Ts-transformer-preval-macro NPM

ts-transformer-preval

NOT STABLE, consider that when using it

Pre-evaluate code at build-time.

Inspired by babel-plugin-preval & forced by the behaviour of next.js (in detail the getInitialProps function on client side).

This small transformer came to live.

The problem

You need to do some dynamic stuff, but don't want to do it at runtime. Or maybe you want to do stuff like read the filesystem to get a list of files and you can't do that in the browser.

babel-plugin-preval#the-problem

The solution

This allows you to specify some code that gets transpiled, runs in Node and whatever your run function returns in there will be swapped. For example:

import { preval } from 'ts-transformer-preval-macro';

const data = preval`async function run() {
   return 5+5; 
}`;

becomes

const data = 10;

Something more fancy?

import IContentBlock from '../interfaces/IContentBlock';
import { preval } from 'ts-transformer-preval-macro';

const content: IContentBlock[] = preval`
import {FetchData, getContentPiece} from '../services/DataService';
import IContentBlock from '../interfaces/IContentBlock';

async function run() {
    const fetcher = new FetchtData();

    const contentBlocks: IContentBlock[] = getContentPiece<IContentBlock>(
        await fetcher.getContentBlocks({
            'fields.identifier[match]': 'article.'
        })
    );

    return contentBlocks;
}`;

becomes

import IContentBlock from '../interfaces/IContentBlock';

const content: IContentBlock[] = [
  {
    identifier: 'article.typescript.transformer.preval',
    title: 'How to write a typescript transformer',
    content: '...',
    slug: 'howto-write-a-typescript-transformer'
  },
  {
    identifier: 'article.typescript.transformer.101',
    title: 'Typescript transformer 101',
    content: '...',
    slug: 'typescript-transformer-101'
  }
];

Install

npm i ts-transformer-preval-macro

Configuration

mode

type: string default: development

Switch basic settings between production and development.

cacheActivated

type: boolean default: false

If activated results of pre-evaluation will be cached until code is changed. production and development mode have separated caches.

debug

type: boolean default: false

If activated evaluated code won't be deleted so you can run & inspect it manually.

Usage

With loaders

With ts-loader

{
    loader: 'ts-loader',
    options: {
        getCustomTransformers: () => {
            return {
                before: [
                    prevalTransformer({
                        cacheActivated: true,
                        mode: isProduction ? 'prod' : 'dev',
                        debug: false,
                    })(),
                ],
            };
        }
    }
}