hard binding(explicit binding)
<!DOCTYPE html>
<html>
<head>
<script>
function foo() {
console.log(this.bar);
}
var obj = { bar: "bar" };
var obj2 = { bar: "bar2" };
var orig = foo;
foo = function(){ orig.call(obj); };
foo();//"bar"
foo.call(obj2);//"bar"
</script>
</head>
<body>
<!-- page content -->
</body>
</html>