maxDifferenceBefore.js

function biggestGap(values) {
  
  function diff(curr, before) {
    return curr - Math.min.apply(null, before);
  }
  
  function diffs(values) {
    return values.map(function(x, i, ar) {
      return diff(x, ar.slice(0, i));
    });
  }
  
  return Math.max.apply(null, diffs(values));
}

// OR Thanks David Chambers
// max :: [a] -> a
const max = Function.apply.bind(Math.max, Math);

// biggestGap :: [Number] -> Number
const biggestGap = xs => max(xs.map((x, idx, xs) => max(xs.slice(idx)) - x));

Finds the biggest gap between elements in an array. Where the smaller element must be in front of the larger.