src/arrays/pairs.js

d3.pairs = function(array) {
  var i = 0, n = array.length - 1, p0, p1 = array[0], pairs = new Array(n < 0 ? 0 : n);
  while (i < n) pairs[i] = [p0 = p1, p1 = array[++i]];
  return pairs;
};