markdown-it/lib/rules_inline/newline.js

// Proceess '\n'

module.exports = function escape(state) {
  var pmax, max, pos = state.pos;

  if (state.src.charCodeAt(pos) !== 0x0A/* \n */) { return false; }

  pmax = state.pending.length - 1;
  max = state.posMax;

  // '  \n' -> hardbreak
  // Lookup in pending chars is bad practice! Don't copy to other rules!
  // Pending string is stored in concat mode, indexed lookups will cause
  // convertion to flat mode.
  if (pmax >= 0 && state.pending.charCodeAt(pmax) === 0x20) {
    if (pmax >= 1 && state.pending.charCodeAt(pmax - 1) === 0x20) {
      state.pending = state.pending.replace(/ +$/, '');
      state.push({
        type: 'hardbreak',
        level: state.level
      });
    } else {
      state.pending = state.pending.slice(0, -1);
      state.push({
        type: 'softbreak',
        level: state.level
      });
    }

  } else {
    state.push({
      type: 'softbreak',
      level: state.level
    });
  }

  pos++;

  // skip heading spaces for next line
  while (pos < max && state.src.charCodeAt(pos) === 0x20) { pos++; }

  state.pos = pos;
  return true;
};