I have an array of objects that I initialize like so:
const balances = Array(5).fill({})
Later, I modify the balance for the first object using:
balances[0]['USD'] = 1000
I expect this to change only the balance for the first index. Instead, the 'USD' key is set for all of the elements:
balances // [{USD: 1000}, {USD: 1000}, {USD: 1000}, {USD: 1000}, {USD: 1000}]
// when I expected this:
balances // [{USD: 1000}, {}, {}, {}, {}]
You're filling the array with the same object. (We can see this by using the === strict equality operator.)
> balances = Array(5).fill({})
(5) [{…}, {…}, {…}, {…}, {…}]
> balances[0] === balances[1]
true
You'll need to construct an object for each slot - an FP (if not necessarily pretty or efficient) way to do this would be
> balances = Array(5).fill(null).map(() => ({}))
(5) [{…}, {…}, {…}, {…}, {…}]
> balances[0] === balances[1]
false
A non-FP, old-school, imperative but plenty fast (79% faster than the one above, by one quick benchmark) way is simply
var balances = [];
for(let i = 0; i < 5; i++) balances.push({});
You can use Array.from with a mapping function to fill the array with different objects.
const balances = Array.from({length: 5}, _=>({}));
filluses the same exact value for every slot in the array. So a reference to the same object.