const parse = require('./parse') const eq = require('./eq') const diff = (version1, version2) => { if (eq(version1, version2)) { return null } else { const v1 = parse(version1) const v2 = parse(version2) const hasPre = v1.prerelease.length || v2.prerelease.length const prefix = hasPre ? 'pre' : '' const defaultResult = hasPre ? 'prerelease' : '' for (const key in v1) { if (key === 'major' || key === 'minor' || key === 'patch') { if (v1[key] !== v2[key]) { return prefix + key } } } return defaultResult // may be undefined } } module.exports = diff