How does javascript eval handle 'this'

I ran tests in Firefox 3.6.11, if that matters, an开发者_如何学Pythond eval misbehaves in the context of call and apply. It somehow jumps over the current 'this' object. Why?

dojo.provide("yal-js.tests.javascript");

function evaltest () {
    var dis=this;
    // it works now... returns 2 on call and apply
    return eval("(function() {return this.testValue;}).call(dis);");
    // this, however, didn't work: it returned 1, not 2
    //return eval("(function() {return this.testValue;})();");
}
function controltest() {
    return this.testValue;
}

var testValue=1;
var testObj={testValue: 2};

doh.register("tests.javascript",
    new TFRunGroup(

        ["direct",
            function () {doh.assertEqual(1,controltest());} ],
        ["call",
            function() {doh.assertEqual(2, controltest.call(testObj) );}],
        ["apply",
            function() {doh.assertEqual(2, controltest.apply(testObj) );}],
        ["eval direct",
            function () {doh.assertEqual(1,evaltest());} ],
        ["eval call",
            function() {doh.assertEqual(2, evaltest.call(testObj) );}],
        ["eval apply",
            function() {doh.assertEqual(2, evaltest.apply(testObj) );}]
        ));

this at javascript is an object that called function, when you use object.function() then this is object, when you use function.call(object,...) then this is object, when you use function.apply(object,...) then this is object, when you use new constructor(...) then this is new constructed object, otherwise this is the global object in brower it is window.

call takes in, as its first argument, the value of this (see here). So in the first line that works

return eval("(function() {return this.testValue;}).call(dis);");

you are passing it dis which points to evaltest, so this.testValue points to evaltest.testValue.

In the second line that doesn't work

return eval("(function() {return this.testValue;})();");

you are passing it null for this, so this.testValue is set to the window's this.testValue object.