How To Use JavaScript Filter Function

Sharing is caring 🙂

The filter() method is a higher-order function in programming that allows you to create a new array with only the elements that meet a specified criterion. It is a common pattern in functional programming, and is used to extract and manipulate data from an array or other collection. The filter() method takes a callback function as an argument, which is applied to each element in the array. If the callback function returns true for a given element, that element is included in the new array. If the callback function returns false, the element is excluded from the new array.

The filter() method is available in many programming languages and libraries, including JavaScript, Python, Ruby, and Java. It is often used in combination with other array methods, such as map() and reduce(), to transform and manipulate data.

How to Filter an Array of Objects

To filter an array of objects in JavaScript, you can use the Array.prototype.filter() method. This method takes a callback function as an argument, which is called for each element in the array. The callback function should return a boolean value indicating whether the element should be included in the filtered array.

Here is an example of how you can use the filter() method to filter an array of objects:

const products = [
  { name: 'Apple', category: 'Fruit', price: 0.99 },
  { name: 'Banana', category: 'Fruit', price: 0.59 },
  { name: 'Tomato', category: 'Vegetable', price: 0.29 },
  { name: 'Potato', category: 'Vegetable', price: 0.39 },
  { name: 'Beef', category: 'Meat', price: 4.99 },
  { name: 'Chicken', category: 'Meat', price: 3.99 }
];

const cheapProducts = products.filter(product => product.price < 1);

console.log(cheapProducts);
// Output: [{ name: 'Apple', category: 'Fruit', price: 0.99 }, 
// { name: 'Banana', category: 'Fruit', price: 0.59 }, 
// { name: 'Tomato', category: 'Vegetable', price: 0.29 }, 
// { name: 'Potato', category: 'Vegetable', price: 0.39 }]

In this example, the filter() method is used to create a new array containing only the products with a price less than 1. The callback function passed to filter() checks the price property of each object in the products array and returns true if the price is less than 1, and false otherwise.

Examples of Filtering an Array of Objects by Property Value

To filter an array of objects by a property value in JavaScript, you can use the Array.prototype.filter() method in combination with the Object.prototype.hasOwnProperty() method.

Here is an example of how you can use these methods to filter an array of objects by a property value:

const products = [
  { name: 'Apple', category: 'Fruit', price: 0.99 },
  { name: 'Banana', category: 'Fruit', price: 0.59 },
  { name: 'Tomato', category: 'Vegetable', price: 0.29 },
  { name: 'Potato', category: 'Vegetable', price: 0.39 },
  { name: 'Beef', category: 'Meat', price: 4.99 },
  { name: 'Chicken', category: 'Meat', price: 3.99 }
];

const fruitProducts = products.filter(product => product.hasOwnProperty('category') && product.category === 'Fruit');

console.log(fruitProducts);
// Output: [{ name: 'Apple', category: 'Fruit', price: 0.99 }, 
// { name: 'Banana', category: 'Fruit', price: 0.59 }]

In this example, the filter() method is used to create a new array containing only the products with a category property equal to “Fruit”. The callback function passed to filter() first checks if the product object has a category property using the hasOwnProperty() method, and then checks if the category property is equal to “Fruit”.

You can also use the Array.prototype.find() method to filter an array of objects by a property value. This method returns the first element in the array that satisfies the provided testing function.

Here is an example of how you can use the find() method to filter an array of objects by a property value:

const products = [
  { name: 'Apple', category: 'Fruit', price: 0.99 },
  { name: 'Banana', category: 'Fruit', price: 0.59 },
  { name: 'Tomato', category: 'Vegetable', price: 0.29 },
  { name: 'Potato', category: 'Vegetable', price: 0.39 },
  { name: 'Beef', category: 'Meat', price: 4.99 },
  { name: 'Chicken', category: 'Meat', price: 3.99 }
];

const fruitProduct = products.find(product => product.hasOwnProperty('category') && product.category === 'Fruit');

console.log(fruitProduct);
// Output: { name: 'Apple', category: 'Fruit', price: 0.99 }

How to Filter an Array of Objects by Key

  1. Using the Array.prototype.filter() method:
const filteredArray = array.filter(object => object.key === value);

This method creates a new array with all elements that pass the test implemented by the provided function. In this case, the function checks if the key property of the object equals the specified value.

  1. Using the Array.prototype.reduce() method:
const filteredArray = array.reduce((filtered, object) => {
  if (object.key === value) {
    filtered.push(object);
  }
  return filtered;
}, []);

This method applies a function to each element in the array and reduces the array to a single value. In this case, the function checks if the key property of the object equals the specified value, and if it does, it pushes the object to the filtered array.

