i have a array with objects in it. I need to create multiple arrays with objects grouped by one of the properties value. To clarify here is my array:

var people = [
    new Person("Scott", "Guthrie", 38),
    new Person("Scott", "Johns", 36),
    new Person("Scott", "Hanselman", 39),
    new Person("Jesse", "Liberty", 57),
    new Person("Jon", "Skeet", 38)
];

I want to have one array with all the people with first name Scott, one with all with the first name Jesse and so on. Any suggestions will be helpful.

i have a array with objects in it. I need to create multiple arrays with objects grouped by one of the properties value. To clarify here is my array:

var people = [
    new Person("Scott", "Guthrie", 38),
    new Person("Scott", "Johns", 36),
    new Person("Scott", "Hanselman", 39),
    new Person("Jesse", "Liberty", 57),
    new Person("Jon", "Skeet", 38)
];

I want to have one array with all the people with first name Scott, one with all with the first name Jesse and so on. Any suggestions will be helpful.

• javascript
• arrays
• object
• group-by

• 1 Have a look at: Array.prototype.filter() or Array.prototype.reduce() – Andreas Commented Jan 16, 2016 at 9:32

2 Answers 2

To do that, you can loop through the array entries (in any of several ways, let's use forEach here) and create an object or Map keyed by the names you e across, with the value being the arrays of entries for those names.

Here's an example using an object:

// Create an object with no prototype, so it doesn't have "toString"
// or "valueOf", although those would be unlikely names for people to have
var nameArrays = Object.create(null);

// Loop the people array
people.forEach(function(person) {
    // Get the name array for this person's name, if any
    var nameArray = nameArrays[person.firstName];
    if (!nameArray) {
        // There wasn't one, create it
        nameArray = nameArrays[person.firstName] = [];
    }
    // Add this entry
    nameArray.push(person);
});

Live Example:

function Person(firstName, lastName, age) {
  this.firstName = firstName;
  this.lastName = lastName;
  this.age = age;
}
var people = [
  new Person("Scott", "Guthrie", 38),
  new Person("Scott", "Johns", 36),
  new Person("Scott", "Hanselman", 39),
  new Person("Jesse", "Liberty", 57),
  new Person("Jon", "Skeet", 38)
];

// Create an object with no prototype, so it doesn't have "toString"
// or "valueOf", although those would be unlikely names for people to have
var nameArrays = Object.create(null);

// Loop the people array
people.forEach(function(person) {
  // Get the name array for this person's name, if any
  var nameArray = nameArrays[person.firstName];
  if (!nameArray) {
    // There wasn't one, create it
    nameArray = nameArrays[person.firstName] = [];
  }
  // Add this entry
  nameArray.push(person);
});

// Show results
Object.keys(nameArrays).sort().forEach(function(firstName) {
  snippet.log(
    "People named " + firstName +
    ": " +
    nameArrays[firstName].map(function(person) {
      return person.firstName + " " + person.lastName;
    }).join(", ")
  );
});

<!-- Script provides the `snippet` object, see http://meta.stackexchange./a/242144/134069 -->
<script src="//tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

ES2015 adds Maps to the language, so in an ES2015 environment we might use a Map rather than an object for our nameArrays:

// Creating the map:
let nameArrays = new Map();

// Getting an array:
nameArray = nameArrays.get(person.firstName);

// Adding an array:
nameArrays.set(person.firstName, []);

// Looping through the map:
for (let [firstName, nameArray] of nameArrays) {
    // Use `firstName` and `nameArray` here
}

This is a more generic solution. getGroupedBy(persons, key) returns an array of arrays with the grouped persons by the key.

function Person(firstName, lastName, age) {
    this.firstName = firstName;
    this.lastName = lastName;
    this.age = age;
}

var people = [
    new Person("Scott", "Guthrie", 38),
    new Person("Scott", "Johns", 36),
    new Person("Scott", "Hanselman", 39),
    new Person("Jesse", "Liberty", 57),
    new Person("Jon", "Skeet", 38)
];

function getGroupedBy(persons, key) {
    var groups = {}, result = [];
    persons.forEach(function (a) {
        if (!(a[key] in groups)) {
            groups[a[key]] = [];
            result.push(groups[a[key]]);
        }
        groups[a[key]].push(a);
    });
    return result;
}

document.write('<pre>' + JSON.stringify(getGroupedBy(people, 'firstName'), 0, 4) + '</pre>');
document.write('<pre>' + JSON.stringify(getGroupedBy(people, 'lastName'), 0, 4) + '</pre>');
document.write('<pre>' + JSON.stringify(getGroupedBy(people, 'age'), 0, 4) + '</pre>');