Here is another way of achieving that goal:

var array = [{
  name: "foo1",
  value: "val1"
}, {
  name: "foo1",
  value: [
    "val2",
    "val3"
  ]
}, {
  name: "foo2",
  value: "val4"
}];

var output = array.reduce(function(o, cur) {

  // Get the index of the key-value pair.
  var occurs = o.reduce(function(n, item, i) {
    return (item.name === cur.name) ? i : n;
  }, -1);

  // If the name is found,
  if (occurs >= 0) {

    // append the current value to its list of values.
    o[occurs].value = o[occurs].value.concat(cur.value);

    // Otherwise,
  } else {

    // add the current item to o (but make sure the value is an array).
    var obj = {
      name: cur.name,
      value: [cur.value]
    };
    o = o.concat([obj]);
  }

  return o;
}, []);

console.log(output);

Your mapping of keys/entries is pretty close. It might be easiest to do something like this though:

const objectOnes = [
  {
    one: null,
    two: "Two",
    three: null
  },
  {
     a: 123,
     b: undefined,
  },
];

const objectTwos = [
  {
    one: "One",
    two: null,
    three: "Three"
  },
  {
      b: 456,
  }
];

function merge(a, b) {
    const result = { ...a }; // copy a
    for (const key in result) {
        // Let's say we want to pick the non-null/undefined value
        if (result[key] !== null && result[key] !== undefined) continue;
        result[key] = b[key]; // copy from b
    }
    return result;
}

const merged = objectOnes.map((obj, i) => merge(obj, objectTwos[i]));
console.log(merged);

You should use lodash's defaultsDeep

_.defaultsDeep({ 'user': { 'name': 'barney' } }, { 'user': { 'name': 'fred', 'age': 36 } });
// → { 'user': { 'name': 'barney', 'age': 36 } }