Given a sample JSON array of objects, like this....
[
{
"firstName": "Bob",
"lastName": "Smith",
"city": "Preston",
"st": "Florida",
"age": 15
},
{
"firstName": "Tom",
"lastName": "Jones",
"city": "Springfield",
"st": "Illinois",
"age": 34
},
{
"firstName": "Mary",
"lastName": "Hart",
"city": "Miami",
"st": "Florida",
"age": 22
},
{
"firstName": "Jenny",
"lastName": "Dixon",
"city": "Palm City",
"st": "Florida",
"age": 26
}
]
What is the best way to get the number of occurrences based on the groupings of a particular property? So, let's say I want to produce a JSON object that has each unique state ("st") and the number of occurrences....
[
{
"st": "Illinois",
"count": 1
},
{
"st": "Florida",
"count": 3
}
]
I can do it manually using a for-let, looping through the array, tracking the values as I loop, etc. But I'm sure there's a more efficient way using ES6. Can you please help me out? Thanks.
You can use Array#reduce to keep a tally
let counts = json.reduce((b, a) => {
let index = b.findIndex(j => j.st === a.st);
if (index > -1) b[index].count++;
else b.push({st: a.st, count: 1});
return b;
}, [])
#UPDATE: As mentioned by @epascarello, there is a more efficient way to go about this, removing the findIndex loop and using Object.values
const results = Object.values(json.reduce((obj, item) => {
obj[item.st] = obj[item.st] || { st: item.st, count: 0 };
obj[item.st].count++;
return obj;}, {}))
let json = [{
"firstName": "Bob",
"lastName": "Smith",
"city": "Preston",
"st": "Florida",
"age": 15
},
{
"firstName": "Tom",
"lastName": "Jones",
"city": "Springfield",
"st": "Illinois",
"age": 34
},
{
"firstName": "Mary",
"lastName": "Hart",
"city": "Miami",
"st": "Florida",
"age": 22
},
{
"firstName": "Jenny",
"lastName": "Dixon",
"city": "Palm City",
"st": "Florida",
"age": 26
}
]
let counts = json.reduce((b, a) => {
let index = b.findIndex(j => j.st === a.st);
if (index > -1) b[index].count++;
else b.push({st: a.st, count: 1});
return b;
}, [])
console.log(counts)
const results = Object.values(json.reduce((obj, item) => {
obj[item.st] = obj[item.st] || { st: item.st, count: 0 };
obj[item.st].count++;
return obj;}, {}))
console.log(results)