating that this can not run.