Accepts a js file containing a JSON-compliant array of filenames and looks through each file for comma-separated terms. The result is a list of files that contain references to the terms.

ManyFilesGrepper.js

/**
 *
 * ManyFilesGrepper.js
 * Accepts a js file containing a JSON-compliant array of
 * filenames and looks through each file for comma-separated terms.
 * The result is a list of files that contain references to
 * the terms.
 *
 * Usage:
 * >node ManyFilesGrepper.js path/to/list/of/files.js terms,to,match [path/to/file/to/write/to.js]
 */

var fs       = require('fs'),
    _        = require('/usr/local/lib/node_modules/lodash'),
    q        = require('/usr/local/lib/node_modules/q'),
    listFile = process.cwd() + '/' + process.argv[2],
    terms    = process.argv[3].split(','),
    outputFile = process.argv[4] ? process.cwd() + '/' + process.argv[4] : undefined;

fs.readFile(listFile, 'utf8', function (err, data) {
  var list = JSON.parse(data);

  var mfg = new ManyFilesGrepper(list, terms, outputFile);

});

var ManyFilesGrepper = function (list, terms, outputFile) {

  var _list  = list,
      _terms = terms,
      _outputFile = outputFile;

  var returnable = {
    init : function () {

      this.process();

    },

    process: function () {
      var self = this,
          filesGotten = [];

      _.forEach(_list, function (filePath) {
        filesGotten.push(self.getFileData(filePath));
      });

      q.all(filesGotten)
        .then(this.addUpMatchOutputs.bind(this))
        .then(this.writeToFile.bind(this));
    },

    /**
     * Promise adapter for standard readFile
     * @param {string} filePath Path to file
     */
    getFileData: function (filePath) {
      var def = q.defer();

      fs.readFile(filePath, 'utf8', function (err, data) {
        def.resolve({data: data, filePath: filePath});
      });

      return def.promise;
    },

    /**
     * Cycle through returned array of file contents
     * and summate the cases where instances were found
     * @param {array} dataArr Q.all() generated array of values (from array of promises)
     */
    addUpMatchOutputs: function (dataArr) {

      var combinedText,
          self = this;

      combinedText = _.reduce(dataArr, function (output, data) {
        return output + self.outputMatchesForFile(data.filePath, self.matchTerms(data.data));
      });

      return combinedText;
    },

    /**
     * Convenience method for matching multiple terms
     * in a file.
     * @param {string} fileData File data to search through.
     */
    matchTerms: function (fileData) {
      var i,
          matching = {},
          matchingLine;

      for (i = 0; i < _terms.length; i++) {
        matchingLine = this.matchTerm(fileData, _terms[i]);

        if (_.size(matchingLine)) {
          matching[_terms[i]] = _.cloneDeep(matchingLine);
        }
      }

      return matching;
    },

    /**
     * Find terms in a body of text, and return their line
     * numbers as well as the line they exist on.
     * @param {string} fileData File data to search through.
     * @param {string} term String to match.
     */
    matchTerm: function (fileData, term) {
      var fileDataArr = this.splitTextIntoArrayOfLines(fileData),
          matchingLines = {};

      _.forEach(fileDataArr, function (line, key) {
        if (line.indexOf(term) > -1) {
          matchingLines[key] = line;
        }
      });

      return matchingLines;
    },

    /**
     * Cycle through returned array of file contents
     * and summate the cases where instances were found
     * @param {string} dataArr Q.all() generated array of values (from array of promises)
     */
    outputMatchesForFile: function (filePath, matches) {
      var text = '';

      text += '\nMatches in:' + filePath + '\n\n';

      _.forEach(matches, function (match, term) {
        _.forEach(match, function (line, number) {
          text += number + '[' + term + ']:' + line + '\n';
        });
      });

      return text;

    },

    /**
     * Write results to a file or output in console, depending on existence of argument.
     * @param {string} text Body of text to write.
     */
    writeToFile: function (text) {
      if (_outputFile) {
        fs.writeFile(_outputFile, text, function (err) {
          console.log('Saved in ' + _outputFile);
        });
      } else {
        console.log(text);
      }
    },

    /**
     * Breaks multiline text into
     * @param {string} fileData File data to search through.
     * @param {string} term String to match.
     */
    splitTextIntoArrayOfLines: function (text) {
      return text ? text.split('\n') : undefined;
    }
  };

  returnable.init();

  return returnable;
};