what does the this keyword do in javascript code example

Example 1: js this in object

var wF = {
    w : 560,
    h : function() { return (312 - 42) / (560 / this.w) + 42; }
};

alert(wF.h())

Example 2: this javascript

// Im Webbrowser ist das window Objekt das globale Objekt:
console.log(this === window); // true

a = 37;
console.log(window.a); // 37

this.b = "MDN";
console.log(window.b);  // "MDN"
console.log(b);         // "MDN"