How can I sum properties from two objects?

You can use reduce for that, below function takes as many objects as you want and sums them by key:

var obj1 = {
  a: 12,
  b: 8,
  c: 17
};

var obj2 = {
  a: 12,
  b: 8,
  c: 17
};

var obj3 = {
  a: 12,
  b: 8,
  c: 17
};


function sumObjectsByKey(...objs) {
  return objs.reduce((a, b) => {
    for (let k in b) {
      if (b.hasOwnProperty(k))
        a[k] = (a[k] || 0) + b[k];
    }
    return a;
  }, {});
}

console.log(sumObjectsByKey(obj1, obj2, obj3));

A little bit deeper, all you want as long as objects are equivalent!

const arr = [{
    a: 12,
    b: { a: 12, c: { a: 12 } },
    c: 17
  },
  {
    a: 12,
    b: { a: 12, c: { a: 12 } },
    c: 17
  },
  {
    a: 12,
    b: { a: 12, c: { a: 12 } },
    c: 17
  }
];

const deepMergeSum = (obj1, obj2) => {
  return Object.keys(obj1).reduce((acc, key) => {
    if (typeof obj2[key] === 'object') {
      acc[key] = deepMergeSum(obj1[key], obj2[key]);
    } else if (obj2.hasOwnProperty(key) && !isNaN(parseFloat(obj2[key]))) {
      acc[key] = obj1[key] + obj2[key]
    }
    return acc;
  }, {});
};

const result = arr.reduce((acc, obj) => acc = deepMergeSum(acc, obj));
console.log('result: ', result);

Try this.

let t1 = 
{ 
  a:12,
  b:8,
  c:17
};

let t2 = 
{ 
  a:2,
  b:4,
  c:1
};

function sum(ob1, ob2) {
  let sum = {};

  Object.keys(ob1).forEach(key => {
    if (ob2.hasOwnProperty(key)) {
      sum[key] = ob1[key] + ob2[key]
    }  
  })
  return sum;
}

sum(t1, t2);

https://jsfiddle.net/fbnt2vhe/

How can I sum properties from two objects?

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