Clash Royale CLAN TAG

Assignment by reference confusion

I have a 'trades' object that includes a reference to orderBook.BTCUSDT. My intention is to change 'trades' when orderBook.BTCUSDT is changed.

However, changing orderBook.BTCUSDT does not work. But changing orderBook.BTCUSDT.asks does.

Why?

orderBook = {'BTCUSDT': {'asks':[1,2,3,5], 'bids':[6,7,8,9]}};
trades = {"one": orderBook.BTCUSDT};
orderBook.BTCUSDT = 1234; // does not work
console.log(trades);

/* Output:
{
"one": {
"asks": [
1,
2,
3,
5
],
"bids": [
6,
7,
8,
9
]
}
}
*/

orderBook = {'BTCUSDT': {'asks':[1,2,3,5], 'bids':[6,7,8,9]}};
trades = {"one": orderBook.BTCUSDT};
orderBook.BTCUSDT.asks = 1234; // works
console.log(trades);

/* Output:
{
"one": {
"asks": 1234,
"bids": [
6,
7,
8,
9
]
}
}
*/

Edit after Axiac and Artur responses

After reading responses from Axiac and Artur, I found another way to ask the question. Why does the first code block work but not the second? Why should I have to add another level to the object by using 'prices'? Seems like both are trying to do the same thing (replace an object with another object but keep the reference), just at different levels.

orderBook = {BTCUSDT: { prices: {'asks':[1,2,3,5], 'bids':[6,7,8,9]}}};
trades = {one: orderBook.BTCUSDT};
orderBook.BTCUSDT.prices = {'asks':[11,12,13,15], 'bids':[16,17,18,19]}; // trades.one.BTCUSDT.prices is updated as expected
console.log(trades);

orderBook = {BTCUSDT: {'asks':[1,2,3,5], 'bids':[6,7,8,9]}};
trades = {one: orderBook.BTCUSDT};
orderBook.BTCUSDT = {'asks':[11,12,13,15], 'bids':[16,17,18,19]}; // trades.one.BTCUSDT is NOT updated as expected
console.log(trades);

Number

Object

Object

trades

3 Answers
3

orderBook = {'BTCUSDT': {'asks':[1,2,3,5], 'bids':[6,7,8,9]}};
trades = {"one": orderBook.BTCUSDT};


1. orderBook.BTCUSDT = 1234;

2.orderBook.BTCUSDT.asks = 1234;

This statement:

trades = {"one": orderBook.BTCUSDT};


makes trades.one refer to the same object as orderBook.BTCUSDT do (an object having the properties asks and bids). This way, the object can be accessed using two variables (trades.one and orderBook.BTCUSDT).

trades.one

orderBook.BTCUSDT

asks

bids

trades.one

orderBook.BTCUSDT

trades.one and orderBook.BTCUSDT are different entities and they are not related in any way. It just happens that after the statement above they point to the same object.

trades.one

orderBook.BTCUSDT

The next statement:

orderBook.BTCUSDT = 1234; // does not work


puts a different value in orderBook.BTCUSDT and breaks the link between it and the object. The object having the asks and bids properties can now be accessed only by using the trades.one variable.

orderBook.BTCUSDT

asks

bids

trades.one

Hopefully this will help you to understand visually why you have this behavior.

In the first picture you can see how the objects are referenced.

In the second picture you can see that by doing this orderBook.BTCUSDT = 1234 you cut the connection of BTCUSDT and the object it was pointing
before, so it does not point anymore, but the trades is still referencing that object.

orderBook.BTCUSDT = 1234

BTCUSDT

trades

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.