Assume that not explicitly mentioned lower versions are unsupported.

lib/preprocessCaniuseData.js

@@ -22,13 +22,17 @@ module.exports = function preprocessCaniuseData(stats) {
     Object.keys(stats).forEach(caniuseId => {
         let highestVersionSeen;
         let highestVersionIsSupported;
+        let lowestVersionSeen;
 
-        function registerHighVersion(version, isSupported) {
+        function registerVersion(version, isSupported) {
             version = addZeroesIfMinorOrPatchIsMissing(version);
             if (!highestVersionSeen || semver.gt(version, highestVersionSeen)) {
                 highestVersionSeen = version;
                 highestVersionIsSupported = isSupported;
             }
+            if (!lowestVersionSeen || semver.lt(version, lowestVersionSeen)) {
+                lowestVersionSeen = version;
+            }
         }
 
         isSupportedByCaniuseIdAndVersion[caniuseId] = {};
@@ -39,11 +43,11 @@ module.exports = function preprocessCaniuseData(stats) {
             } else if (/^\d+(\.\d+){0,2}$/.test(version)) {
                 // Exact major or major.minor or major.minor.patch version
                 isSupportedByCaniuseIdAndVersion[caniuseId][version] = isSupported;
-                registerHighVersion(version, isSupported);
+                registerVersion(version, isSupported);
             } else {
                 const matchRange = version.match(/^(\d+(?:\.\d+){0,2})-(\d+(?:\.\d+){0,2})$/);
                 if (matchRange) {
-                    registerHighVersion(matchRange[2], isSupported);
+                    registerVersion(matchRange[2], isSupported);
                     const range = new semver.Range(matchRange[1] + ' - ' + matchRange[2]);
                     (checkSupportedRangeByCaniuseId[caniuseId] = checkSupportedRangeByCaniuseId[caniuseId] || []).push((major, minor, patch) => {
                         if (range.test(buildVersion(major, minor, patch))) {
@@ -62,6 +66,13 @@ module.exports = function preprocessCaniuseData(stats) {
                 }
             });
         }
+        if (lowestVersionSeen) {
+            (checkSupportedRangeByCaniuseId[caniuseId] = checkSupportedRangeByCaniuseId[caniuseId] || []).push((major, minor, patch) => {
+                if (semver.lt(buildVersion(major, minor, patch), highestVersionSeen)) {
+                    return false;
+                }
+            });
+        }
     });
 
     return function checkSupported(caniuseId, major, minor, patch) {

test/index.js

@@ -42,14 +42,14 @@ describe('with an empty policy', function () {
     });
 });
 
-describe('with a browser that caniuse-db has data about in a version that is not explicitly mentioned', function () {
+describe('with a browser that caniuse-db has data about in a version that is older than all the explicitly mentioned ones', function () {
     // Chrome 1
     beforeEach(() => {
         userAgentString = 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/525.19 (KHTML, like Gecko) Chrome/1.0.154.36 Safari/525.19';
     });
 
-    it('should leave the CSP string unchanged', function () {
-        return expect('script-src somewhere.com/with/a/path', 'to come out as', 'script-src somewhere.com/with/a/path');
+    it('should assume that CSP is not supported and strip the header', function () {
+        return expect('script-src somewhere.com/with/a/path', 'to come out as', undefined);
     });
 });
 

test/preprocessCaniuseData.js

@@ -42,12 +42,12 @@ describe('preprocessCaniuseData', function () {
 
     it('should let exact major.minor versions through', function () {
         expect({ie: { '11.1': 'y' }}, 'to support', 'ie', 11, 1)
-            .and('to be undecided about', 'ie', 11, 0);
+            .and('not to support', 'ie', 11, 0);
     });
 
     it('should let exact major.minor.patch versions through', function () {
         expect({ie: { '11.1.2': 'y' }}, 'to support', 'ie', 11, 1, 2)
-            .and('to be undecided about', 'ie', 11, 1);
+            .and('not to support', 'ie', 11, 1);
     });
 
     it('should let exact major versions through', function () {
@@ -57,14 +57,14 @@ describe('preprocessCaniuseData', function () {
     it('should explode version ranges spanning several minor versions', function () {
         expect({ie: { '10.1-10.2': 'y' }}, 'to support', 'ie', 10, 1)
             .and('to support', 'ie', 10, 2)
-            .and('to be undecided about', 'ie', 10, 0);
+            .and('not to support', 'ie', 10, 0);
     });
 
     it('should explode version ranges spanning several major and minor versions', function () {
         expect({ie: { '10.3-12.1': 'y' }}, 'to support', 'ie', 10, 3)
             .and('to support', 'ie', 10, 20)
             .and('to support', 'ie', 10, 20, 4)
-            .and('to be undecided about', 'ie', 10, 0)
+            .and('not to support', 'ie', 10, 0)
             .and('to support', 'ie', 11, 0)
             .and('to support', 'ie', 11, 20)
             .and('to support', 'ie', 12, 0)
@@ -76,7 +76,7 @@ describe('preprocessCaniuseData', function () {
             .and('to support', 'ie', 11)
             .and('not to support', 'ie', 10, 0)
             .and('not to support', 'ie', 10, 1)
-            .and('to be undecided about', 'ie', 10, 2)
+            .and('not to support', 'ie', 10, 2)
             .and('to support', 'ie', 10, 3)
             .and('to support', 'ie', 10, 20)
             .and('to support', 'ie', 11, 0)
@@ -127,6 +127,48 @@ describe('preprocessCaniuseData', function () {
         });
     });
 
+    describe('when there is only data about newer versions of the browser being queried', function () {
+        describe('when the lowest known version is given as major', function () {
+            it('should assume that the older version is not supported when the higher version is not', function () {
+                expect({ie: { '10': 'n' }}, 'not to support', 'ie', 9);
+            });
+
+            it('should assume that the older version is not supported when the higher version is', function () {
+                expect({ie: { '10': 'y' }}, 'not to support', 'ie', 9);
+            });
+        });
+
+        describe('when the highest known version is given as major.minor', function () {
+            it('should assume that the older version is not supported when the higher version is not', function () {
+                expect({ie: { '10.5': 'n' }}, 'not to support', 'ie', 9);
+            });
+
+            it('should assume that the older version is not supported when the higher version is', function () {
+                expect({ie: { '10.5': 'y' }}, 'not to support', 'ie', 9);
+            });
+        });
+
+        describe('when the highest known version is given as major.minor.patch', function () {
+            it('should assume that the older version is not supported when the higher version is not', function () {
+                expect({ie: { '10.5.8': 'n' }}, 'not to support', 'ie', 9);
+            });
+
+            it('should assume that the older version is not supported when the higher version is', function () {
+                expect({ie: { '10.5.8': 'y' }}, 'not to support', 'ie', 9);
+            });
+        });
+
+        describe('when the highest known version is given as a range', function () {
+            it('should assume that the older version is not supported when the higher version is not', function () {
+                expect({ie: { '10.5.2-10.5.8': 'n' }}, 'not to support', 'ie', 9);
+            });
+
+            it('should assume that the older version is not supported when the higher version is', function () {
+                expect({ie: { '10.5.2-10.5.8': 'y' }}, 'not to support', 'ie', 9);
+            });
+        });
+    });
+
     describe('when there is no data about the browser being queried', function () {
         it('should return undefined', function () {
             expect({}, 'to be undecided about', 'ie', 10, 0);