Filter method in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the filter method do in JavaScript?

The filter method creates a new array with all elements that pass a test implemented by a provided function.

beginner

How do you use the filter method syntax?

You call array.filter(callback(element, index, array)). The callback returns true to keep the element, or false to remove it.

beginner

What type of value must the callback function return when used with filter?

The callback must return a boolean value: true to keep the element, false to exclude it from the new array.

beginner

Can the filter method change the original array?

No, filter does not modify the original array. It returns a new array with the filtered elements.

beginner

Give a simple example of using filter to get even numbers from an array.

Example:<br><code>const numbers = [1, 2, 3, 4];<br>const evens = numbers.filter(n =&gt; n % 2 === 0);<br>console.log(evens); // [2, 4]</code>

What does the filter method return?

A A new array with elements that pass the test

B The original array modified

C A single element from the array

D Undefined

The filter method returns a new array containing only the elements that pass the test in the callback function.

Which value should the callback function return to keep an element?

A true

B false

C null

D undefined

Returning true keeps the element in the new filtered array.

Does filter change the original array?

A Only if you assign it back

B Yes, it modifies the original array

C No, it returns a new array

D Sometimes, depends on the callback

filter always returns a new array and does not modify the original.

What will [1, 2, 3].filter(n => n > 2) return?

A [1, 2, 3]

B [3]

C [1, 2]

D []

Only the number 3 is greater than 2, so the new array contains [3].

Which of these is a valid use of filter?

A array.filter(element => element.length)

B array.filter(element => element * 2)

C array.filter(element => console.log(element))

D array.filter(element => element === 5)

The callback must return a boolean. element === 5 returns true or false, so it is valid.

Explain how the filter method works and what it returns.

Think about how you pick items from a list based on a rule.

Write a simple example using filter to select only strings longer than 3 characters from an array.

Use <code>str.length &gt; 3</code> inside the callback.