Smart Preprocessor

A smart preprocessor for Node.js.

• License: MIT
• Current status: beta

Smart Preprocessor allows you to preprocess a Javascript module, to build modified versions of the module, or even load a modified version of the module at run-time (module.preprocessorRequire()).

While it is inspired by the C/C++ preprocessor, the syntax is different to be more in phase with the Javascript's spirit.

Its syntax is hidden behind the comment mark, so the original source file is perfectly operational without any pre-processing

Common use cases:

• you want a lot of logs in development mode, but you don't even want that the production code had to filter them out with dozens of if statements.

• your source code uses specific features of an engine, namely io.js, but you have specified an alternative code that is compatible with node.js.

• you have some server code that works fine, you want to build a browser-compatible version

Be sure to check the recommendations.

smart-preprocessor: the CLI program

Smart Preprocessor can be invoked as a Command Line utility to build an alternative version of your code.

After installing it globally, using npm install -g smart-preprocessor, we can run it from everywhere.

The syntax is simple:

smart-preprocessor <source-file> [dest-file] [--switch1 [value1]] [--switch2 [value2]] [...].

If dest is not given, the standard output will be assumed. It's useful if we have to pipe that into another program.

All switches are identifiers we have used in our source-file. See below.

Any alpha-numric string can be used as a switch. Just try to be consistent with other projects.

Some examples:

• smart-preprocessor main.js main.debug.js --debug: build the main.debug.js file from main.js, using the debug parameter
• smart-preprocessor main.js main.trace.js --loglevel trace: build the main.trace.js file from main.js, setting the loglevel parameter to 'trace'

Require a module and pre-process it on-the-fly

Smart Preprocessor can require a module while pre-processing it on-the-fly.

module.preprocessorRequire( modulePath , switches , [ options ] )

• modulePath: string the module file path to load
• switches: object an object containing the preprocessor switches
• options: object optional, contains some options, available options are:
  • multi: if the module is required multiple times with different switches objects, multiple instances of the module will be spawned. Without this options, subsequent require will use the first instance even if the switches object has different options. Some node.js module execute code at require-time, that's why the default behaviour is to share only one instance, just like a normal require() does.

var spp = require( 'smart-preprocessor' ) ;	// Load the smart preprocessor module

var myModule = module.preprocessorRequire( 'my-module' , { config1: true , config2: 4 } ) ;

The .preprocessorRequire() method is added to the module prototype itself, that way, ISO behavior with vanilla require() is guaranted. Also this method is accessible from files that do not require 'smart-preprocessor' directly.

Recommendations / Good practices

• Your source code should be working without any preprocessing. That's what make Preprocessor.js a bad thing, Javascript must run unprocessed, out of the box.
• In fact, your source code should be your standard / production version
• Use runtime module.preprocessorRequire() only for development, debugging, or any kind of fail-safe or emergency mode, that's not a good practice to use it for production running in standard mode.
• As you can see, you cannot define a preprocessor switch to false, null, undefined or an empty string. There is nothing close to an else statement either. Why? Because invoking Smart Preprocessor without any switch MUST be equivalent to the original unprocessed code.

Preprocessor Commands Syntax

The Javascript source file should contains some preprocessor command. All preprocessor command are hidden into comments.

In fact, many commands work by commenting or uncommenting a line or a block.

A preprocessor command start with a preprocessor mark.

A preprocessor mark is a comment mark followed by a #.

For inline command, this will start with //#.

For multi-line command, this will start with either /*# or //*#, see below the differences. Multi-line commands stop at //*/.

Conditional comment syntax

//# <expression> : <inline-code>

If expression is true, then the code is uncommented.

Example, source code of hello.js:

console.log( 'Hello' ) ;
//# debug : console.log( myVar ) ;
console.log( 'world!' ) ;

After running in a shell the command smart-preprocessor hello.js hello.pproc.js --debug, you get a hello.pproc.js file with this content:

console.log( 'Hello' ) ;
console.log( myVar ) ;
console.log( 'world!' ) ;

If you had typed smart-preprocessor hello.js hello.pproc.js --whatever, hello.pproc.js would had been:

console.log( 'Hello' ) ;
//console.log( myVar ) ;
console.log( 'world!' ) ;

... note that the preprocessor command had been removed entirely.

<inline-code> //# <expression> !

If expression is true, then the code is commented.

Example, source code of hello.js:

console.log( 'Hello' ) ;
console.log( myVar ) ;	//# quiet !
console.log( 'world!' ) ;

After running in a shell the command smart-preprocessor hello.js hello.pproc.js --quiet, you get a hello.pproc.js file with this content:

console.log( 'Hello' ) ;
//console.log( myVar ) ;
console.log( 'world!' ) ;

If you had typed smart-preprocessor hello.js hello.pproc.js --whatever, hello.pproc.js would had been:

console.log( 'Hello' ) ;
console.log( myVar ) ;
console.log( 'world!' ) ;

/*# <expression> :\n <multiline-code> \n//*/

If expression is set, the code is uncommented.

Example, source code of hello.js:

console.log( 'Hello' ) ;
/*# debug :
console.log( myVar1 ) ;
console.log( myVar2 ) ;
//*/
console.log( 'world!' ) ;

After running in a shell the command smart-preprocessor hello.js hello.pproc.js --debug, you get a hello.pproc.js file with this content:

console.log( 'Hello' ) ;
//*
console.log( myVar1 ) ;
console.log( myVar2 ) ;
//*/
console.log( 'world!' ) ;

If you had typed smart-preprocessor hello.js hello.pproc.js --whatever, hello.pproc.js would had been:

console.log( 'Hello' ) ;
/*
console.log( myVar1 ) ;
console.log( myVar2 ) ;
//*/
console.log( 'world!' ) ;

//*# <expression> :\n <multiline-code> \n//*/

If expression is true, the code is commented.

