Group an array of objects by key with Javascript

I feel so privileged being able to use Linq group by statements when I’m working in C# to quickly and easily group a list of objects; however, when it comes to Javascript Arrays, it’s not quite so easy.

Let’s solve that today by creating a new groupBy Array prototype function that leverages the reduce function.

• Group By Basics
• Group an array with the Reduce function
• Grouping Objects with Array.prototype.reduce() function
• Creating Array.prototype.groupBy() function
• Grouping Objects with Object.prototype.hasOwnProperty()
• Grouping Arrays with Array.prototype.indexOf()
• Grouping Objects with ES6 Map/Set Methods
• Frequently Asked Questions

Group By Basics

Group By is a feature that allows you to group items in an array based on a key value. It’s useful for creating lists of similar objects.

In JavaScript, there are two ways to group data. You can use an object with keys as the basis for grouping, or you can use an array. Let’s take a look at how each works.

Group an array with the Reduce function

The reduce function works by iterating through the array for us, but really importantly it also passes the resulting value from the previously executed iteration.

This will allow us to dynamically build up our list of keys.

Let’s begin by looking at example array that I want to leverage this new groupBy function with.

var cars = [
{
'make': 'audi',
'model': 'r8',
'year': '2022'
}, {
'make': 'audi',
'model': 'rs5',
'year': '2023'
}, {
'make': 'ford',
'model': 'mustang',
'year': '2022'
}, {
'make': 'ford',
'model': 'fusion',
'year': '2021'
}, {
'make': 'kia',
'model': 'optima',
'year': '2021'
},
];

Now, I want the output array to be group the cars up by the make, e.g.

var cars = {
'audi': [
{
'model': 'r8',
'year': '2022'
}, {
'model': 'rs5',
'year': '2023'
},
],
'ford': [
{
'model': 'mustang',
'year': '2022'
}, {
'model': 'fusion',
'year': '2021'
}
],
'kia': [
{
'model': 'optima',
'year': '2021'
}
]
};

Grouping Objects with Array.prototype.reduce() function

Arrays are used to store data in JavaScript. You can add new elements to an existing array using push(). If you want to remove an element from an array, use splice(), and if you want to replace one element with another, use slice().

If you’re using Node.js as your background, I’ve got a great article on Solving No Access-Control-Allow-Origin with Node js and Express in case you ever encounter this error during your development process.

Creating Array.prototype.groupBy() function

And finally to accomplish this I will create a new Array prototype function called groupBy that uses the reduce function to iterate the array and build up a new array grouped by the car make value:

Array.prototype.groupBy = function(key) {
return this
.reduce((hash, obj) => {
if(obj[key] === undefined) return hash;
return Object.assign(hash, { [obj[key]]:( hash[obj[key]] || [] ).concat(obj)})
}, {})
};

To use this function with the previously created cars variable:

console.log(cars.groupBy('make'));

Grouping Objects with Object.prototype.hasOwnProperty()

An object has properties that are stored inside its own property list. These properties are called “own” because they belong only to the object itself.

If we wanted to find out whether a particular property was owned by an object, we would call the object’s.hasOwnProperty method.

Grouping Arrays with Array.prototype.indexOf()

In JavaScript, arrays are ordered collections of objects. They are similar to lists in other programming languages.

To access the first element of an array, use the index 0. To access the second element, use 1, and so on.

Grouping Objects with ES6 Map/Set Methods

In order to group an object together, we can use map() and set() methods. These two methods are part of the ECMAScript 6 specification.

They allow us to transform arrays into other collections. We can also use these methods to convert objects into other types.

Frequently Asked Questions

How do you group an array of objects through a key?

The efficient way of groupby array of objects in js is through reduced functions. The reduce() method executes a reducer function on each component and outputs the corresponding value in the array, resulting in one value.

How do I group an array of objects in JavaScript?

Use reduce() to combine array objects from JavaScript. The easiest way to groupby objects with JavaScript is to call reduce() function.

Using this method, the reduction function is executed for the array element containing the a single output value.

Can I use groupBy js?

ArrayGrouping() organizes the objects with a simple Javascript object whereas the array. GroupByToMap() set them in maps. You can access them with the corejs library by using Poly Fill.

How can I group an array of objects?

The best method to organize a collection of objects from the JavaScript arrays is by using Reduce() functions. This function uses the reduction function of the array element and produces an output value.

Examples: The following list of products contains the two categories: name and category.

How do you group an array in JavaScript?

The group() method grouped the call array element according to the string value returned by the test function supplied. Each returned object has a unique property for all groups, and an array contains the items within each of those groups.

The above technique should be applied if groups are represented using string strings.

Can object keys be arrays?

Objects. Keys() returns an array with enumerations which correspond directly with objects. It is similar to looping over objects properties manually.

How do you Groupby in JavaScript?

The most effective way to group objects by key is by using the Reduce function in js. The Reduce() methods execute reducer functions on all the data points on an array and result in an output value of the same value.

How do you categorize data in JavaScript?

The post examines how the values of JavaScript are classified through the hidden property [[Class]]. IsArrays().

How do you use reduce in js?

Javascript array reduce() A reduce() function is executed for an array element. Reduce() returns only the results of the method.

The reduced() function cannot use the function on an empty element array. Reduced methods don’t update the original rows.

How do you group objects in an array?

The easiest approach of grouping an item with a key on an array of js items is used. The reduced() method executes a reduced function on all the array elements resulting in one result.