Question

/*
* Copy the enumerable properties of p to o, and return o.
* If o and p have a property by the same name, o's property is overwritten.
* This function does not handle getters and setters or copy attributes.
*/
 function extend(o, p) {
       for(prop in p) { // For all props in p.
           o[prop] = p[prop]; // Add the property to o.
      }
      return o;
}



/*
* Return a new object that holds the properties of both o and p.
* If o and p have properties by the same name, the values from o are used.
*/
     function union(o,p) { return extend(extend({},o), p); }

I think for union, he meant “values from p are used”.

I did the test on Chrome. Am I wrong? Sorry. I tend to be very caution when I am learning, especially this is the #1 book for Javascript, and 6ed is recent.

var o = {x:1}

var p = {x: 2}

function extend(o,p){

for(prop in p)
    o[prop] = p[prop];
return o;

}

function union(o,p){

return extend(extend({},o),p);

var g = union(o,p)

g.x

2

Thank you.

Answer 1

yeah it should read the properties from p are kept and o are overwritten.

Though when writing this code it is a little safer to do this:

for(var prop in obj) {
    if(obj.hasOwnProperty(prop)) {
        // now you know it is actually a property on obj and not inherited from elsewhere
    }
}