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reversible-preproc v1.0.3

A preproc system with a powerful condition language. It uses annotated comments to switch off code, for convenient in place switching beween configurations.


1. Outline

This package is a library providing core functionality for a preprocessor, e.g., it can be used to implement preprocessing in a stream, or a CLI program. (For example, the npm package reversible-preprocess-cli is a CLI program implemented with this module -- soon to be published ).

The "reversible" moniker indicates that it is suitable for lightweight switching back and forth between configuations (e.g., in-place). The deactivated regions are marked with annotated comments (//!!) enabling the preprocessor to remove them for a different configuration. Repeated applications of the same defines are idempotent.

Regexp is not used. Neither is all text searched. Only lines beginning with comment marks (e.g. //), are further processed as potential processing command lines. This makes processing relatively fast.

2. interface

The module exports a class ReversiblePreproc

class ReversiblePreproc {
    constructor(defines = {}, options = defaultOptions) {...}
    line(line, callback = null) {...}
export default ReversiblePreproc

2.1 constructor(defines, options)

2.1.1 defines

  • Normally defines is an object providing the property names and values to be referenced/tested by conditional statements embedded in the file/stream to be processed.
  • As a special case the top level may the single single string "*". The effect is to evaluate all conditionals as true, i.e. nothing is commented out.
  • The top level cannot be an array. Lower levels arrays are allowed.
  • These top level property names are forbidden: true, false, null, undefined, def, ndef, this.
  • Values are restricted to strings, numbers, and the special values true. false, and null.

An example defines:

    "DEBUG" : 2,
    "configs": [{ 

2.1.2 options

  • commentMark: <string> [default "//"]
    • only lines beginning with this commentMark will be further examined for preprocessor commands
    • reversibleCommentIndicator: [default "!!"]
  • the preprocessor disable a regions by prepending with commentMark followed by reversibleCommentIndicator, e.g., //!!
  • testMode: <boolean> [default false]
    • only output preprocess command start lines prepended by true or false, according to the condtion evaluation.
  • debugOutput:
    • prepend lines with the line number of the command condition whose range they are under, if any. (Unlikely to be much use other than developer testing)

2.2 line(line, callback = null)

This function performs stateful line by line processing

2.2.1 Return value

  • When the passed argument callback is null it return a pair [err, outputLine]. err will be null if there is no error to report. Otherwise err will be an instance of Error, and the value of outputline is undetermined.
  • When callback is not null it must be a function taking the the two arguments err and outputline in that order.

2.2.2. Stateful line processing logic

The function to perform stateful preprocessing on each line of input. From the perspective of the preprocessor, input lines are categorized as follows:

  • command start line
  • command end line
  • non-command line

A command start line has the form

[commentMark][almost arbitrary fill]if<<CONDITIONAL-STATEMENT>>

For example, in the context of the example defines written above:

//  if<<configs[select].A>>

A command end line has the same form but without if and the CONDITIONAL-STATEMENT. E.g,

//  <<>>

The command end line marks the end of the range of the nearest command start line above it which is not already matched by a command end line.

Hence, command start and end line pairs can be nested. However, they cannot overlap.

The form and action of the `CONDITIONAL-STATEMENT is explained in detail below.

When the CONDITIONAL-STATEMENT evaluates to true, the ordinary lines in its range will be output without change. However when it evaluates to false, the ordinary lines in its range will be prepended with the commentMark and reversibleComment indicator.

When commands are nested, the inner overrides the outer for the duration of its range.

For example:

Before processing:

// if<<true>>
Mother duck and her chicks cross the road to get to the water. 
John can 
// if <<false>> 
hardly wait to get home and can't 
// <<>>
stop the car in time.
// <<>>

After processing:

// if<<true>>
Mother duck and her chicks cross the road to get to the water. 
John can 
// if <<false>> 
//!! hardly wait to get home and can't 
// <<>>
stop the car in time.
// <<>>


Two forms of conditional statement are enabled:

  • psuedo javascript
    • doesn't use javascript eval
    • parsed by jsep into abstract parse tree form, then evaluated by a simple interpreter.
  • actual javascript
    • uses javascript eval
    • client defined function is evald with defines passed as argument.

The pseudo javascript is more brief and readable, but the actual javascript is almighty.

2.3.1 Psuedo javascript conditional statement

The 'pseudo javascript' uses the npm module 'jsep' to parse, and then an interpreter to execute the parsed data. Example:

// if<<DEBUG>1 && configs[select].B>=1>>

which would evaluate to false using the above defines example.

  • Round parentheses ( and ) are allowed for grouping logic.
  • Square brackets [ and ] are allowed for property access as an alternative to . or where the identifier is a variable and . can't be used.
  • Allowed binary operators are: <= < > >= == === != !== && ||
  • Allowed unary operators are: !
  • Predefined keywords are: true, false, null, undefined, def, ndef

Just like Javascript referencing a nonexistent property may or may not result in an exception.
(As explained above, exceptions will passed back in the result of the line function.)

  • Two predefined functions are provided:

  • def() evaluates to wither true or false, regardless of the property value.

  • true is the result when no exception occurs and PROPERTY-EXPRESSION does not resolve to undefined

  • false is the result otherwise.

  • def() never results in an error value being passed back from line().

  • ndef() simply returned the logical inverse of def(), i.e. !def()

Example usage:

// if<< def(configs[select].C) && configs[select].C===1 >>


// if<< def(configs[2]) >> 

which are respectively true and false given the above defines example above.

Note: The jsep module is not a dependency listed in package.json because it has been bundled directly into this module.

2.3.2 Actual javascript conditional statement

These have the form


where : serves to differentiate from a pseudo javascipt statement.

JAVASCRIPT-FUNCTION-OF-ONE-ARG will be evald with defines passed as the arg. For example, alternates to the pseudo javascipt examples provided above:

// if<<:(D)=>{ return D.configs[select].C!==undefined && configs[select].C===1 } >>


// if<<: (D)=>{ return D.configs.length > 2 }
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