--- /dev/null
+'use strict';
+var semver = require('semver');
+
+module.exports = function (a, b) {
+ if (semver.gt(a, b)) {
+ return null;
+ }
+
+ a = semver.parse(a);
+ b = semver.parse(b);
+
+ for (var key in a) {
+ if (key === 'major' || key === 'minor' || key === 'patch') {
+ if (a[key] !== b[key]) {
+ return key;
+ }
+ }
+
+ if (key === 'prerelease' || key === 'build') {
+ if (JSON.stringify(a[key]) !== JSON.stringify(b[key])) {
+ return key;
+ }
+ }
+ }
+
+ return null;
+};