add sourcemaps support for @if directives.

README.md

@@ -274,6 +274,15 @@ The syntax type of source string to preprocess. There are 3 main syntax variants
    `ts`, `tsx`, `peg`, `pegjs`, `jade`, `styl`
  - `coffee`, aliases: `bash`, `shell`, `sh`
 
+##### options.supportSourceMaps
+Type: `Boolean`
+Default: `false`
+
+**Note:** Currently only the @if directive is supported.
+
+When set to `true`, the preprocessor will replace excluded lines with empty lines, by that maintaining correct line
+mapping when sourcemaps are generated. 
+
 ### preprocessFile(srcFile, destFile[, context[, callback[, options]]])
 
 Preprocesses a `sourceFile` and saves the result to `destFile`. Simple wrapper around `fs.readFile()` and `fs.writeFile()`.

lib/preprocess.js

@@ -82,6 +82,7 @@ function preprocess(src, context, typeOrOptions) {
     options.fileNotFoundSilentFail = typeOrOptions.fileNotFoundSilentFail || options.fileNotFoundSilentFail;
     options.srcEol = typeOrOptions.srcEol || options.srcEol;
     options.type = delim[typeOrOptions.type] || options.type;
+    options.supportSourceMaps = typeOrOptions.supportSourceMaps || false;
   }
 
   context = copy(context);
@@ -160,17 +161,30 @@ function preprocessor(src, context, opts, noRestoreEol) {
     });
   }
 
+  function compensateDirectiveRemoval(content){
+    return opts.supportSourceMaps ? opts.srcEol + content + opts.srcEol : content;
+  }
+
+  function getSourceMapCompatiableReplacementBlock(content) {
+    if (opts.supportSourceMaps) {
+      var lineCount = content.split(/\r\n|\r|\n/).length;
+      return Array(lineCount).join(opts.srcEol);
+    } else {
+      return '';
+    }
+  }
+
   if (opts.type.if) {
     rv = replaceRecursive(rv, opts.type.if, function (startMatches, endMatches, include, recurse) {
       var variant = startMatches[1];
       var test = (startMatches[2] || '').trim();
       switch(variant) {
         case 'if':
-          return testPasses(test,context) ? recurse(include) : '';
+          return compensateDirectiveRemoval(testPasses(test,context) ? recurse(include) : getSourceMapCompatiableReplacementBlock(include));
         case 'ifdef':
-          return typeof getDeepPropFromObj(context, test) !== 'undefined' ? recurse(include) : '';
+          return compensateDirectiveRemoval(typeof getDeepPropFromObj(context, test) !== 'undefined' ? recurse(include) : getSourceMapCompatiableReplacementBlock(include));
         case 'ifndef':
-          return typeof getDeepPropFromObj(context, test) === 'undefined' ? recurse(include) : '';
+          return compensateDirectiveRemoval(typeof getDeepPropFromObj(context, test) === 'undefined' ? recurse(include) : getSourceMapCompatiableReplacementBlock(include));
         default:
           throw new Error('Unknown if variant ' + variant + '.');
       }

test/sourcemaps.spec.js

@@ -0,0 +1,27 @@
+'use strict';
+
+var chai = require('chai'),
+  pp = require('../lib/preprocess');
+
+chai.should();
+
+describe('when the sourcemaps option is true it should maintain the line count', function () {
+
+  describe('with @if directive', function () {
+    var input = "a\n" +
+      "// @if NODE_ENV=='production' \n" +
+      "b\n" +
+      "c\n" +
+      "// @endif \n" +
+      "d";
+
+    it('when condition evals to true', function () {
+      pp.preprocess(input, {NODE_ENV: 'production'}, {type: 'js', supportSourceMaps: true}).should.equal("a\n\nb\nc\n\nd");
+    });
+
+    it('when condition evals to false', function () {
+      pp.preprocess(input, {NODE_ENV: 'dev'}, {type: 'js', supportSourceMaps: true}).should.equal("a\n\n\n\n\nd");
+    });
+  });
+
+});