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] };
});
var hash = {}, result = [];
arr.forEach(function(el){
if(hash[el.price]){
hash[el.price].numElements++;
}else{
result.push(hash[el.price]={price:el.price,numElements:1});
}
});
May use a hash table for price lookup. Or with reduce and find:
arr.reduce((res,{price})=>
(( res.find(el=>el.price===price) || res[res.push({price,numElements:0})-1] )
.numElements++,res)
);
You can use try this:
let arr = [
{ price: 10 },
{ price: 10 },
{ price: 10 },
{ price: 10 },
{ price: 20 },
{ price: 20 },
]
let result = []
let counter = {}
arr.forEach( el => {
if (!counter[el.price]) counter[el.price] = 1
else counter[el.price]++
console.log(counter[el.price])
})
for (let id in counter) {
result.push({numElements: counter[id], price: id})
}
Assuming that the data comes sorted on price property, with a single .reduce() you may do as follows;
var data = [{ price: 10 }, { price: 10 }, { price: 10 }, { price: 10 }, { price: 20 }, { price: 20 }],
result = data.reduce((r,d,i) => i ? r[r.length-1].price === d.price ? (r[r.length-1].numElemenets++, r)
: (r.push(Object.assign({}, d, {numElemenets: 1})),r)
: [Object.assign({}, d, {numElemenets: 1})], {});
console.log(result);You could look up the price in the result array and if not found insert a new object.
var data = [{ price: 10 }, { price: 10 }, { price: 10 }, { price: 10 }, { price: 20 }, { price: 20 }],
grouped = data.reduce((r, { price }) => {
var t = r.find(p => price === p.price);
t || r.push(t = { numElements: 0, price });
t.numElements++;
return r;
}, []);
console.log(grouped);