copyright 2020, craigphicks, ISC license

MiniPreproc

Outline

• A simple, light, and fast text preprocessor implemented as a node transform stream.
• Speed is ensured by only pre-processing upto a STOP command. After the STOP command, the MiniPreproc transform stream stops parsing lines and simply passes each "chunk" to output with no processing. That is as fast a transform stream can possibly work.
• The available commands are IF, ELSE, ENDIF, and STOP. Only one level of IF condition is allowed.
• An IF command is followed by a property: IF{{<key>}}, with true/false looked up in dictionary passed in by the caller.
• Each command must be prefixed by //-- from the start of the line, with no spaces before the command. (Currently the choice of prefix is fixed.)
• There is a progmatic interface createPreprocStream. (Currently there is no CLI interface provided). See API section for details.
• The strip option allows all directives to be removed, so the output looks clean.
• When strip is false, the preproccer output can be run through the preprocessor again with any defines and yield the correct result (*).
  • *Yes, it is possible to write input which confuses the preprocessor (e.g. extra //--), however that is only possible before the first STOP command, so it is not a problem in practice.

Install

mini-preproc would most likely be used as a dev tool, so installation as a dev depency is demonstrated:

npm install mini-preproc --save-dev

Example

Suppose an input file ./demo-test.txt with content:

...before
//--IF{{RELEASE}}
//--const someModule=require('some-module');
//--const RELEASE_MODE=true;
//--ELSE
const someModule=require('./some-module.js');
const RELEASE_MODE=false;
//--ENDIF
//--STOP
...after

The following example CLI program demo-cli.js pipes stdin through the stream created by createPreprocStream, to stdout, while also passing CLI parameters to that stream.

'use strict';
const {createPreprocStream}=require('mini-preproc');
async function mpp(defines,strip){
  process.stdin.pipe(
    createPreprocStream(
      defines,{strip:strip}))
    .on('error',(e)=>{
      console.error(e.message);
      process.exitCode=1;
    })
    .pipe(process.stdout);
}
var defines=JSON.parse(process.argv[2]);
var strip=!!JSON.parse(process.argv[3]);
mpp(defines,strip);

Some runs of the program:

$ node demo-cli.js '{"RELEASE":true}' true < ./demo-test.txt
...before
const someModule=require('some-module');
const RELEASE_MODE=true;
...after

$ node demo-cli.js '{"RELEASE":false}' true < ./demo-test.txt
...before
const someModule=require('./some-module.js');
const RELEASE_MODE=true;
...after

$ node demo-cli.js '{"RELEASE":true}' false < ./demo-test.txt
...before
//--IF{{RELEASE}}
const someModule=require('some-module');
const RELEASE_MODE=true;
//--ELSE
//--const someModule=require('./some-module.js');
//--const RELEASE_MODE=true;
//--ENDIF
//--STOP
...after

$ node demo-cli.js '{"RELEASE":false}' false < ./demo-test.txt
...before
//--IF{{RELEASE}}
//--const someModule=require('some-module');
//--const RELEASE_MODE=true;
//--ELSE
const someModule=require('./some-module.js');
const RELEASE_MODE=true;
//--ENDIF
//--STOP
...after

A typescript version of the program is listed in the appendix

WARNING: If using vscode debugger, output to process.stdout is NOT normally shown in the 'DEBUG CONSOLE' window.

API

• import
  • const miniPreproc=require('mini-preproc');
• miniPreproc.createPreprocStream(defines,options)
  • return value is a node transform stream, suitable for piping
  • defines is an object containing zero or more property-value pairs which are used to evaluate the preprocessor IF conditions in the stream's input text. An absent property evaluates to false. Other values are converted to true or false according to normal javascript rules
    • See Preprocessor syntax for details about preprocesser directives.
  • options has a single valid property: strip. When strip is true all the command lines are removed from output. Otherwise they remain.
  • the returned stream may throw an error on illegal preprocessor syntax. The error is of class MiniPreprocError, derived from type Error.

Preprocessor syntax

• All directives are on a single line, //--<directive> with no spaces before //--, and <directive> in [IF,ELSE,ENDIF,STOP].
• Any text outside of an IF-ELSE-ENDIF block of lines is passed through.
• Any text after the first STOP directive line is passed through.

Consider:

//--IF{{<key>}} 
<text-block-true>
//--ELSE
<text-block-false>
//--ENDIF

• If (<key> in defines) evaluates to true

  • If options.strip evaluates to true
    • Output

    <<text-block-true>> with any `//--` line prefixes removed 
    

  • else if options.strip evaluates to false
    • Output

    //--IF{{<key>}} 
    <text-block-true> with '//--' line prefixes removed
    //--ELSE
    <text-block-false> with '//--' line prefixes required
    //--ENDIF
    

• else if (<key> in defines) evaluates to false

  • If options.strip evaluates to true
    • Output

    <<text-block-false>> with any `//--` line prefixes removed 
    

  • else if options.strip evaluates to false
    • Output

    //--IF{{<key>}} 
    <text-block-true> with '//--' line prefixes required
    //--ELSE
    <text-block-false> with '//--' line prefixes removed
    //--ENDIF
    

• examples of <text-block-*> with '//--' line prefixes removed

  • Case: Has //-- prefix

  //--xxxx
  

  becomes

  xxx
  

  • Case: Already doesn't have //-- prefix

  yyyy
  

  stays as

  yyyy
  

• examples of <text-block-false> with '//--' line prefixes required

  • Case: Already has //-- prefix

  //--xxxx
  

  stays as

  //--xxxx
  

  • Case: Doesn't have //-- prefix

  yyyy
  

  becomes

  //--yyyy
  

Typescript

The typescript declaration are bundled with the package. (They are not provided seperately under npm @types).

An example typescript program using the mini-preproc typescript delared argument types is listed in the appendix.

version changes

1.1.0

• typescript definitions added
• supports "node": ">=10.13.0"

Appendix

Printing to process.stdout from vscode in debug mode.

When a program is started by the vscode debugger, output to process.stdout is NOT shown in the 'DEBUG CONSOLE' window. See 'outputCapture' at https://code.visualstudio.com/docs/nodejs/nodejs-debugging . However, the easiest solution is to start the program from outside the debugger with node --inspect-brk my-prog.js and then attach that process from vscode - the output to process.stdout will then show correctly in the terminal from which node was invoked.

Typescript version of the example program

'use strict'
import mpp=require('mini-preproc'); 
async function mppCli(defines:mpp.Defs,strip:boolean){
  process.stdin.pipe(
    mpp.createPreprocStream(
      defines,{strip:strip}))
    .on('error',(e)=>{
      console.error(e.message);
      process.exitCode=1;
    })
    .pipe(process.stdout);
}
var defines=JSON.parse(process.argv[2]);
var strip=!!JSON.parse(process.argv[3]);
mppCli(defines,strip);