Language-neutral file preprocessor

preprocess.js

/**
 * Preprocess a file's contents represented as a string.
 *
 * This will look for lines containing custom preprocessor directives embedded
 * in a string. The directives take the form of psuedo-HTML tags, which begin
 * with <@@ COND @@> and end with </@@>, where COND is any valid JavaScript
 * expression that is used to test whether the text between the tags
 * should be output or not.
 * 
 * Within the COND expressions, the variable __ (double underscore)
 * will be available to access the data passed to the preprocessor.
 * 
 * Nested directive are allowed.
 * 
 * Any characters outside of the markers is completely ignored, so you can
 * surround them with whatever commenting syntax is appropriate for the host
 * language. You can even add comments for the directives themselves in order
 * to help keep track of nesting levels, etc.
 * 
 * Lines containing directives are never output. Therefore, it is not possible
 * to inline a directive.
 */
function preprocess(inStr, data = {}) {
  return Function('__', [
    'const out = [];',
    ...inStr.split('\n').map(line => {
      let m;
      return (m = /^(\s*).*<@@\s*(.*)\s*@@>/g.exec(line)) ?
        `${m[1]}if (${m[2]}) {` :
      (m = /^(\s*).*<\/@@>/g.exec(line)) ?
        `${m[1]}}` :
      (m = /^(\s*)/g.exec(line)) ?
        `${m[1]}out.push("${
          line.replace(/[\\"']/g, '\\$&').replace(/\u0000/g, '\\0')
        }");` :
      '';
    }),
    'return out;'
  ].join('\n'))(data).join('\n');
}

let test = `
if (a == b) {
  // <@@ !__.TEST @@>
  console.log('a == b');
  // </@@> !TEST
}

if (c == d) {
  // <@@ __.TEST @@>
  console.log("c == \\" d");
  //   <@@ __.TEST2 @@>
  hello();
  //   </@@> TEST2
  // </@@> TEST
}
  `;

console.log(preprocess(test, {TEST: true, TEST2: false}));