markdown-it/lib/rules_inline/text_collapse.js

// Merge adjacent text nodes into one, and re-calculate all token levels
//
'use strict';


module.exports = function text_collapse(state) {
  var curr, last,
      level = 0,
      tokens = state.tokens,
      max = state.tokens.length;

  for (curr = last = 0; curr < max; curr++) {
    // re-calculate levels
    level += tokens[curr].nesting;
    tokens[curr].level = level;

    if (tokens[curr].type === 'text' &&
        curr + 1 < max &&
        tokens[curr + 1].type === 'text') {

      // collapse two adjacent text nodes
      tokens[curr + 1].content = tokens[curr].content + tokens[curr + 1].content;
    } else {
      if (curr !== last) { tokens[last] = tokens[curr]; }

      last++;
    }
  }

  if (curr !== last) {
    tokens.length = last;
  }
};