Reduce Array of Objects with Dynamic Keys

Question

This seems like it should be pretty simple but my searching hasn't turned anything up.

I have an array of objects that looks like this:

[{"A":4,"B":2},{"A":2,"B":1},{"A":3,"B":1},{"A":2,"B":1,"C":1}]

I want to flatten it down to something that looks like this (what I would think of as a reduce function):

{"A": 11, "B": 5, "C": 1}

The starting array of objects is the product of the transformation of a much larger and more complex starting object. The important detail is that the keys themselves are of arbitrary values (i.e. you might see 'D's and 'E's but the code itself needs to be blind to this). I've gotten to this point using Underscore.js and I'm hoping to find similarly clean, functional-style solution.

You need to post your attempt(s) please. And given the features built-in with ES5 there is no need for underscore — Dexygen
– Dexygen, Commented Jan 22, 2020 at 21:54

Nina Scholz · Accepted Answer · 2020-01-22 21:52:02Z

1

Get the key/value pairs and update the sum.

var data = [{ A: 4, B: 2 }, { A: 2, B: 1 }, { A: 3, B: 1 }, { A: 2, B: 1, C: 1 }],
    result = data.reduce((r, o) => (Object.entries(o).forEach(([k, v]) => r[k] = (r[k] || 0) + v), r), {});

console.log(result);

answered Jan 22, 2020 at 21:52

Nina Scholz

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3 Comments

Paul Murray Over a year ago

Yeah, that'll do it. I had been wondering if it was going to require an inner for-each on the key sets. Thanks for the help!

Paul Murray Over a year ago

Is there a reason (beyond code clarity) to pass in the initial value of '{ }' rather than starting the accumulator with the first element?

Nina Scholz Over a year ago

it follows the non mutating approach of the data. whithout startValue, the first object is mutated.

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