diff(v1, v2)

README.md

@@ -269,6 +269,9 @@ strings that they parse.
   `v2` is greater.  Sorts in ascending order if passed to `Array.sort()`.
 * `rcompare(v1, v2)`: The reverse of compare.  Sorts an array of versions
   in descending order when passed to `Array.sort()`.
+* `diff(v1, v2)`: Returns difference between two versions by the release type
+  (`major`, `premajor`, `minor`, `preminor`, `patch`, `prepatch`, or `prerelease`),
+  or null if the versions are the same.
 
 
 ### Ranges

semver.js

@@ -464,6 +464,33 @@ function inc(version, release, loose, identifier) {
   }
 }
 
+exports.diff = diff;
+function diff(version1, version2) {
+  if (eq(version1, version2)) {
+    return null;
+  } else {
+    var v1 = parse(version1);
+    var v2 = parse(version2);
+    if (v1.prerelease.length || v2.prerelease.length) {
+      for (var key in v1) {
+        if (key === 'major' || key === 'minor' || key === 'patch') {
+          if (v1[key] !== v2[key]) {
+            return 'pre'+key;
+          }
+        }
+      }
+      return 'prerelease';
+    }
+    for (var key in v1) {
+      if (key === 'major' || key === 'minor' || key === 'patch') {
+        if (v1[key] !== v2[key]) {
+          return key;
+        }
+      }
+    }
+  }
+}
+
 exports.compareIdentifiers = compareIdentifiers;
 
 var numeric = /^[0-9]+$/;

test/index.js

@@ -13,6 +13,7 @@ var lte = semver.lte;
 var satisfies = semver.satisfies;
 var validRange = semver.validRange;
 var inc = semver.inc;
+var diff = semver.diff;
 var replaceStars = semver.replaceStars;
 var toComparators = semver.toComparators;
 var SemVer = semver.SemVer;
@@ -411,6 +412,34 @@ test('\nincrement versions test', function(t) {
   t.end();
 });
 
+test('\ndiff versions test', function(t) {
+//  [version1, version2, result]
+//  diff(version1, version2) -> result
+  [['1.2.3', '0.2.3', 'major'],
+    ['1.4.5', '0.2.3', 'major'],
+    ['1.2.3', '2.0.0-pre', 'premajor'],
+    ['1.2.3', '1.3.3', 'minor'],
+    ['1.0.1', '1.1.0-pre', 'preminor'],
+    ['1.2.3', '1.2.4', 'patch'],
+    ['1.2.3', '1.2.4-pre', 'prepatch'],
+    ['0.0.1', '0.0.1-pre', 'prerelease'],
+    ['0.0.1', '0.0.1-pre-2', 'prerelease'],
+    ['1.1.0', '1.1.0-pre', 'prerelease'],
+    ['1.1.0-pre-1', '1.1.0-pre-2', 'prerelease'],
+    ['1.0.0', '1.0.0', null]
+
+  ].forEach(function(v) {
+    var version1 = v[0];
+    var version2 = v[1];
+    var wanted = v[2];
+    var found = diff(version1, version2);
+    var cmd = 'diff(' + version1 + ', ' + version2 + ')';
+    t.equal(found, wanted, cmd + ' === ' + wanted);
+  });
+
+  t.end();
+});
+
 test('\nvalid range test', function(t) {
   // [range, result]
   // validRange(range) -> result