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Javascript in 100 bits

Course by zooboole,

Last Updated on 2025-01-28 08:04:00

Lesson 34 - JavaScript Array Methods – filter()

The filter() method creates a new array containing elements that pass a specific condition defined by a callback function.

Explanation

  • filter() loops through an array and includes only elements that return true from the callback function.
  • It does not modify the original array.

Syntax:

let filteredArray = array.filter(function(element, index, array) {
  // return true to keep the element, false to exclude
});
  • element: The current element being processed.
  • index (optional): The index of the current element.
  • array (optional): The array filter was called upon.

Example 1: Filtering Even Numbers

let numbers = [1, 2, 3, 4, 5, 6];

let evens = numbers.filter(num => num % 2 === 0);

console.log(evens); // Output: [2, 4, 6]
  • Only even numbers are kept.

Example 2: Filtering Objects by Property

let users = [
  { name: 'Alice', age: 25 },
  { name: 'Bob', age: 17 },
  { name: 'Charlie', age: 30 }
];

let adults = users.filter(user => user.age >= 18);

console.log(adults);
// Output: [{ name: 'Alice', age: 25 }, { name: 'Charlie', age: 30 }]
  • Filters users who are 18 or older.

Example 3: Removing Falsy Values

let mixedArray = [0, "Hello", false, "", 42, null, "JavaScript"];

let truthyValues = mixedArray.filter(Boolean);

console.log(truthyValues); // Output: ["Hello", 42, "JavaScript"]
  • Removes falsy values like 0, false, "", and null.

Try Your Hand

Steps:

  1. Create a File:

    • Name it lesson34-filter.html.

  2. Write the Code:

    • Copy the following code:

      <!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Lesson 34 - filter() Method</title>
</head>
<body>
<h2>Even Numbers</h2>
<ul id="evenList"></ul>

<script>
   let numbers = [10, 15, 20, 25, 30, 35];
   let evens = numbers.filter(num => num % 2 === 0);

   let list = document.getElementById("evenList");

   evens.forEach(num => {
       let li = document.createElement("li");
       li.textContent = num;
       list.appendChild(li);
   });
</script>
</body>
</html>

  3. Save the File.

  4. Open it in a Browser to See the Even Numbers.

Experiment

  1. Filter an array of names to include only names starting with the letter "A".
  2. Given an array of numbers, filter out all numbers less than 50.
  3. From an array of objects representing tasks, filter out only completed tasks.

Key Takeaways

  • filter() returns a new array based on a true/false condition.
  • The original array remains unchanged.
  • Useful for refining data sets.