Added cron expression validation

no issue

- CRON format is the most common one used for job scheduling and is well known to most developers
- This will become one of supported formats for job scheduling

ghost/job-manager/lib/is-cron-expression.js

@@ -0,0 +1,32 @@
+const parser = require('cron-parser');
+
+/**
+ * Checks if expression follows supported CRON format as follows
+ * e.g.:
+ *  "2 * * * *" where:
+ *
+ *  "*    *    *    *    *    *"
+ *   ┬    ┬    ┬    ┬    ┬    ┬
+ *   │    │    │    │    │    |
+ *   │    │    │    │    │    └ day of week (0 - 7) (0 or 7 is Sun)
+ *   │    │    │    │    └───── month (1 - 12)
+ *   │    │    │    └────────── day of month (1 - 31)
+ *   │    │    └─────────────── hour (0 - 23)
+ *   │    └──────────────────── minute (0 - 59)
+ *   └───────────────────────── second (0 - 59, optional)
+ *
+ * @param {String} expression in CRON format
+ *
+ * @returns {boolean} wheather or not the expression is valid
+ */
+const isCronExpression = (expression) => {
+    try {
+        parser.parseExpression(expression);
+
+        return true;
+    } catch (err) {
+        return false;
+    }
+};
+
+module.exports = isCronExpression;

ghost/job-manager/package.json

@@ -24,6 +24,7 @@
     "sinon": "9.2.1"
   },
   "dependencies": {
+    "cron-parser": "2.17.0",
     "fastq": "1.9.0",
     "p-wait-for": "3.1.0"
   }

ghost/job-manager/test/is-cron-expression.test.js

@@ -0,0 +1,19 @@
+// Switch these lines once there are useful utils
+// const testUtils = require('./utils');
+require('./utils');
+
+const isCronExpression = new require('../lib/is-cron-expression');
+
+describe('Is cron expression', function () {
+    it('valid cron expressions', function () {
+        should(isCronExpression('* * * * *')).be.true();
+        should(isCronExpression('1 * * * *')).be.true();
+        should(isCronExpression('* * 12-15 * * *'), 'Range should be 0-23').be.true();
+    });
+
+    it('invalid cron expressions', function () {
+        should(isCronExpression('123 * * * *')).not.be.true();
+        should(isCronExpression('a * * * *')).not.be.true();
+        should(isCronExpression('* * 12-36 * * *'), 'Invalid range should be 0-23').not.be.true();
+    });
+});