Explaining dictionary merges

dict-merge.js

var obj  = {
    child : {
        a : '0',
        b : '2'
    }
};
var extra = {
    b : '3'
};

obj.child = {
    // The ... operator "spreads" a dictionary, and as such merges it into the dictionary currently being defined
    ...(
        // This evaluates to the existing nested object, or an empty object if on doesn't exist 
        obj.child || {}
    ),
    ...extra
};

console.log(obj.child.b);
// 3