Using Filter Javascript for Arrays

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Arrays are everywhere in JavaScript. Whether you’re building an app, managing data, or just sorting through a grocery list, arrays make life easier. And what’s one of the handiest tools to work with arrays? The filter() method!

But don’t worry. This won’t be a dull coding lecture. We’ll make it fun and simple. Let’s dive in!

TL;DR

The JavaScript filter() method helps you create a new array by pulling out only the stuff you want. It doesn’t change the original array. Instead, it lets you grab just what you need using a function. Think of it like a coffee filter: it keeps the good stuff and leaves the rest behind.

Why Use filter()?

Filtering is useful when you need a subset of data. Not all items in an array are always useful. You might want:

  • Only even numbers
  • Only adults out of a list of people
  • Just the completed tasks

filter() to the rescue!

How Does It Work?

The filter() method loops through an array and applies a test (a function). If the test passes (returns true), the item is kept. If the test fails (returns false), it’s dropped.

Here’s a super simple example:


const numbers = [1, 2, 3, 4, 5];
const evenNumbers = numbers.filter(function(num) {
  return num % 2 === 0;
});
console.log(evenNumbers); // [2, 4]

What’s going on?

  • We have an array of numbers.
  • We run each number through a test: Is it even?
  • We keep only the ones that pass the test.

Easy, right?

Arrow Functions Make It Slick

Need it even cleaner? Use arrow functions:


const evenNumbers = numbers.filter(num => num % 2 === 0);

That’s it. Slim and readable!

Real-Life Examples

Let’s see how this helps in everyday code life. Ready?

Filter Only Adults


const people = [
  { name: "Emily", age: 17 },
  { name: "Jake", age: 25 },
  { name: "Sarah", age: 19 },
  { name: "Tom", age: 16 }
];

const adults = people.filter(person => person.age >= 18);
console.log(adults);
// [{ name: "Jake", age: 25 }, { name: "Sarah", age: 19 }]

filter() lets us pick only the people aged 18 or older. Boom. The party is age-restricted 🎉.

Filter Completed Tasks


const tasks = [
  { task: "Buy milk", done: true },
  { task: "Walk dog", done: false },
  { task: "Read book", done: true }
];

const completed = tasks.filter(task => task.done);
console.log(completed);
// [{ task: "Buy milk", done: true }, { task: "Read book", done: true }]

The code above sniffs out only the tasks marked as done. Very helpful when you need to show a progress report or encourage users.

Break It Down

The function inside filter() gets three arguments:

  1. item – The current item being looked at
  2. index – Its position in the array (optional)
  3. array – The full array (optional)

Let’s use all three:


const fruits = ["apple", "banana", "cherry", "apple"];
const filtered = fruits.filter((fruit, index, array) => {
  console.log(`Looking at: ${fruit}, at index ${index}`);
  return fruit === "apple";
});
console.log(filtered);
// ["apple", "apple"]

filter() gives full control. Power in your hands!

Filter Is Non-Destructive

Important note: filter() does NOT change the original array. 


const nums = [10, 20, 30];
const under20 = nums.filter(n => n < 20);
console.log(nums);      // [10, 20, 30]
console.log(under20);   // [10]

That’s great if you like to keep your data safe.

Use With Chaining

You can chain filter() with other array methods!


const people = [
  { name: "Alice", age: 25 },
  { name: "Bob", age: 23 },
  { name: "Carl", age: 17 }
];

const names = people
  .filter(p => p.age >= 18)
  .map(p => p.name);

console.log(names); // ["Alice", "Bob"]

First, we filter out non-adults. Then we grab just their names. Nice and clean!

Common Gotchas

  • forEach vs filter: Don’t use filter() if you don’t care about the result. That’s what forEach() is for.
  • Returns a new array: Always. It never mutates.
  • Always return a boolean: Your test function must return true or false. Nothing else will do.

When to Use filter()

Here’s when filter() is your BFF:

  • You want to collect only the items that match some rule
  • You want a smaller version of the array
  • You want to avoid changing the original data

Bonus: Custom Filter Function

Want to create your own version of filter()? Let’s do a fun experiment: 


Array.prototype.myFilter = function(callback) {
  const result = [];
  for (let i = 0; i < this.length; i++) {
    if (callback(this[i], i, this)) {
      result.push(this[i]);
    }
  }
  return result;
};

const nums = [1, 2, 3, 4];
const odds = nums.myFilter(num => num % 2 !== 0);
console.log(odds); // [1, 3]

Look at you – making custom tools like a JavaScript wizard 🧙‍♂️.

Wrap Up

The filter() method is a small tool with huge power. If you master it, working with arrays becomes a breeze. Whether you’re trimming data, pulling out the goodies, or just cleaning things up – filter() is the perfect teammate.

  • It’s easy to read
  • It’s safe (doesn’t mess up originals)
  • It works well with other methods like map()

So next time you look at an array and think, “I only want a few of these,” you know what to reach for – the filter() function!