Trying to understand JS concurrency

Sorella's JS

function sum(x, cc) {
    setTimeout(function() {
        cc(x.l + x.r);
    }, 100);
}

var a = {
        l: 1,
        r: 3
    },
    b = {
        l: 4
    };
    
var A = sum(a, function(r) {
    sum({
        l: b.l,
        r: r
    }, print);
});

var B = setTimeout(function() {
    a.l = 3;
}, 10);

var C = setTimeout(function() {
    b.l = 6;
}, 100);