Adding a simple nested field to object

Question

I know this is very basic but i just can't solve it. I simply need to add a field to an object inside an object.

   var shipping = {}
   shipping["only"]["price"]  = $(this).val();

Keep saying that i can't set price of undefined.

Everywhere i checked, i am allowed to do this nested assignment.

means shipping["only"] is undefined. And looking at your shipping object, there is no properties in it. — epascarello
– epascarello, Commented May 26, 2020 at 18:50
try this :var shipping = {only:{}} shipping["only"]["price"] = $(this).val(); — Rahul
– Rahul, Commented May 26, 2020 at 18:57
What is the problem with shipping.only = {price : $(this}.val()}; ? — Vahid Alimohamadi
– Vahid Alimohamadi, Commented May 26, 2020 at 19:02
@VahidAlimohamadi because i dont want to override .only, it might be already exist, or might not, and i want to update it. — paul seems
– paul seems, Commented May 28, 2020 at 8:49
so you can: shipping.only = { ...shipping.only, price: $(this).val() } — Vahid Alimohamadi
– Vahid Alimohamadi, Commented May 28, 2020 at 10:09

Jacob · Accepted Answer · 2020-05-26 18:58:32Z

1

You've got the semantics wrong for what your code means. This:

shipping["only"]["price"] = $(this).val();

...sort of translates to:

Evaluate $(this).val() (we'll call this R for the sake of convenience)
Evaluate shipping["only"] (we'll call this O)
Assign the price property of O to the value R.

On step 2, because shipping is the value {}, shipping["only"] is undefined, and this is why you get the error that it can't assign to the property price of undefined.

If your intent is to create an object only property and set the price value of that new object, you'll want to do something more like:

const shipping = {
  only: {
    price: $(this).val()
  }
};

...or you can do two assignment statements, one to populate .only and then one to populate the sub-property:

shipping.only = {};
shipping.only.price = $(this).val();

answered May 26, 2020 at 18:58

Jacob

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2 Comments

paul seems Over a year ago

thanks Jacob, the thing is that this object is dynamic and global, and i don't want to rewrite it , just to add a field. How can i add a field to an existing object without override it?

Jacob Over a year ago

I'm not quite sure what you're asking. Perhaps you mean you don't know whether a property already exists or not? If that's the case, check if the property exists before overwriting. For example shipping.only = shipping.only || {}, or the less esoteric if (!shipping.only) shipping.only = {};

max · Accepted Answer · 2020-05-26 19:37:28Z

1

You can use defineProperty()

var shipping = {}

Object.defineProperty( shipping, 'only', {

    value: { price: $(this).val() },
    writable: true,
    enumerable: true,
    configurable: true

});

edited May 26, 2020 at 19:37

answered May 26, 2020 at 19:19

max

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