#metaprogramming

Each year I make a list of the best posts I think I made for the year not in the most popular posts sidebar (on obscurejavascript.tumblr.com as of late December 2019). Since popular doesn't necessarily mean better. The list for 2019 is below:

1. Replaying JavaScript Functions Without Variables (Or Libraries) There are many implications of Functions also being Objects in JavaScript, but one of the most interesting is that you can cache the last value of a function call use a property on itself. This post explains how that is done.

2. What is the Point of Var in Modern JavaScript? Even with let and const now supported in all modern JavaScript environments, there are still some rare use cases where var is necessary.

3. Metaprogramming Classes in JavaScript Compared to many programming languages, even other popular scripting languages, JavaScript tends to be very flexible. Without using any custom eval code - just standard non-verbose syntax, it is possible to create classes from templates. Which can include things like dynamic method names and other variations.

4. String Substitutions In Console Strings One of the little known features in the console object is support for string substitutions. Since most environments display console output of objects in an easy to inspect way (sometimes even with GUI features), this is very useful for debuggging.

5. Quickly Swapping Values Swapping values in objects or even variables is possible now with just one line of code due to destructuring assignments.

Posts of Previous Years

Best Posts Of 2018

Best Posts Of 2017

Best Posts Of 2016

Sometimes when defining very similar objects, it can make sense to generate them all at once. For example here is how 3 classes of similar functions can be generated from an array and a single class definitions:

const classTypes = ['A', 'B', 'C']; for (let type of classTypes) { global[`class${type}`] = class { [`describe${type}`]() { return `A class of type ${type}`; }; } } const objA = new classA(); console.log(objA.describeA()); // A class of type A const objB = new classB(); console.log(objB.describeB()); // A class of type B const objC = new classC(); console.log(objC.describeC()); // A class of type C

global should be whatever the top level value (or object) that you want to attach the classes too. So window for browsers. In most cases, method names would remain the same across all classes, but I wanted to demonstrate that like everything else, those can have a unique name directly input.

I could not find a direct compatibility table of the support for those dynamic method names, only general support tables](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions#Browser_compatibility). So instead I verified the above works manually with window in the latest Chrome, Firefox, Safari and MS Edge in addition to Node 10+ with global.

Metaprogramming in C# has historically relied on reflection (slow at runtime) or T4 templates (clunky build integration). C# 9 changes the game with Source Generators. They allow you to hook into the compilation process, inspect your code, and generate new C# files on the fly that get compiled with your project. How It Works flowchart LR SourceCode[Your C# Code] --> Compiler[Roslyn…