Using JSON to create instances of an object

Question

I'm trying to learn to program. I've gone through a list of tutorials sites online and I'm stuck on a thing that I think is extremely important for me to understand.

My questions:

1. (This is what I'd like to understand most) In my for...in loop, why is creating new "obj" objects using the "Contacts" constructor working? It seems like I would need a different name each time I loop so that I don't overwrite the object that I've created the pass before. If this is correct, how do I do this if I don't know anything about the number or value of contacts ahead of time? Additionally, why does the title of the any of the objects in the console logs not say obj? Am I confused about what it means to create an instance of an object? Are the names of these instances unimportant?

2. Why are all of the properties undefined? Should referencing properties from the temporary "i" variable work?

Creating objects from an unknown total of data entries seems really important. Unfortunately, places like Codecademy fall short here. You always manually create new instances of objects with hardcoded names they give you. But what would happen if there were two of the same name?

Thanks so much for any help I may get on this. Don't hold back from telling me anything else silly that I may be doing.

Here is a link to a console screenshot - https://i.sstatic.net/uvova.png

var TestApp = {};

// my data... taken from wherever
TestApp.jsonContacts = {
    contact1: {
        name: "Ethan",
        age: 24
    },
    contact2: {
        name: "Evan",
        age: 30
    },
    contact3: {
        name: "Paul",
        age: 9000
    }
};

// I know this is silly, just let me pretend...
TestApp.jsonStrung = JSON.stringify(TestApp.jsonContacts);

TestApp.globalContactList = [];

// my constructor function to create instances of Contact
TestApp.Contact = function(name, age){
    this.name = name;
    this.age = age;
    TestApp.globalContactList.push(this);
};

// where I'm taking data and creating new Contact objects
TestApp.instantiateObjects = function(){
    // I know this is silly, just let me pretend...
    var jsonUnstrung = JSON.parse(TestApp.jsonStrung);
    // I think I'm looping through the first set of objects sitting in jsonContacts
    for (var i in jsonUnstrung) {
        var obj = new TestApp.Contact(i.name, i.age);
        console.log(obj);
    }
    console.log(TestApp.globalContactList);
};

TestApp.instantiateObjects();

Answer 1

In my for...in loop, why is creating new "obj" objects using the "Contacts" constructor working?

A variable is just a holding place for a value. It is not the value itself. If you overwrite a variable with a new value, the previous value it held will continue to exist as long as something is holding a reference to it; it's simply that the variable won't be referring to it anymore.

It seems like I would need a different name each time I loop so that I don't overwrite the object that I've created the pass before.

No, you don't. One variable is fine, as long as you do something with the value before you assign the variable to the next value.

If this is correct, how do I do this if I don't know anything about the number or value of contacts ahead of time?

It doesn't matter how many you have.

Additionally, why does the title of the any of the objects in the console logs not say obj?

console.log() prints out the value that is passed to it. It doesn't care (doesn't know) anything about the variable that you pass to it.

Am I confused about what it means to create an instance of an object?

It seems so. Creating an instance of an object allocates some memory to store that object's values. A variable allows you to gain access to that allocated object.

Are the names of these instances unimportant?

Yes, object instances don't have names.

Why are all of the properties undefined?

i holds the property names of the object you are iterating through, so in this case, the strings "contact1", "contact2", "contact3". These strings don't have a name or age property, so your constructor is receiving two undefined values.

Should referencing properties from the temporary "i" variable work?

You need to use i as a property name to access the property values:

var obj = new TestApp.Contact(jsonUnstrung[i].name, jsonUnstrung[i].age);

In general, it's a good idea not to have side-effects like TestApp.globalContactList.push(this); in a constructor. There may be cases where doing so makes sense, but most of the time, removing that line and doing this would be preferable:

for (var i in jsonUnstrung) {
    var contact = jsonUnstrung[i];
    var obj = new TestApp.Contact(contact.name, contact.age);
    console.log(obj);
    TestApp.globalContactList.push(obj);
}