Use reduce to chunk flat array into pairs

make-pairs.js

[1,1,2,2,3,3,4,4,5].reduce( function(a, b, i) {
        if (i === 1) {
          return [[a, b]];
        } else {
          if (i % 2 === 0) {
            return a.concat([[b]]); 
          } else {
            return a.splice(0, a.length-1).concat([a[a.length-1].concat(b)]); 
          }
        }
      });