  1. Using a for loop:
const filteredArray = [];
for (let i = 0; i < array.length; i++) {
  if (array[i].key === value) {
    filteredArray.push(array[i]);
  }
}

This method iterates over the elements in the array and checks if the key property of the object equals the specified value. If it does, it pushes the object to the filteredArray.

How to Filter an Array on a Value

To filter an array based on a specific value in JavaScript, you can use the Array.prototype.filter() method. This method creates a new array with all elements that pass the test implemented by the provided function.

For example, consider an array of numbers called numbers:

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

To create a new array with only the elements that are greater than 5, you could do the following:

const filteredNumbers = numbers.filter(number => number > 5);

This will create a new array called filteredNumbers that contains the elements 6, 7, 8, 9, and 10.

You can also use the Array.prototype.filter() method with arrays of objects. For example, consider an array of objects called users:

const users = [
  {id: 1, name: 'John', age: 30},
  {id: 2, name: 'Jane', age: 25},
  {id: 3, name: 'Bob', age: 35},
  {id: 4, name: 'Alice', age: 40}
];

To create a new array with only the users whose age is greater than 30, you could do the following:

const filteredUsers = users.filter(user => user.age > 30);

This will create a new array called filteredUsers that contains the objects for Bob and Alice.

Using Filter and Map

Here is an example of using the Array.prototype.filter() and Array.prototype.map() methods together in JavaScript:

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

// Use filter() to get only the even numbers
const evenNumbers = numbers.filter(number => number % 2 === 0);

// Use map() to double the value of each even number
const doubledEvenNumbers = evenNumbers.map(number => number * 2);

console.log(doubledEvenNumbers); // [4, 8, 12, 16, 20]

In this example, the Array.prototype.filter() method is used to create a new array called evenNumbers that contains only the even numbers from the original numbers array. Then, the Array.prototype.map() method is used to create a new array called doubledEvenNumbers that contains the values of the even numbers from the evenNumbers array, but doubled.

The Array.prototype.filter() and Array.prototype.map() methods are both useful for transforming arrays and creating new ones based on certain criteria. They can be used together or independently, depending on your needs.

How to Filter an Object by Key

To filter an object by key in JavaScript, you can use the Object.keys() method to get an array of the object’s keys, and then use the Array.filter() method to create a new array with only the keys that meet the specified criteria.

Here’s an example of how you might do this:

const obj = {
  key1: 'value1',
  key2: 'value2',
  key3: 'value3'
};

const filteredKeys = Object.keys(obj).filter(key => key.startsWith('key'));

console.log(filteredKeys); // ['key1', 'key2', 'key3']

This will create a new array filteredKeys that contains only the keys of the obj object that start with the string 'key'.

You can also use the Array.map() method to transform the keys in the array, or the Array.reduce() method to create a new object with only the keys that meet the specified criteria.

For example:

const filteredObj = filteredKeys.reduce((acc, key) => {
  acc[key] = obj[key];
  return acc;
}, {});

console.log(filteredObj); // { key1: 'value1', key2: 'value2', key3: 'value3' }

This will create a new object filteredObj that contains only the key-value pairs from the obj object where the key starts with the string 'key'.

How to Filter an Object by Value

To filter an object by value in JavaScript, you can use the Object.entries() method to get an array of the object’s key-value pairs, and then use the Array.filter() method to create a new array with only the key-value pairs that meet the specified criteria.

Here’s an example of how you might do this using an object with Xbox game names as the values:

const xboxGames = {
  game1: 'Halo',
  game2: 'Gears of War',
  game3: 'Forza',
  game4: 'Fable'
};

const filteredGames = Object.entries(xboxGames).filter(([key, value]) => value.startsWith('F'));

console.log(filteredGames); // [['game3', 'Forza'], ['game4', 'Fable']]

This will create a new array filteredGames that contains only the key-value pairs from the xboxGames object where the value starts with the letter 'F'.

You can also use the Array.map() method to transform the key-value pairs in the array, or the Array.reduce() method to create a new object with only the key-value pairs that meet the specified criteria.

For example:

const filteredObj = filteredGames.reduce((acc, [key, value]) => {
  acc[key] = value;
  return acc;
}, {});

console.log(filteredObj); // { game3: 'Forza', game4: 'Fable' }

This will create a new object filteredObj that contains only the key-value pairs from the xboxGames object where the value starts with the letter 'F'.

Sharing is caring 🙂