I have an array
[
{ price: 10 },
{ price: 10 },
{ price: 10 },
{ price: 10 },
{ price: 20 },
{ price: 20 },
]
and I want it transformed into
[
{ numElements: 4, price: 10 },
{ numElements: 2, price: 20 },
]
I have tried using arr.reduce((prev, curr) => ..., []) to accomplish this, but I can't figure out how to do it.
A traditional method might use a for/loop to wrangle the data, but these days JavaScript has a number of functional methods that can help. This code uses reduce and map. To get your data in the format you want is a two stage process.
First, use reduce to create a hash table using the price as a key (because you know the each price is going to be unique:
const obj = arr.reduce((p, c) => {
// If price exists as a key its value by 1
// otherwise set it to 1.
p[c.price] = ++p[c.price] || 1;
return p;
}, {});
OUTPUT
{
"10": 4,
"20": 2
}
As it stands you've got a perfectly good object that you can access by the key/price and I would probably just stop there:
obj['10'] // 4
But if you want to get that data into the format in your question, map over the object keys to return an array of new objects.
const out = Object.keys(obj).map(key => {
return { price: +key, numElements: obj[key] };
});