将字符串分成两部分，长度几乎相同

var str = "Thisasdfasdfasdfasdfasdf is a sample string",
  len = str.length;

var ind = str.indexOf(' ', Math.floor(str.length / 2) - 1);
ind = ind > 0 ? ind : str.lastIndexOf(' ');

var str1 = str.substr(0, ind),
  str2 = str.substr(ind);

document.write(str1 + ' <br /> ' + str2);

function split(str, noOfWords) {
  // Set no. of words to 2 by default when nothing is passed
  noOfWords = noOfWords || 2;

  var len = str.length; // String length
  wordLen = Math.floor(len / noOfWords); // Approx. no. of letters in each worrd

  var words = [],
    temp = '',
    counter = 0;

  // Split the string by space and iterate over it
  str.split(' ').forEach(function(v) {
    // Recalculate the new word length
    wordLen = Math.floor((len - words.join(' ').length) / (noOfWords - counter));

    // Check if word length exceeds
    if ((temp + v).length < wordLen) {
      temp += ' ' + v;
    } else {
      // Add words in the array
      words.push(temp.trim());

      // Increment counter, used for word length calculation
      counter++;
      temp = v;
    }
  });

  // For the last element
  temp.trim() && words.push(temp.trim());
  return words;
}

var str = "This is a sample string. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Eos quae error ab praesentium, fugit expedita neque ex odio veritatis excepturi, iusto, cupiditate recusandae harum dicta dolore deleniti provident corporis adipisci.";

var words = split(str, 10);

console.log(words);
document.write('<pre>' + JSON.stringify(words, 0, 2) + '</pre>');