title: how to find difference between two array using lodash/underscore in nodejs
body: <p>I have two arrays of arrays and am trying to find the difference.</p>
  
  <pre><code>var a = [[ 11, 24, 28, 38, 42, 44 ],
    [ 7, 19, 21, 22, 29, 38 ],
    [ 2, 21, 27, 30, 33, 40 ],
    [ 6, 11, 12, 21, 34, 48 ],
    [ 1, 10, 17, 31, 35, 40 ],
    [ 1, 18, 26, 33, 36, 45 ],
    [ 15, 21, 22, 24, 38, 46 ],
    [ 5, 17, 21, 27, 29, 41 ],
    [ 3, 7, 12, 16, 20, 28 ],
    [ 9, 12, 13, 18, 30, 37 ],
    [ 3, 19, 21, 31, 33, 46 ],
    [ 6, 11, 16, 18, 20, 34 ],
    [ 1, 3, 11, 13, 24, 28 ],
    [ 12, 13, 16, 40, 42, 46 ],
    [ 1, 3, 5, 36, 37, 41 ],
    [ 14, 15, 23, 24, 26, 31 ],
    [ 7, 13, 14, 15, 27, 28 ]];
  
  var b = [[ 4, 7, 9, 21, 31, 36 ],
    [ 2, 5, 6, 12, 15, 21 ],
    [ 4, 7, 8, 15, 38, 41 ],
    [ 11, 24, 28, 38, 42, 44 ],
    [ 7, 19, 21, 22, 29, 38 ]];
  </code></pre>
  
  <p>How would I find:</p>
  
  <pre><code>c = [[ 2, 21, 27, 30, 33, 40 ],
    [ 6, 11, 12, 21, 34, 48 ],
    [ 1, 10, 17, 31, 35, 40 ],
    [ 1, 18, 26, 33, 36, 45 ],
    [ 15, 21, 22, 24, 38, 46 ],
    [ 5, 17, 21, 27, 29, 41 ],
    [ 3, 7, 12, 16, 20, 28 ],
    [ 9, 12, 13, 18, 30, 37 ],
    [ 3, 19, 21, 31, 33, 46 ],
    [ 6, 11, 16, 18, 20, 34 ],
    [ 1, 3, 11, 13, 24, 28 ],
    [ 12, 13, 16, 40, 42, 46 ],
    [ 1, 3, 5, 36, 37, 41 ],
    [ 14, 15, 23, 24, 26, 31 ],
    [ 7, 13, 14, 15, 27, 28 ]];
  </code></pre>
  
  <p>I had tried underscore:</p>
  
  <pre><code>_ = require('underscore');
  _.difference(a,b);
  </code></pre>
  
  <p>But it doesn't work.</p>
  
  <p>I also tried lodash:</p>
  
  <pre><code>_ = require('lodash');
  _.differenceBy(a,b);
  </code></pre>
  
  <p>but it doesn't work either.</p>
  
  <p>What am I doing wrong here?</p>