JavaScript - cannot set property of undefined

Question

My code:

var a = "1",
b = "hello",
c = { "100" : "some important data" },
d = {};

d[a]["greeting"] = b;
d[a]["data"] = c;

console.debug (d);

I get the following error:

Uncaught TypeError: Cannot set property 'greeting' of undefined.

I'm trying to do something similar to an associative array. Why isn't this working?

Answer 1

you never set d[a] to any value.

Because of this, d[a] evaluates to undefined, and you can't set properties on undefined.

If you add d[a] = {} right after d = {} things should work as expected.

Alternatively, you could use an object initializer:

d[a] = {
    greetings: b,
    data: c
};

Or you could set all the properties of d in an anonymous function instance:

d = new function () {
    this[a] = {
        greetings: b,
        data: c
    };
};

---

If you're in an environment that supports ES2015 features, you can use computed property names:

d = {
  [a]: {
    greetings: b,
    data: c
  }
};

Answer 2

You have to set d[a] to either an associative array, or an object:

• d[a] = [];
• d[a] = {};

---

Without setting, this is what's happening:

d[a] == undefined, so you're doing undefined['greeting']=b; and by definition, undefined has no properties. Thus, the error you received.

Answer 3

The object stored at d[a] has not been set to anything. Thus, d[a] evaluates to undefined. You can't assign a property to undefined :). You need to assign an object or array to d[a]:

d[a] = [];
d[a]["greeting"] = b;

console.debug(d);