Write a function “groupBy(array, callback)”

Question

I have a JavaScript task where I have to implement a function \"groupBy\", which, when given an array of objects and a function, returns an object w

Answer 1

 var list = [{id: "102", name: "Alice"},
              {id: "205", name: "Bob", title: "Dr."},
              {id: "592", name: "Clyde", age: 32}];

console.log(groupBy(list, function(i) { return i.id; }));
console.log(groupBy(list, function(i) { return i.name.length; }));

Here's a link to working example: https://codepen.io/pablo-tavarez/pen/NwjrEZ?editors=0012

function groupBy(list, callback) {
  var output = {};
  var key
    , i = 0;

  // iterate over list of objects
  for ( ; i < list.length; i++ ) {
    // pass the current item to callback and get (current) key
    key = callback(list[i]);

    if (output[key]) {
      // handle duplicate keys without overriding -- (optionally make all key value Arrays)
      output[key] = [output[key]];
      output[key].push(list[i]);
    }
    else {
      output[key] = list[i];
    }
  }

  return output;
}