Question

I am trying to understand something very basic. If I have an object like this:

var topics = {}

And I do this:

topics[name] = ["chapter 1", "chapter 2", "chapter 3"];

When I log this object, I don’t see the name attribute. What have I done exactly? Have I created a key called name with the value of an array?

Of course I know I can do that by just doing

topics.name =  ["chapter 1", "chapter 2", "chapter 3"];

But then what is this doing?

topics[name] = ["chapter 1", "chapter 2", "chapter 3"];

Could someone please clarify?

Answer 1

When you use the [] notation it expects an expression in between, that translates to a string.

Using name not enclosed in quotes 'name' it assumes you are using a variable called name.

Which in your case is undefined.

The correct usage would be

topics["name"] = ["chapter 1", "chapter 2", "chapter 3"];

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If you want to use a variable you can do things like

var prop = 'name';
var topics = {};

topics[prop] = ["chapter 1", "chapter 2", "chapter 3"];

this will create the name property on the object.. useful for dynamic/automatic creation/filling of objects..