How to Group an Array of Objects by Key in JavaScript
In this article, we will explore how to group an array of objects by a specific key in JavaScript. This is a common task when working with data, as it allows us to organize and manipulate information in a more efficient way.
Problem Description
Let's start by understanding the problem at hand. We have an array of car objects, each containing properties such as "make", "model", and "year". The goal is to group these car objects by their "make" property and create a new array of objects based on this grouping.
Solution Approach
To solve this problem, we can use various JavaScript methods and techniques. One of the popular approaches is to use the reduce() method in combination with an empty object as the initial value. Here is an example of how we can achieve this:
const cars = [
{
'make': 'audi',
'model': 'r8',
'year': '2012'
}, {
'make': 'audi',
'model': 'rs5',
'year': '2013'
}, {
'make': 'ford',
'model': 'mustang',
'year': '2012'
}, {
'make': 'ford',
'model': 'fusion',
'year': '2015'
}, {
'make': 'kia',
'model': 'optima',
'year': '2012'
},
];
const groupedCars = cars.reduce((acc, obj) => {
const key = obj.make;
if (!acc[key]) {
acc[key] = [];
}
acc[key].push({
model: obj.model,
year: obj.year
});
return acc;
}, {});
console.log(groupedCars);
Explanation
Let's break down the solution step by step:
- We start by declaring an empty object called "groupedCars" to store our final result.
- We use the reduce() method on the "cars" array, which iterates over each car object.
- Within the reduce() method, we create a new variable called "key" and assign it the value of the "make" property of the current car object.
- We check if the "groupedCars" object already has a key that matches the current car's make. If not, we create an empty array at that key.
- We then push a new object containing the car's "model" and "year" into the corresponding array of the "groupedCars" object.
- Finally, we return the updated "groupedCars" object at the end of each iteration.
- The reduce() method accumulates the results and returns the final "groupedCars" object.
If we run the above code snippet, we will get the desired output:
{
'audi': [
{
'model': 'r8',
'year': '2012'
}, {
'model': 'rs5',
'year': '2013'
}
],
'ford': [
{
'model': 'mustang',
'year': '2012'
}, {
'model': 'fusion',
'year': '2015'
}
],
'kia': [
{
'model': 'optima',
'year': '2012'
}
]
}
Alternative Approaches
While the above solution works perfectly fine, there are alternative approaches you can consider depending on your needs and preferences. Here are a few examples:
Using Array.prototype.reduce() and an ES6 Map
const cars = [
{
'make': 'audi',
'model': 'r8',
'year': '2012'
}, {
'make': 'audi',
'model': 'rs5',
'year': '2013'
}, {
'make': 'ford',
'model': 'mustang',
'year': '2012'
}, {
'make': 'ford',
'model': 'fusion',
'year': '2015'
}, {
'make': 'kia',
'model': 'optima',
'year': '2012'
},
];
const groupedCars = Array.from(cars.reduce(
(acc, obj) => acc.set(obj.make, [...(acc.get(obj.make) || []), { model: obj.model, year: obj.year }]),
new Map()
)).reduce(
(acc, [key, value]) => Object.assign(acc, { [key]: value }),
{}
);
console.log(groupedCars);
This approach utilizes the ES6 Map data structure, which allows for efficient key-value pair storage and retrieval. We first create a new Map object, where the keys are the car makes and the values are arrays of car objects. We then convert the Map object into a regular array using Array.from(). Finally, we use another reduce() method to convert the array back into an object with the desired grouping.
Using Lodash groupBy()
If you have Lodash available in your project, you can leverage its groupBy() function to achieve the same result:
const cars = [
{
'make': 'audi',
'model': 'r8',
'year': '2012'
}, {
'make': 'audi',
'model': 'rs5',
'year': '2013'
}, {
'make': 'ford',
'model': 'mustang',
'year': '2012'
}, {
'make': 'ford',
'model': 'fusion',
'year': '2015'
}, {
'make': 'kia',
'model': 'optima',
'year': '2012'
},
];
const groupedCars = _.groupBy(cars, 'make');
console.log(groupedCars);
The groupBy() function in Lodash takes an array and a key as parameters, and returns an object where the keys are the unique values of the specified key in the array, and the values are arrays of objects with that key value.
Conclusion
In conclusion, grouping an array of objects by a key is a useful operation in JavaScript that allows us to organize and process data more efficiently. By leveraging methods like reduce() or libraries like Lodash, we can easily group objects based on a specific property and create new arrays or objects with the desired grouping.
I hope this article provided a clear explanation of how to solve the given problem. Feel free to experiment with the code examples and adapt them to your specific needs.