setTimeout is a method of window object. “this” context refer to the window. But you can change its context in the following way. setTimeout does not support call method.

setTimeout(function() { alert(this); }, 0); //window
//{}, function() { alert(this); }, 0); //not supported
setTimeout((function() { alert(this); }).call({}), 0); //object 

eval is also a method of Global object, in the case of browse, this is window object. But is context defined by its Containing context. Eval also does not support call method

function Foo() {
    this.TestContext = function() {
        eval("alert(this==window);"); //show false
        setTimeout(function() { alert(this == window); }, 0);  //show true

var f = new Foo();

eval("alert(this);");{}, "alert(this);"); 
//firefox does not support this, but IE support, but it does not change context