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JavaScript array filter

last modified February 15, 2021

JavaScript array filter tutorial shows how to filter arrays in JavaScript. The filter function creates a new array with all elements that pass predicate function.

An array is a collection of a number of values. The array items are called elements of the array.

Predicate

Predicate in general meaning is a statement about something that is either true or false. In programming, predicates represent single argument functions that return a boolean value.

JS filter function

The filter function creates a new array with all elements that satisfy the given predicate.

filter(predicate, [ctx])

The filter function takes the predicate as its first parameter. The second parameter is optional; it is a helper context object. Inside the predicate, we can access the properties of the context object with the this keyword.

JS array filter example

In the first example, we filter numbers in an array.

positive.js
let nums = [4, -5, 3, 2, -1, 7, -6, 8, 9];

let pos_nums = nums.filter((e) => e > 0);
console.log(pos_nums);

We have an array of numbers. With the filter function, we create a new array that contains only positive numbers.

let pos_nums = nums.filter((e) => e > 0);

In this case, the predicate is an anonymous function which returns true for values greater than zero.

$ node positive.js 
[ 4, 3, 2, 7, 8, 9 ]

JS array filter context

In the next example, we use a context object in the filtering.

filter_range.js
function isInRange(val) {

    return val >= this.lower && val <= this.upper;
}

let range = {
    lower: 1,
    upper: 10
};

let data = [-2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];

let res = data.filter(isInRange, range);
console.log(res);

We have an array of values. We specify the lower and upper bounds for the filtering in the context object.

function isInRange(val) {

    return val >= this.lower && val <= this.upper;
}

We access the properties of the context via the this keyword.

let range = {
    lower: 1,
    upper: 10
};

This is the context object.

let res = data.filter(isInRange, range);

We pass the context as the second parameter.

$ node filter_range.js 
[
  1, 2, 3, 4,  5,
  6, 7, 8, 9, 10
]

JS array filter datatype

The typeof operator returns a string indicating the type of the operand.

filter_datatype.js
function isNumber(value) {
    if (typeof value === 'number') {
        return true;
    }
}

let data = [10, null, "30", 1.4, 'falcon', undefined, true, 17];

let res = data.filter(isNumber);
console.log(res);

In the example, the data array has elements of various data types. We filter out all numbers.

function isNumber(value) {
    if (typeof value === 'number') {
        return true;
    }
}

In the isNumber predicate, we check for numeric values using the typeof operator.

$ node filter_datatype.js 
[ 10, 1.4, 17 ]

JS array filter objects

We have an array of objects. We filter the array based on the object property.

filter_by_city.js
const users = [
  { name: 'John', city: 'London', born: '2001-04-01' },
  { name: 'Lenny', city: 'New York', born: '1997-12-11' },
  { name: 'Andrew', city: 'Boston', born: '1987-02-22' },
  { name: 'Peter', city: 'Prague', born: '1936-03-24' },
  { name: 'Anna', city: 'Bratislava', born: '1973-11-18' },
  { name: 'Albert', city: 'Bratislava', born: '1940-12-11' },
  { name: 'Adam', city: 'Trnava', born: '1983-12-01' },
  { name: 'Robert', city: 'Bratislava', born: '1935-05-15' },
  { name: 'Robert', city: 'Prague', born: '1998-03-14' }
];


let res = users.filter(user => user.city === 'Bratislava');
console.log(res);

The example filters out all users who live in Bratislava.

$ node filter_by_city
[
  { name: 'Anna', city: 'Bratislava', born: '1973-11-18' },
  { name: 'Albert', city: 'Bratislava', born: '1940-12-11' },
  { name: 'Robert', city: 'Bratislava', born: '1935-05-15' }
]

We can filter the objects by multiple properties.

filter_by_city_name.js
const users = [
  { name: 'John', city: 'London', born: '2001-04-01' },
  { name: 'Lenny', city: 'New York', born: '1997-12-11' },
  { name: 'Andrew', city: 'Boston', born: '1987-02-22' },
  { name: 'Peter', city: 'Prague', born: '1936-03-24' },
  { name: 'Anna', city: 'Bratislava', born: '1973-11-18' },
  { name: 'Albert', city: 'Bratislava', born: '1940-12-11' },
  { name: 'Adam', city: 'Trnava', born: '1983-12-01' },
  { name: 'Robert', city: 'Bratislava', born: '1935-05-15' },
  { name: 'Robert', city: 'Prague', born: '1998-03-14' }
];

let res = users.filter(user => user.city === 'Bratislava' && user.name.startsWith('A'));
console.log(res);

We filter all users who live in Bratislava and whose names begins with 'A'.

$ node filter_by_city_name.js 
[
  { name: 'Anna', city: 'Bratislava', born: '1973-11-18' },
  { name: 'Albert', city: 'Bratislava', born: '1940-12-11' }
]

In the next example, we are going to filter by age. For this, we need the moment library.

$ nmp i moment

We install moment library.

filter_by_age.js
const moment = require('moment');

const users = [
  { name: 'John', city: 'London', born: '2001-04-01' },
  { name: 'Lenny', city: 'New York', born: '1997-12-11' },
  { name: 'Andrew', city: 'Boston', born: '1987-02-22' },
  { name: 'Peter', city: 'Prague', born: '1936-03-24' },
  { name: 'Anna', city: 'Bratislava', born: '1973-11-18' },
  { name: 'Albert', city: 'Bratislava', born: '1940-12-11' },
  { name: 'Adam', city: 'Trnava', born: '1983-12-01' },
  { name: 'Robert', city: 'Bratislava', born: '1935-05-15' },
  { name: 'Robert', city: 'Prague', born: '1998-03-14' }
];

let res = users.filter(user => getAge(user.born) > 40);

console.log(res);


function getAge(dt) {

    return moment.duration(moment() - moment(dt, 'YYYY-MM-DD', true)).years();
}

The example filters out all users older than 40 years.

$ node filter_by_age.js 
[
  { name: 'Peter', city: 'Prague', born: '1936-03-24' },
  { name: 'Anna', city: 'Bratislava', born: '1973-11-18' },
  { name: 'Albert', city: 'Bratislava', born: '1940-12-11' },
  { name: 'Robert', city: 'Bratislava', born: '1935-05-15' }
]

In this tutorial, we have covered filtering of JavaScript arrays.

List all JavaScript tutorials.