JavaScript - cannot set property of undefined
JavascriptJavascript Problem Overview
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?
Javascript Solutions
Solution 1 - Javascript
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
}
};
Solution 2 - Javascript
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.
Solution 3 - Javascript
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);
Solution 4 - Javascript
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
Solution 5 - Javascript
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);
Solution 6 - Javascript
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
Solution 7 - Javascript
> 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:''
}