当分配给变量 object 时，变量的行为如何？

Question

The application gets an object from the HTML form via the post method like this:

code:

app.post("/foo", (req, res)=> {

  const data = req.body;
  const itemId = req.body.id;

  console.log(data);
  console.log(itemId);

});

console.log() results are:

{
  brand: 'foo',
  model: 'foo',
  amount: '10',
  id: '629f3149ffc8f9dcfa56e403'
}
629f3149ffc8f9dcfa56e403

On the console, the whole object and the item's value are shown perfectly.

Now when I apply delete to remove the id property from const data before assigning req.body.id to itemId : (position1)

app.post("/foo", (req, res)=> {

  const data = req.body;
  // position 1
  delete data.id;
  
  const itemId = req.body.id;
  

  console.log(data);
  console.log(itemId);

})

I get undefined for itemId :

{
  brand: 'foo',
  model: 'foo',
  amount: '10',
  
}
undefined

But if I use delete after assigning req.body.id to itemId : (position2)

app.post("/foo", (req, res)=> {

  const data = req.body;
  
  const itemId = req.body.id;
  // position 2
  delete data.id;
  

  console.log(data);
  console.log(itemId);

})

I get the itemId in the console .

{
  brand: 'foo',
  model: 'foo',
  amount: '10',
}
629f3149ffc8f9dcfa56e403

The Question is why and/or how this happens?

I embrace any edit on the Question's title/or body for better understanding for anybody. Thanks.

Answer 1

Objects and values are treated differently in Javascript assignments.

data and req.body are objects , and after the assignment

const data = req.body;

they are the same object. The object is not copied, but a new pointer to the same object is created.

Now

const itemId = req.body.id;
delete data.id;

first copies the value of id into a new variable and then deletes the original ( data.id is the same as req.body.id ). The copy survives.

delete data.id;
const itemId = req.body.id;

first deletes the original id and then attempts to copy its value. But by then, it is gone.