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Extract unique objects from array of objects by property in JavaScript

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在 JavaScript 中按属性从对象数组中提取唯一对象

In this tutorial, we will learn to extract unique objects from an array of objects by their properties. Sometimes, we need to filter objects from an array of objects based on specific properties.

For example, we have an id as the key that identifies the object. Therefore, we need to ensure that the array only contains a single object with a single id. If two or more objects contain the same primary key value, this can lead to problems with object unique identification.

Here we will learn different ways to filter all unique objects from an array based on specific properties.

Using maps and for loops

In JavaScript, Map can store unique key-value pairs. Furthermore, we can get any value from the Map using the Map's keys in O(1) time complexity. So, we will iterate over the array of objects and store the key-value pairs in a Map. Additionally, we will check if the Map contains an object with a specific property value; we will not add the object to the array of new objects.

grammar

You can use map to extract unique objects from an array by following the following syntax.

const employees = [
   { emp_id: 1, emp_name: "Shubham", emp_age: 22 },
   { emp_id: 1, emp_name: "Joe", emp_age: 23 },
];     
var map = new Map();
for (let employee of employees) {
   map.set(employee["emp_id"], employee);
}
var iteratorValues = map.values();
var uniqueEmployess = [...iteratorValues];

In the above syntax, we use mapping to store objects with unique emp_id.

algorithm

Step 1 - Create an object array containing multiple objects with duplicate emp_id.
Step 2 - Create a Map() object using the Map constructor.
Step 3 - Use a for-of loop to iterate through each object of the "employees" array.
Step 4 - For the map, use emp_id as the key and the entire object as the value. Within a for-of loop, use the Map object's set() method to set the object to the value of the emp_id key.
Step 5 - The map is an iterator. So, to get all the values of the iterator, apply values() method.
Step 6 - Copy all objects from iteratorValues to uniqueEmployees array using spread operator.
Step 7 - uniqueEmployees array contains all objects with unique emp_id, user can iterate over it to get object values.

Example

In the example below, we implement the algorithm for extracting unique objects by attributes using Map and for-of loop. We have created an array of employees containing four different objects with duplicate emp_id values and we are using the above algorithm to extract all the unique objects.

<html>
<body>
   <h3>Extracting the unique objects by the emp_id attribute of an array of objects using the <i> Map and for-loop. </i></h3>
   <div id="output"></div>
   <script>
      let output = document.getElementById("output");
      // creating the array of employees, which contains multiple objects.
      const employees = [
         { emp_id: 1, emp_name: "Shubham", emp_age: 22 },
         { emp_id: 2, emp_name: "Joe", emp_age: 23 },
         { emp_id: 2, emp_name: "Sam", emp_age: 62 },
      ];
      var map = new Map();
      for (let employee of employees) {
         // setting up the employee object for unique emp_id value
         map.set(employee["emp_id"], employee);
      }
      var iteratorValues = map.values();
      var uniqueEmployess = [...iteratorValues];
      for (let uniqueEmp of uniqueEmployess) {
         output.innerHTML += "The new unique object values is </br>";
         output.innerHTML +=  "emp_id :- " + uniqueEmp.emp_id +
            ", emp_name :-  " + uniqueEmp.emp_name +
            ", emp_age :- " + uniqueEmp.emp_age +" </br> ";
      }
   </script>
</body>
</html>

Use array.filter() method and Map

We can use the filter() method to filter the values in the array. It takes a callback function as a parameter and filters the values based on the boolean value returned by the callback function.

Like the example above, we will use a map to store the attribute value and check if the map already contains the value. If so, we move on; otherwise, we add the values to the map and filter arrays.

grammar

Users can filter all objects with unique attribute values by following the following syntax.

var map = new Map();
let uniqueObjects = websites.filter((web) => {
   if (map.get(web.website_name)) {
      return false;
   }
   map.set(web.website_name, web);
   return true;
});

algorithm

Step 1 - Create a map using a Map() object.
Step 2 - Use the filter() method to filter values from the array.
Step 3 - Pass the callback function as an argument to the filter method that takes the network as a parameter. The network is an object in the reference array.
Step 4 - Check if the map already contains website_name as a key, then continue by returning false from the callback function.
Step 5 - If the map does not contain website_name, add website_name and object to the map as key-value pairs and return true to filter them out in the uniqueObjects array.

Example

In this example, we use the filter() method to filter all website objects by website_name from the array. In the output, the user can observe that the filter() method returns only two objects containing unique website_names.

<html>
<body>
   <h3>Extracting the unique objects by the website_name attribute of an array of objects using the <i> Map and filter() method. </i></h3>
   <div id="output"></div>
   <script>
      let output = document.getElementById("output");
         //creating the array of websites object. A single website can have multiple domains
      const websites = [
         { website_name: "TutorialsPoint", domain: "tutorialspoint.com" },
         { website_name: "TutorialsPoint", domain: "qries.com" },
         { website_name: "Tutorix", domain: "tutorix.com" },
      ];
      var map = new Map();
      let uniqueObjects = websites.filter((web) => {
         if (map.get(web.website_name)) {
            return false;
         }
         // If website_name is not found in the map, return true.
         map.set(web.website_name, web);
         return true;
      });
      // iterating through the unique objects and printing its value
      for (let web of uniqueObjects) {
         output.innerHTML += "The new unique object values is </br>";
         output.innerHTML += "website_name :- " + web.website_name +
            ", domain :-  " +  web.domain +  " </br> ";
     }
   </script>
</body>
</html>

This tutorial teaches us two methods to extract unique objects through specific attribute values. We are using a map in both methods but using different iterator methods to iterate over the array of objects. Users can iterate over the array using a for-of loop or the filter() method.

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Extract unique objects from array of objects by property in JavaScript

Using maps and for loops

grammar

algorithm

Example

Use array.filter() method and Map

grammar

algorithm

Example

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