add: support indexOf for IE8. refactor: indexOf and splice.

src/echo.js

@@ -1,21 +1,57 @@
+// support Array.indexOf for IE8 : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf
+if (!Array.prototype.indexOf) {
+  Array.prototype.indexOf = function (searchElement , fromIndex) {
+    var i,
+        pivot = (fromIndex) ? fromIndex : 0,
+        length;
+
+    if (!this) {
+      throw new TypeError();
+    }
+
+    length = this.length;
+
+    if (length === 0 || pivot >= length) {
+      return -1;
+    }
+
+    if (pivot < 0) {
+      pivot = length - Math.abs(pivot);
+    }
+
+    for (i = pivot; i < length; i++) {
+      if (this[i] === searchElement) {
+        return i;
+      }
+    }
+    return -1;
+  };
+}
+
+
 window.Echo = (function (window, document, undefined) {
 
   'use strict';
 
   var store;
+  
+  var _windowHeight = function() {
+    return window.innerHeight || document.documentElement.clientHeight;
+  }
 
   var _inView = function (img) {
     var coords = img.getBoundingClientRect();
-    return (coords.top >= 0 && coords.left >= 0 && coords.top) <= (window.innerHeight || document.documentElement.clientHeight);
+    return (coords.top >= 0 && coords.left >= 0 && coords.top) <= _windowHeight();
   };
+  
 
   var _pollImages = function () {
     for (var i = 0; i < store.length; i++) {
       var self = store[i];
       if (_inView(self)) {
         self.src = self.getAttribute('data-echo');
-        if ([].indexOf && [].slice.call(store).indexOf(self) !== -1) {
-          [].slice.call(store).splice(i, 1);
+        if (store.indexOf(self) !== -1) {
+          store.splice(i, 1);
         }
       }
     }