Javascript : Guess output of the code

Guess output of the code.

Code:

//first question

var obj = {

    a: 1,

    b: 2,

    add: function () {

        return this.a + this.b;

    }

}

var r = obj.add();

console.log("r=" + r);

//second question

var obj1 = {

    a: 1,

    b: 2,

    add: function () {

        return this.a + this.b;

    }

}

var s = obj1.add;

console.log("s=" + s());

//third question

var s1 = obj1.add;

s2 = s1.bind(obj1);

console.log("s2=" + s2());

Output:

        r=3

        s=NaN

s2=3

Notes:

wrt obj1.add here s contains the function, but the connection

to obj1 is not retained.In non-strict mode, JavaScript sets: this = globalThis