Write the function "groupBy (array, callback)"

Question

I have a JavaScript task where I need to implement the "groupBy" function , which, when it sets an array of objects and a function, returns an object into which input objects are called, caused by the result of calling fn on each of them.

Essentially, I need to write the function "groupBy (array, callback)", returns an object into which the following is returned.

For instance:

  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 new to callback functions and would like some tips / hints to just get started. Even links to good tutorials would be greatly appreciated.

Many thanks!

Answer 1

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; }));

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Answer 2

a reduce() . . a ( ) , , c ( ) - , .

, :

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

function groupBy(list, Fn) {
    return list.reduce((a, c) => (a[Fn(c)] ? a[Fn(c)].push(c) : a[Fn(c)] = [c], a), {})
}

var t = groupBy(list, function(i) { return i.id; });
console.log(t)

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

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Answer 3

: http://rextester.com/KROB29161

function groupBy(list, callback) {
  // Declare a empty object
  var obj = {};

  // We then loop through the array
  list.forEach(function(item) {
    // We define the key of the object by calling the 
    // callback with the current item
    var key = callback(item);

    // If the field exists, add this item to it
    if (obj[key]) {
      obj[key] = obj[key].concat(item);
    } else {

      // If the field does not exist, create it and this value
      // as an array 
      obj[key] = [item];
    }
  });
  return obj;
}

Answer 4

Lodash _.groupBy, , . doc .

Answer 5

 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 is a link to a 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;
}

Answer 6

function groupBy(array, callback) {
  var object = {};

  for (var i = 0; i < array.length; i++) {
    var item = array[i];
    var key = callback(item);

    if (object.hasOwnProperty(key)) {
      object[key].push(item);
    } else {
      object[key] = [item];
    }
  }

  return object;
};

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

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

console.log(byId);

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

console.log(byNameLength);

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