parens are for invoking a function. While it's possible that person1.age is a function that needs to be invoked, it's more likely to be a number.

In the question, I said it was a property, so we just look up the property using a dot. If the property happened to be a function, we could invoke it, but I didn't ask you to.

Yes. Unless you can't:

    var class_dict = {hermione: {hair: 'bushy brown', pet: 'Crookshanks'},
                      harry: {hair: 'black', pet: 'Hedwig'},
                      ron: {hair: 'red', pet: 'Scabbers'}};
    var name = prompt('person to look up? ');
    var person = class_dict[name];          // can't use dot here
    console.log(person.hair, person.pet);   // can use dot here

It just means that the programmer doesn't know what the key is when they are writing the program. An example is the previous case, where we are looking up a student in the class dictionary.

Good question. The Object.keys() function is similar to the Python .keys() method: it returns a list of the keys of the dictionary.

Given the class_dict above, we can get a list of students in the class like this:

    var students = Object.keys(class_dict);
    console.log(students);
    students.forEach(name => {
        let s = class_dict[name];
        console.log(`${name} has ${s.hair} hair and owns ${s.pet}`);
    });

It's rare to delete data, so while it's good to know about, it's less important .

Array methods handle missing elements in different ways:

    var scores = [ 91, 88, 72, 85, 92 ];
    delete scores[2];          // drop the 72
    y = scores.map(n => n+1);  // generates missing element
    scores.forEach(n => console.log(n));  // skips missing element

Loops handle them differently as well:

for( i in scores ) { console.log(scores[i]); }
for( i = 0; i < scores.length; i++ ) { console.log(scores[i]); }

pop and shift usually don't leave any holes in the array, so they are less of a concern.

Glad to hear it!