javascript - Write a function "groupBy(array, callback)"

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 where the input objects are keyed by the result of calling the fn on each of them.

Essentially, I have to write a function, "groupBy(array, callback)", returns an object where the following is returned.

For example:

  var list = [{id: "102", name: "Alice"},
              {id: "205", name: "Bob", title: "Dr."},

              {id: "592", name: "Clyde", age: 32}];

  groupBy(list, function(i) { return i.id; });

Returns:

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

  }

Example 2:

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

Returns:

  {
    "3": [{id: "205", name: "Bob", title: "Dr."}],

    "5": [{id: "102", name: "Alice"},
          {id: "592", name: "Clyde", age: 32}]
  }

I'm still quite new to callback functions, and would like some tips/advice to simply get started. Even links to good tutorials would be greatly appreciated.

Answer

Solution with pure JS:

var list = [{
    id: "102",
    name: "Alice"
  },
  {
    id: "205",
    name: "Bob",
    title: "Dr."
  },

  {
    id: "592",
    name: "Clyde",
    age: 32
  }
];

function groupBy(array, callback) {
  return array.reduce(function(store, item) {
    var key = callback(item);

    var value = store[key] || [];
    store[key] = value.concat([item]);
    return store;
  }, {})
}

console.log('example 1: ', groupBy(list, function(i) { return i.id; }));
console.log('example 2: ', groupBy(list, function(i) { return i.name.length; }));