validate iat param and document the function

lib/psSupported.js

@@ -1,3 +1,9 @@
 var semver = require('semver');
 
+/**
+ * Checks if the current Node.js version is supported by this library.
+ *
+ * @returns {boolean}
+ */
+
 module.exports = semver.satisfies(process.version, '^6.12.0 || >=8.0.0');

lib/timespan.js

@@ -1,18 +1,32 @@
 var ms = require('ms');
 
+/**
+ *
+ * - Returns the timespan of *time* from *iat*
+ * - if *iat* is undefined it return the timespan of time from current time
+ * - if params are invalid, it returns undefined
+ *
+ * @param {string|number} time -> 1m,1h...
+ * @param {number} iat -> in ms
+ * @returns {number} in s
+ *
+ */
 module.exports = function (time, iat) {
-  var timestamp = iat || Math.floor(Date.now() / 1000);
+
+  if (typeof iat != 'number')
+    return
+
+  var timestamp = iat || Math.floor(Date.now() / 1000)
 
   if (typeof time === 'string') {
-    var milliseconds = ms(time);
-    if (typeof milliseconds === 'undefined') {
-      return;
-    }
-    return Math.floor(timestamp + milliseconds / 1000);
+    var milliseconds = ms(time)
+    if (typeof milliseconds === 'undefined')
+      return
+    return Math.floor(timestamp + milliseconds / 1000)
   } else if (typeof time === 'number') {
-    return timestamp + time;
+    return timestamp + time
   } else {
-    return;
+    return
   }
 
 };