A functional javascript approach to a basic markdown parser. Inspired by a real interview question.

BasicMarkdownParserExample.js

var _ = require('underscore')
var lines = [
  "#The test",
  "###Hello World",
  "This is a new paragraph.",
  "It has a second line",
  "",
  "This is my new paragraph"
];

var htmlTemplate = _.template("<<%= tag %>><%= content %></<%= tag %>>");

function getFirstCharacter (line) { 
  return line[0];
}

function getHeadingLevel (line) { 
  var level = 0;
  _.every(line, function (letter) {
    if(_.isEqual(letter, "#")) {
      level++;
      return true;
    }
    return false;
  });
  return level;
};

function parseHeader (line) { 
  var level = getHeadingLevel(line);
  return htmlTemplate({
    tag: "h" + level, 
    content: line.substring(level, line.length)
  });
};

function parseNewLine (line) {
  return "<br />";
};

function parseParagraph (line) {
  return "";
};

function parse (line) {
  switch (getFirstCharacter(line)) {
          case '#':
          return parseHeader(line);
          case undefined:
          return parseNewLine(line);
          default:
          return parseParagraph(line);
  }
}

function appendLineToBuffer (line, buffer) {
  if(_.isEmpty(buffer)) {
    return line;
  }
  return buffer + ' ' + line;
}

function addOutstandingBufferToOutput (buffer, output) {
  if(!_.isEmpty(buffer)) {
    var newOutput = _.clone(output);
    newOutput.push(htmlTemplate({
      tag: "p", 
      content: buffer
    }));
    return newOutput;
  } else {
    return output;
  }
}

function parseMarkdown (lines) {
  var buffer = "";
  var result = _.reduce(lines, function (output, line) {
    var newData = parse(line);
    if(!_.isEmpty(newData)) {
      var newOutput = addOutstandingBufferToOutput(buffer, output);
      buffer = "";
      newOutput.push(newData);
      return newOutput;
    } else {
      buffer = appendLineToBuffer(line, buffer);
      return output;
    }
  }, []);
  return addOutstandingBufferToOutput(buffer, result).join("\n");
}

console.log(parseMarkdown(lines));