JavaScript - this understanding

//nodejs中的this和在浏览器中javascript中的this是不一样
//nodejs在函数中this指向的是global对象,全局中的this默认是一个空对象
//浏览器中this指向Windows

var a ='a in global';

function testThis() {
    this.a='a in testThis()';
    this.say = function(){
        //var that =this;
        //console.dir(that);
        console.log(this.a);

        function m(){
            //console.dir(that);
            //console.dir(this);
            console.log(this.a);
        }

        m();
        //this.m();
        //m.call(this);
    }
}

//test
var test = new testThis();
test.say();
console.log(this.a);
testThis();
console.log(this.a);

function m(){
    console.dir(this);
}

console.dir(this);

m();