Get the sum of numbers from array objects

Let's first understand what reduce is? Consider this following custom implementation:

const arr = [{ a: 1 }, { b: 5 }, { c: 2 }];

const reduced = (array, instructionToCombine, buildingUpOn) => {
  for (let i = 0; i < array.length; i++) {
    buildingUpOn = instructionToCombine(buildingUpOn, array[i][Object.keys(array[i])]);
  }
  return buildingUpOn;
}

const instructionToCombine = (a, b) => a + b;

const ans = reduced(arr, instructionToCombine, 0);
console.log(ans)

Now, you got how JavaScript default reduce method works.

You're missing the initialValue (buildingUpOn in my implementation). Know that, initialValue is not required parameter, rather it's optional. Yet,

If no initialValue is supplied, the first element in the array will be used as the initial accumulator value and skipped as currentValue - MDN

But you treated otherwise that's why didn't get the expected answer. acc is a object for the first iteration, but rest of the iteration it's a number but you treated like a object which returned undefined and, sum of a Number with undefined returns NaN.

If you still need to achieve this without passing the initialValue, it's very possible by keeping acc an object this way:

const arr = [{ a: 1 }, { b: 5 }, { c: 2 }];

const app = (arr) => {
    const r = arr.reduce((acc, nextValue) => {
       
        const newValue = acc[Object.keys(acc)] + nextValue[Object.keys(nextValue)];
        return {x: newValue};
    })
    return r[Object.keys(r)];
}
console.log(app(arr))

arr.map(obj => Object.values(obj)).flat().reduce((acc, next) => acc += next)

const arr = [{ a: 1 }, { b: 5 }, { c: 2 }]

const arrayMap = arr.map(obj => Object.values(obj)).flat().reduce((acc, next) => acc += next)

console.log(arrayMap)

This is so simple, you making it difficult. You need values not keys and to make sure use Number() function

const arr = [{ a: 1 }, { b: 5 }, { c: 2 }];

const app = (arr) => {
    const r = arr.reduce((acc, nextValue) => {
        return acc += Number(Object.values(nextValue))
    },0)
    return r
}
console.log(app(arr))

It is good to point that you want to return a number from reducer, and out initial value type number as a second parameter in your function

const app = (arr) => {
        const r = arr.reduce((acc, nextValue) => {
          console.log(nextValue[Object.keys(nextValue)]);
          return acc + nextValue[Object.keys(nextValue)];
        }, 0);
        return r;
      };
      console.log(app(arr));

you need to add initialValue 0 to reduce

const app = (arr) => arr.reduce((acc, nextValue) => {
        return acc + Object.values(nextValue)[0]
    } ,0)
   
console.log(app(arr))

You need a start value of zero and the first key of the keys array, not the whole array.

For summing take the accumulator directly.

const arr = [{ a: 1 }, { b: 5 }, { c: 2 }];
const app = (arr) => {
    const r = arr.reduce((acc, nextValue) => {
        return acc + nextValue[Object.keys(nextValue)[0]];
    }, 0);
    return r;
};

console.log(app(arr));