transform nested array into array of objects using higher order function

Question

I currently have this input:

[
  [['firstName', 'Joe'], ['lastName', 'Blow'], ['age', 42], ['role', 'clerk']],
  [['firstName', 'Mary'], ['lastName', 'Jenkins'], ['age', 36], ['role', 'manager']]
];

The output should be as follows:

[
  { firstName: 'Joe', lastName: 'Blow', age: 42, role: 'clerk' },
  { firstName: 'Mary', lastName: 'Jenkins', age: 36, role: 'manager' }
];

I wrote the following code:

function transformEmployeeData(array) {
  let newArr = [];

  for(var i = 0; i< array.length; i++) {
    let newObj = {}
    for(var j = 0; j< array[i].length; j++) {
      newObj[array[i][j][0]] = array[i][j][1]
    }
    newArr.push(newObj);
  }

  return newArr; 
}

However, I want to try and use the .map() and .reduce() functions. How can I implement them?

Nick Parsons · Accepted Answer · 2020-02-01 13:09:25Z

The easiest way is to probably use .map() and then use Object.fromEntries() as your mapping function like so:

const arr = [
  [['firstName', 'Joe'], ['lastName', 'Blow'], ['age', 42], ['role', 'clerk']],
  [['firstName', 'Mary'], ['lastName', 'Jenkins'], ['age', 36], ['role', 'manager']]
];

const res = arr.map(Object.fromEntries);
console.log(res);

However, if you really want to use .reduce(), then you can use .map() and .reduce() to build your objects for you:

const arr = [
  [['firstName', 'Joe'], ['lastName', 'Blow'], ['age', 42], ['role', 'clerk']],
  [['firstName', 'Mary'], ['lastName', 'Jenkins'], ['age', 36], ['role', 'manager']]
];

const res = arr.map(
  entries => entries.reduce((acc, [key, value]) => ({...acc, [key]: value}), {})
);
console.log(res);

The above uses .reduce() to iterate over the [key, value] pair arrays in your inner arrays. For each key-value pair, you can destructure (([key, value]) =>) it to pull out the key and the value from the array. Then, you can return an object to be used as the next value for the accumulator. The object you return is a built-up object from acc (which is initially {}) and the current key-value pair. This is done using the spread syntax (...) with computed property names ({[key]: value}).

One final approach would be to map each inner [key, value] to an object:

[ 
  [ { "firstName": "Joe" }, { "lastName": "Blow" }, { "age": 42 }, { "role": "clerk" } ], 
  [ { "firstName": "Mary" }, { "lastName": "Jenkins" }, { "age": 36 }, { "role": "manager" } ] 
]

Then, you can map this array to an array of objects by using Object.assign(), with each argument for Object.assign() being the objects from the inner arrays. This will "merge" each object in the array to form one larger object:

const arr = [
  [['firstName', 'Joe'], ['lastName', 'Blow'], ['age', 42], ['role', 'clerk']],
  [['firstName', 'Mary'], ['lastName', 'Jenkins'], ['age', 36], ['role', 'manager']]
];

const res = arr.map(
  entries => entries.map(([k, v]) => ({[k]: v}))
).map(arr => Object.assign(...arr));
console.log(res);

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