How to update single object property using JSONata transform operator

I'm trying to update a single property within an object using the JSONata transform operator. I don't want to lose any other properties from the initial object.

My input object:

{
  "item": {
    "_id": "097369183479824602",
    "name": "Name",
    "description": "Description",
    "sku": "0230115",
    "quantity": 8
  },
  "quantityRemaining": 6
}

My JSONata expression:

$ ~> | item | { "quantity" : quantityRemaining } |

Expected output:

{
  "item": {
    "_id": "097369183479824602",
    "name": "Name",
    "description": "Description",
    "sku": "0230115",
    "quantity": 6
  },
  "quantityRemaining": 6
}

But the actual object is:

{
  "item": {
    "_id": "097369183479824602",
    "name": "Name",
    "description": "Description",
    "sku": "0230115",
    "quantity": 8
  },
  "quantityRemaining": 6
}

I'm not sure why the value of item.quantity is not being overwritten by quantityRemaining.
Any help is appreciated!

Interestingly, if I simply the expression to the following:

$ ~> | item | { "quantity" : 3 } |

the output is expected:

{
  "item": {
    "_id": "097369183479824602",
    "name": "Name",
    "description": "Description",
    "sku": "0230115",
    "quantity": 3
  },
  "quantityRemaining": 6
}

I'm doing all of this in the https://try.jsonata.org/ playground.

In your expression - where you try and set the quantity - the context is set to the value of item. input.item.quantityRemaining evaluates to undefined and so JSONata seems to discard the value from the expression and use the one from the input.

You could use the $$ variable, which reference the root of the input JSON, to break out of the current context.

https://docs.jsonata.org/programming#built-in-variables

$ ~> | item | { "quantity" : $$.quantityRemaining } |

You can see this working in this JSONata playground.

Note that I'm not super familiar with JSONata, and so there might be a better way to achieve what you're after.