convertToJSON-async.js

const fs = require('fs');

module.exports = {
  convertToJson: (filepath, options, callback) => {
    callback && fs.readFile(filepath, (err, buf) => {
      if (err) { callback(err) }
      callback(null, String(buf).split("\n").map(line => {
        const [glyph, ruby] = line.split(options.separator);
        return { glyph, ruby };
      }));
    })
  }
};

convertToJSON-sync.js

const fs = require('fs');

module.exports = {
  convertToJson: (filepath, options) =>
    String(fs.readFileSync(filepath)).split("\n").map(line => {
      const [glyph, ruby] = line.split(options.separator);
      return { glyph, ruby };
    })
};

still get undefined result with the sync.

test("extractPronunciation()", t => {
  const filepath = "./src/codepoint-ruby.tsv";
  let data = [];
  data = dataminer.convertToJson(filepath, {
    headers: ["glyph", "ruby"],
    separator: "\t"
  });
  console.log(data);
});

I think your issue might comes from something else, here's the output when running it into a console:

In that case p is an absolute path to the tsv file, not a relative one