JavaScript filter

JavaScript filter
JavaScript filter

JavaScript filter is a built-in array method that allows you to create a new array containing elements from the original array that meet a specific condition. It is a powerful tool for filtering data and is commonly used for data manipulation and extracting specific elements from arrays.

Here’s an in-depth explanation of the filter method:

Syntax fo JavaScript filter:

const newArray = array.filter(callback(element, index, array));
  • array: The original array you want to filter.
  • callback: A function that is executed for each element in the array.
  • element: The current element being processed in the array.
  • index (optional): The index of the current element in the array.
  • array (optional): The array on which filter was called.

Return Value:

  • The filter method returns a new array containing all elements from the original array that satisfy the condition specified in the callback function.

How it works:

  • For each element in the array, the callback function is called.
  • If the callback function returns true for an element, that element is included in the new array. If the callback returns false, the element is excluded.

Key Points:

  1. Non-Destructive: The filter method does not modify the original array. It creates a new array with the filtered elements, leaving the original array intact.
  2. Callback Function: The callback function is crucial, as it defines the condition for inclusion in the new array. It should return true for elements that should be kept and false for elements that should be removed.
  3. Index and Array (optional): The index and array parameters in the callback function are optional and can be used for more complex filtering conditions.


const numbers = [1, 2, 3, 4, 5];
const evenNumbers = numbers.filter(num => num % 2 === 0);
console.log(evenNumbers); // Output: [2, 4]

In this example, filter is used to create a new array, evenNumbers, which contains only the elements from the numbers array that meet the condition of being even. The callback function checks if num % 2 === 0 and includes elements that meet this condition in the evenNumbers array.

Overall, the filter method provides an elegant and efficient way to work with arrays, allowing you to selectively extract elements based on a wide range of conditions.

Most used Examples

Certainly, let’s go through JavaScript’s filter function step by step with detailed explanations:

1. Basic Usage:

Filtering elements in an array based on a condition.

const numbers = [1, 2, 3, 4, 5];
const evenNumbers = numbers.filter(num => num % 2 === 0);
console.log(evenNumbers); // Output: [2, 4]

In this example, the filter method is used to create a new array, evenNumbers, which contains only the elements from the numbers array that meet the condition specified in the callback function (in this case, that condition is “num % 2 === 0”).

2. Filtering Objects

Filtering an array of objects.

const products = [
  { name: 'Phone', price: 500 },
  { name: 'Laptop', price: 1000 },
  { name: 'Tablet', price: 300 },
const affordableProducts = products.filter(product => product.price <= 500);
console.log(affordableProducts); // Output: [{ name: 'Phone', price: 500 }, { name: 'Tablet', price: 300 }]

Here, we’re using the filter method to create a new array, affordableProducts, containing objects from the products array where the price property is less than or equal to 500.

3. Using Index

Accessing the index while filtering.

const numbers = [10, 20, 30, 40, 50];
const filteredIndices = numbers.filter((num, index) => index % 2 === 0);
console.log(filteredIndices); // Output: [10, 30, 50]

In this example, filteredIndices is an array of numbers from the numbers array at even indices. The filter callback function takes two arguments, the current element (num) and its index (index), which allows us to filter elements based on their position in the array.

4. Filtering Unique Values

Creating a new array with unique values.

const numbers = [1, 2, 2, 3, 4, 4, 5];
const uniqueNumbers = numbers.filter((num, index, array) => array.indexOf(num) === index);
console.log(uniqueNumbers); // Output: [1, 2, 3, 4, 5]

In this case, uniqueNumbers contains only the unique values from the numbers array. The filter method, combined with indexOf, helps to eliminate duplicate values by checking if an element’s first occurrence matches its current position in the array.

5. Chaining Filters

Applying multiple filter conditions.

const employees = [
  { name: 'Alice', salary: 50000, department: 'HR' },
  { name: 'Bob', salary: 60000, department: 'Engineering' },
  { name: 'Charlie', salary: 70000, department: 'HR' },
  { name: 'David', salary: 80000, department: 'Engineering' },

const highPaidEngineers = employees
  .filter(employee => employee.department === 'Engineering')
  .filter(employee => employee.salary > 70000);

console.log(highPaidEngineers); // Output: [{ name: 'David', salary: 80000, department: 'Engineering' }]

In this example, we first filter employees by department and then by salary. The result, highPaidEngineers, is an array containing employees who are part of the ‘Engineering’ department and have a salary greater than 70000.

These examples illustrate various use cases of the filter method, showcasing its flexibility for extracting specific elements from arrays based on different conditions.

6. Filtering by String Length

Filtering an array of strings based on their length.

const fruits = ['apple', 'banana', 'kiwi', 'strawberry', 'cherry'];
const shortFruits = fruits.filter(fruit => fruit.length < 5);
console.log(shortFruits); // Output: ['kiwi']

In this example, the filter method is used to create an array, shortFruits, that contains only the fruits with a length less than 5 characters.

7. Filtering Null or Undefined Values

Filtering an array to remove null or undefined values.

const data = [42, null, 56, undefined, 23, 0, 17];
const validData = data.filter(item => item !== null && item !== undefined);
console.log(validData); // Output: [42, 56, 23, 0, 17]

Here, validData is an array containing only the elements that are not null or undefined, effectively filtering out these falsy values.

8. Filtering with Regular Expressions

Filtering an array of strings using regular expressions.

const words = ['apple', 'banana', 'kiwi', 'strawberry', 'cherry'];
const regex = /^k/;
const startsWithK = words.filter(word => regex.test(word));
console.log(startsWithK); // Output: ['kiwi']

In this case, we use a regular expression to filter words in the words array. The startsWithK array contains only words that start with the letter ‘k’.

9. Filtering Using a Function

Using a custom filter function for complex conditions.

const numbers = [10, 25, 5, 30, 60, 15];
function isDivisibleBy5AndEven(num) {
  return num % 5 === 0 && num % 2 === 0;
const filteredNumbers = numbers.filter(isDivisibleBy5AndEven);
console.log(filteredNumbers); // Output: [10, 30, 60]

In this example, the isDivisibleBy5AndEven function defines a custom filter condition, and the filteredNumbers array contains numbers that satisfy this condition.

10. Filtering based on Array of Conditions

Filtering based on multiple conditions.

const products = [
  { name: 'Phone', price: 500, category: 'Electronics' },
  { name: 'Shirt', price: 25, category: 'Clothing' },
  { name: 'Laptop', price: 1000, category: 'Electronics' },
  { name: 'Jeans', price: 50, category: 'Clothing' },

const expensiveElectronics = products.filter(product => {
  return product.category === 'Electronics' && product.price > 500;
console.log(expensiveElectronics); // Output: [{ name: 'Laptop', price: 1000, category: 'Electronics' }]

In this case, we’re using a complex filter condition that checks both the product category and price to find expensive electronics.

These additional examples demonstrate the versatility of the filter method in JavaScript for a wide range of filtering scenarios, from simple to complex conditions.


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