filter

Creates a new array with elements that pass the test implemented by the provided callback function. Iterates through each element of the source array and includes only those elements for which the callback function returns a truthy value. The callback receives the current element, its index, and the entire array as arguments.

Signature

const filter: <Type>(array: Type[], callback: (item: Type, index: number, array: Type[]) => boolean) => Type[]

Parameters

Name	Type	Description
array	-	Source array to filter
callback	-	Filtering function to test each element

Returns

New filtered array containing only elements that pass the test

Examples

Filter numbers by condition

import { filter } from '@winglet/common-utils';

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const evens = filter(numbers, num => num % 2 === 0);
console.log(evens); // [2, 4, 6, 8, 10]

const greaterThanFive = filter(numbers, num => num > 5);
console.log(greaterThanFive); // [6, 7, 8, 9, 10]

Filter objects by property

interface User {
  id: number;
  name: string;
  age: number;
  isActive: boolean;
}

const users: User[] = [
  { id: 1, name: 'Alice', age: 30, isActive: true },
  { id: 2, name: 'Bob', age: 25, isActive: false },
  { id: 3, name: 'Charlie', age: 35, isActive: true },
  { id: 4, name: 'Diana', age: 28, isActive: false }
];

const activeUsers = filter(users, user => user.isActive);
console.log(activeUsers); // [{ id: 1, name: 'Alice', ... }, { id: 3, name: 'Charlie', ... }]

const adultsOver30 = filter(users, user => user.age > 30);
console.log(adultsOver30); // [{ id: 3, name: 'Charlie', age: 35, ... }]

Using index and array parameters

const items = ['a', 'b', 'c', 'd', 'e'];

// Filter by index (get every other element)
const everyOther = filter(items, (item, index) => index % 2 === 0);
console.log(everyOther); // ['a', 'c', 'e']

// Filter based on array context
const duplicatesRemoved = filter(items, (item, index, array) =>
  array.indexOf(item) === index
);

Filter strings by criteria

const words = ['apple', 'banana', 'apricot', 'cherry', 'avocado'];

// Filter by starting letter
const startsWithA = filter(words, word => word.startsWith('a'));
console.log(startsWithA); // ['apple', 'apricot', 'avocado']

// Filter by length
const longWords = filter(words, word => word.length > 5);
console.log(longWords); // ['banana', 'apricot', 'cherry', 'avocado']

Complex filtering with multiple conditions

interface Product {
  name: string;
  price: number;
  category: string;
  inStock: boolean;
}

const products: Product[] = [
  { name: 'Laptop', price: 999, category: 'Electronics', inStock: true },
  { name: 'Book', price: 15, category: 'Education', inStock: true },
  { name: 'Phone', price: 699, category: 'Electronics', inStock: false },
  { name: 'Desk', price: 299, category: 'Furniture', inStock: true }
];

// Filter expensive electronics in stock
const expensiveElectronics = filter(products, product =>
  product.category === 'Electronics' &&
  product.price > 500 &&
  product.inStock
);
console.log(expensiveElectronics); // [{ name: 'Laptop', ... }]

Working with nested objects

const employees = [
  { name: 'John', skills: ['JavaScript', 'Python'], department: { name: 'Engineering' } },
  { name: 'Jane', skills: ['Java', 'C++'], department: { name: 'Engineering' } },
  { name: 'Bob', skills: ['Marketing', 'Sales'], department: { name: 'Sales' } }
];

// Filter engineers with JavaScript skills
const jsEngineers = filter(employees, emp =>
  emp.department.name === 'Engineering' &&
  emp.skills.includes('JavaScript')
);
console.log(jsEngineers); // [{ name: 'John', ... }]

Playground

import { filter } from '@winglet/common-utils';

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const evens = filter(numbers, num => num % 2 === 0);
console.log(evens); // [2, 4, 6, 8, 10]

const greaterThanFive = filter(numbers, num => num > 5);
console.log(greaterThanFive); // [6, 7, 8, 9, 10]

Notes

Performance: Uses dynamic array growth with direct index assignment for optimal performance. Time complexity is O(n) where n is the array length.

Callback Parameters: The callback function receives three parameters:

• item: The current element being processed
• index: The index of the current element
• array: The entire source array

Immutability: Does not modify the original array, returns a new filtered array.

Truthy Values: Elements are included if the callback returns any truthy value, not just boolean true.