fix issue #83 (Image Inside a Scrollable div Would not be Loaded)

src/echo.js

@@ -1,20 +1,11 @@
-(function (root, factory) {
-  if (typeof define === 'function' && define.amd) {
-    define(function() {
-      return factory(root);
-    });
-  } else if (typeof exports === 'object') {
-    module.exports = factory;
-  } else {
-    root.echo = factory(root);
-  }
-})(this, function (root) {
-
+(function() {
     'use strict';
 
     var echo = {};
+    var root = {};
 
-  var callback = function () {};
+    var callback = function () {
+    };
 
     var offset, poll, delay, useDebounce, unload;
 
@@ -44,6 +35,7 @@
 
     echo.init = function (opts) {
         opts = opts || {};
+        root = opts.viewport || this || (0,eval)('this')
         var offsetAll = opts.offset || 0;
         var offsetVertical = opts.offsetVertical || offsetAll;
         var offsetHorizontal = opts.offsetHorizontal || offsetAll;
@@ -77,8 +69,8 @@
         var view = {
             l: 0 - offset.l,
             t: 0 - offset.t,
-      b: (root.innerHeight || document.documentElement.clientHeight) + offset.b,
-      r: (root.innerWidth || document.documentElement.clientWidth) + offset.r
+            b: (root.innerHeight || root.clientHeight) + offset.b,
+            r: (root.innerWidth || root.clientWidth) + offset.r
         };
         for (var i = 0; i < length; i++) {
             elem = nodes[i];
@@ -129,6 +121,14 @@
         clearTimeout(poll);
     };
 
+    if ((typeof module !== "undefined" && module !== null) && module.exports) {
+        module.exports = echo;
+    } else if (typeof define === 'function') {
+        define(function() {
             return echo;
-
         });
+    } else {
+        root.echo = echo;
+    }
+}());
+