JavaScript - cannot set property of undefined

My code:

var a = "1",
b = "hello",
c = { "100" : "some important da开发者_JS百科ta" },
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?

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
  }
};

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.

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);

In ES6 you can use the spread operator, if d[a] is undefined it will just ignore, but if it has content it will keep it

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

The accepted answer will override anything in d, which is not the behaviour we would always want

In javascript almost everything is an object, null and undefined are exception.

Instances of Array is an object. so you can set property of an array, for the same reason,you can't set property of a undefined, because its NOT an object

i'd just do a simple check to see if d[a] exists and if not initialize it...

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

    if (d[a] === undefined) {
        d[a] = {}
    };
    d[a]["greeting"] = b;
    d[a]["data"] = c;

    console.debug (d);

d = {} is an empty object right now.

And d[a] is also an empty object.

It does not have any key values. So you should initialize the key values to this.

d[a] = {
greetings:'',
data:''
}

This occurs due to setting value to an object property without properly defining it's structure.

eg: The following code will give such error, because we are assigning value to an array. That can't be possible.

let boatNames = [];
 boats.map(boat => {
    while( i < boats.length){
      boatNames[i].name = boat.boat.name;
      i++;
    }
  })

To make it work, before setting some value to a property (boatNames[i].name = boat.boat.name;) make sure you define it first. like this... boatNames[i] = {};