1  const parse = require('./parse')
 2  const eq = require('./eq')
 3  
 4  const diff = (version1, version2) => {
 5    if (eq(version1, version2)) {
 6      return null
 7    } else {
 8      const v1 = parse(version1)
 9      const v2 = parse(version2)
10      const hasPre = v1.prerelease.length || v2.prerelease.length
11      const prefix = hasPre ? 'pre' : ''
12      const defaultResult = hasPre ? 'prerelease' : ''
13      for (const key in v1) {
14        if (key === 'major' || key === 'minor' || key === 'patch') {
15          if (v1[key] !== v2[key]) {
16            return prefix + key
17          }
18        }
19      }
20      return defaultResult // may be undefined
21    }
22  }
23  module.exports = diff