Example, source code of hello.js:

console.log( 'Hello' ) ;
//*# quiet !
console.log( myVar1 ) ;
console.log( myVar2 ) ;
//*/
console.log( 'world!' ) ;

After running in a shell the command smart-preprocessor hello.js hello.pproc.js --quiet, you get a hello.pproc.js file with this content:

console.log( 'Hello' ) ;
/*
console.log( myVar1 ) ;
console.log( myVar2 ) ;
//*/
console.log( 'world!' ) ;

If you had typed smart-preprocessor hello.js hello.pproc.js --whatever, hello.pproc.js would had been:

console.log( 'Hello' ) ;
//*
console.log( myVar1 ) ;
console.log( myVar2 ) ;
//*/
console.log( 'world!' ) ;

Expressions

In previous examples, we have only used the presence or absence of a switch. However it is possible to compare a switch to a value.

A value is typically a string. However, any string that can be parsed as a float will be converted to a number. The same rule apply for the switch itself.

This is the list of all possible comparisons.

<switch>

This is the simplest check. If the switch is defined, the condition is true.

Please be extremely careful: unlike Javascript, even if a switch's value is 0, the condition will still hold true. This expression does not check if the switch is truthy or falsy, it checks if it exists. There are strong rationales behind this design.

Example:

//# debug : console.log( myVar ) ;

If a debug switch exists, whatever its value, then the code will be uncommented.

<switch> = <value>

This expression is true if the switch exists and if it equals value.

Example:

//# loglevel = trace : console.log( myVar ) ;

If a loglevel switch exists and is set to "trace", then the code will be uncommented.

<switch> > <value>, <switch> >= <value>, <switch> < <value>, <switch> <= <value>

This expression is true if the switch exists, if both the switch and the value are numbers, and if the switch is respectively greater, greater or equals, lesser, lesser or equals than the value.

Example:

//# loglevel >= 3 : console.log( myVar ) ;

If a loglevel switch exists, is a number and is greater than or equal to 3, then the code will be uncommented.

Aliases

It is possible to define aliases. Each alias is bound to a particular switch.

When an expression involves a switch that has aliases, the preprocessor will try to perform alias substitution on both the switch's value and the value against whom the switch is compared.

The syntax of alias definition is //# <switch> # <alias> ~ <value>.

Example:

//# loglevel # error ~ 0
//# loglevel # warning ~ 1
//# loglevel # verbose ~ 2
//# loglevel # trace ~ 3

//# loglevel >= warning : console.log( "Warning: something bad happens!" ) ;

We defined aliases for the loglevel switch. If this switch value is one of error, warning, verbose or trace, it will be replaced respectively by 0, 1, 2 or 3. Then we have a conditional syntax, comparing against warning... warning is replaced by 1 beforehand. Therefore, if the loglevel switch is warning, verbose, trace or any number greater than or equal to 3, the code will be uncommented.

Benefits of aliases:

• it can be thought as a constant
• it can be thought as an enum
• it can be used to perform greater than or lesser than comparisons between strings

Assignment

It is possible to assign a switch's value to a Javascript variable.

The syntax is //# <switch> -> <Javascript-variable>.

Example, code.js:

var runningLogLevel ;
//# loglevel -> runningLogLevel

After running in a shell the command smart-preprocessor code.js code.pproc.js --loglevel 3, you get a code.pproc.js file with this content:

var runningLogLevel ;
runningLogLevel = 3 ;

If you had typed smart-preprocessor code.js code.pproc.js --loglevel warning, code.pproc.js would had been:

var runningLogLevel ;
runningLogLevel = 'warning' ;

If you had typed smart-preprocessor code.js code.pproc.js --loglevel, code.pproc.js would had been:

var runningLogLevel ;
runningLogLevel = true ;

Please note that if the loglevel switch wasn't defined, the whole line would be blank. This is still related to the fact that no switch = original unprocessed source code. So if you had typed smart-preprocessor code.js code.pproc.js --whatever, code.pproc.js would had been:

var runningLogLevel ;

Also you can set up the value to anything, like (assuming the loglevel switch value is 'warning'):

• object: //# loglevel -> myObject.child.prop becomes myObject.child.prop = 'warning' ;
• array: //# loglevel -> myArray[ 5 ] becomes myArray[ 5 ] = 'warning' ;

Finally, it is possible to write conditional assignment. This is simply a mix of the conditional syntax and the assignment syntax.

For example:

//# loglevel = trace -> myVar

... will assign the loglevel switch to myVar only if loglevel === trace.

Another example:

//# loglevel >= 3 -> myVar

... will assign the loglevel switch to myVar only if loglevel is a number and is greater than or equal to 3.

Spacing

Spaces are optional.

This code:

//#debug:console.log('Warning!');

... is strictly equivalent to this code:

//# debug : console.log( 'Warning!' ) ;

However spaces between the comment mark ('//', '/*' or '//*') and the '#' are *NOT* allowed!

This is called the preprocessor mark, and therefore should be preserved.

Thus the current code has no effect:

// # debug : console.log( 'Warning!' ) ;

... it will be considered like a normal comment.