comptime, a conditional compilation typescript plugin

This project is a typescript transformer that provides conditional compilation and compile-time variable substitution in a somewhat type-safe manner.

Note that this plugin is still in a pretty alpha state and will crash on you the first chance it gets. You have been warned.

The documentation here is pretty minimal for now and will be supplemented at a later time. Please just know that there are probably many cave-at that you should be aware of that I didn't have time to detail here.

Note that by design, if you compile without the plugin, the code will have everything included (all the ifs and whatnot) and will run like if Production is true and Env is an empty object.

What it allows you to do

• Conditionally compile code
• Replace variables at compile time with values known then (such as environment variables or package.json values)
• Define compile-time variables and functions directly in your source code
• All of the above done in a type-safe manner

Why

https://github.com/webpack/docs/wiki/list-of-plugins#defineplugin There are some conditional compilation soluthttps://github.com/webpack/docs/wiki/list-of-plugins#definepluginions out there such as the define plugin, but they're all out of typescript's scope.

Consider this :

///ifdef SOME_CONDITION
var myvar = ''
///endif

console.log(myvar)

while this example is stupid, it still highlights the fact that you will never be warned until your code crashes at runtime.

What it (probably) won't do

Macros and code emission are for now out of scope of this project.

While code-rewrite is an interesting feature, it is pretty hard to implement it while still keeping the type logic intact, as macros could potentially do pretty much whatever they want.

How to use

The simplest way to use it is in conjunction with ttypescript. Just add in your tsconfig.json the following after having installed it with npm --save comptime :

// ...
  "compilerOptions": {
    /// ...
    "plugins": [
      { "transform": "comptime/comptime" }
    ]
  }
// ...

There is no need to configure your IDE to use something else than the normal typescript language server ; this plugin affects only the emitted code.

Conditions

Compile-time expressions are those using only values from the comptime namespace or literals (strings, booleans, ...).

import { Comptime } from 'comptime'

if (Comptime.Env.NODE_ENV === 'production') {
  // do things here
} else if (Comptime.Env['NODE_ENV'] === 'dev') {
  // or do that
}

export const MY_VAR = Comptime.Env.NODE_ENV === 'production' ? 'prod-value' : 'dev-value'

Default flags and environment variable

You have the following starting variables to play with :

• Comptime.Production defaults to true, is false if COMPTIME_ENV=dev
• Comptime.Dev defaults to false, is true if COMPTIME_ENV=dev or if DEBUG=<anything>
• Comptime.Debug defaults to false, is true if DEBUG is set
• Comptime.Env is an object with your environment

Creating your own variables

In your code and in a file that you are sure will be loaded first by typescript (typically with a name that sorts before or in a file imported by pretty much everyone) :

import { Comptime } from 'comptime'

// Module augmentation baby !
declare module 'comptime' {
  namespace Comptime {
    // We declare new variables
    export function Test(a: number): string
  }
}

// this function can now be called at compile time in other files.
Comptime.Test = function (a: number) { return '' + a + 3 }