How to add properties from this object to another object?

Question

Given

var obj1 = {
  a: \'cat\'
  b: \'dog\'
};
var obj2 = {
  b: \'dragon\'
  c: \'cow\'
};

How can I add properties from obj2 to

Answer 1

Just check if it exists first and if it doesn't add it!

for (key in obj2) {
    if (!obj1[key]) obj1[key] = obj2[key]
}

As RobG pointed out in the comments, this won't work if your values are falsey (0, false, undefined, null, '', etc.) and it will skip past them. If you are always using strings, like in your example it will be ok, but you might as well be safe and comprehensive:

for (key in obj2) {
    if (!(obj1.hasOwnProperty(key))) {
        obj1[key] = obj2[key]
    }
}