filter
- Node.js course
- Beginner
- Lesson 60/116
- 5 min
Learn to filter elements matching a condition with .filter.
The filter method
The filter method returns a new array with the elements that satisfy a condition. Items that fail the condition are skipped; the original array is again left untouched.
The syntax mirrors array.map(fn): array.filter(fn). Here fn is a predicate — a function that returns a boolean (true or false) for each element. Items for which it returns true stay in the result.
Example: [1, 2, 3, 4].filter(n => n > 2) returns [3, 4], because only 3 and 4 are greater than 2.
filter and predicate
The function passed to filter is called a predicate. It returns a boolean: true keeps the element, false drops it.
The predicate can use the element itself; for example, to keep adults from a list of ages you write ages.filter(age => age >= 18). The more complex the condition, the more readable the predicate should be.
If no element matches, filter returns an empty array — it does not return null or throw. You can rely on this.
Practice Pick the expensive books
Define an array of prices [120, 45, 200, 80]. Use filter to build a new array with only the prices greater than 100, and print the result with console.log.
Starter code
const prices = [120, 45, 200, 80];
// Filter prices above 100 and printSample solution
const prices = [120, 45, 200, 80];
const expensive = prices.filter(p => p > 100);
console.log(expensive);Expected output
[ 120, 200 ]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Filter values above two
const nums = [1, 2, 3, 4];
const big = nums.filter(n => n ____ 2);const nums = [1, 2, 3, 4];
const big = nums.filter(n => n > 2);Explanation: Only values greater than 2 stay; the result is [3, 4].
2 What does filter return?
- true
- false
- A boolean
- A new array Correct answer
Explanation: filter always returns a new array, not a boolean.
3 Match the filter parts
| Term | Meaning |
|---|---|
array.filter(fn) |
Filtering call |
fn |
Predicate |
true |
Result that keeps the item |
false |
Result that drops the item |