无法将属性设置为对象

Question

var data = {};


....

data[someprop][someotherprop] = 'bla bla'

I get

Uncaught TypeError: Cannot read property ... of undefined and Cannot set property of...

Yes, the object doesn't have these properties ..yet, but I'm setting them in that line where I get the error lol. So what's up with that?

Answer 1

You are trying to assign a property to an object that doesn't exist, running that statement is equivalent to

data.someprop.someotherprop the parser won't automatically create data.someprop for you, hence the error.

You have to initialize someprop

data.someprop = {};
data['someprop']['someotherprop'] = 'gw ganteng';

Answer 2

You need to set the properties in diffrent steps like this:

var data = {};


data[someprop] = {}; 
data[someprop][someotherprop] = 'bla bla';

The object data will not have a property named "someprop" before you asssign it.

Answer 3

var data = {};
data[someprop] = {someotherprop:'bla bla'};

otherwise data[someprop] will be undefined

Answer 4

You need to create data[someprop] first:

var data = {};
data[someprop] = {};
data[someprop][someotherprop] = 'bla bla'

http://jsfiddle.net/infernalbadger/zXETB/

Answer 5

if someprop and someotherprop are variables, Richard D's answer is correct, if they are hardcoded strings, you could write your code like this:

var data = {someprop:{someotherprop: 'value'}};

if, however, they are values stored in variables, and you really - really want to cram everything into a single line, try something like this:

var data = {window.someprop : {window.someotherprop: 'value'}};

where window can be replaced by any object, if the someprop value is itself a property of an object:

var data = {someForm.name : { formElement.name : formElement.value}};

Or jQuery objects, since your tag suggest you're using jQuery:

var data= {$('#formId').attr('name'):{$('#formSelect').attr('name'): $('#formSelect').val()}};

Note: the jQuery example is not to be recommended, as the id selector is used twice on the same element. jQuery will scan the DOM-tree twice, so you're better of storing your that #formSelect element in a variable somewhere.