'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; };