js definitive guide 6th -- ch8.2.2

gistfile1.js

'use strict';

var o = { 
    m: function(){
        var self = this; //save this value in a variable
        console.log(this === o); //true
        f();

        function f(){
            console.log(this === o); //false
            console.log(this); //undefined
            console.log(self === o); //true
        }   
    }   
};

o.m();