Useful JavaScript Code Snippets for Common Front‑End Tasks

Get Browser Cookie Value

Use document.cookie to retrieve a specific cookie value.

const cookie = name => `; ${document.cookie}`.split(`; ${name}=`).pop().split(';').shift();

cookie('_ga'); // Result: "GA1.2.1929736587.1601974046"

Convert RGB Color to Hexadecimal

Transform RGB components into a hex string.

const rgbToHex = (r, g, b) => "#" + ((1 << 24) + (r << 16) + (g << 8) + b).toString(16).slice(1);

rgbToHex(0, 51, 255); // Result: #0033ff

Copy Text to Clipboard

Leverage navigator.clipboard.writeText to copy a string.

const copyToClipboard = text => navigator.clipboard.writeText(text);

copyToClipboard("Hello World");

Validate Date Legitimacy

Check whether a given date string can be parsed into a valid Date object.

const isDateValid = (...val) => !Number.isNaN(new Date(...val).valueOf());

isDateValid("December 17, 1995 03:24:00"); // Result: true

Find Day of Year

Calculate which day of the year a date falls on.

const dayOfYear = date => Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24);

dayOfYear(new Date()); // Result: 272 (example)

Capitalize First Letter of a String

JavaScript lacks a built‑in method, so use a custom function.

const capitalize = str => str.charAt(0).toUpperCase() + str.slice(1);

capitalize("follow for more"); // Result: Follow for more

Calculate Days Between Two Dates

Return the absolute difference in days.

const dayDif = (date1, date2) => Math.ceil(Math.abs(date1.getTime() - date2.getTime()) / 86400000);

dayDif(new Date("2020-10-21"), new Date("2021-10-22")); // Result: 366

Clear All Cookies

Iterate over document.cookie and set each cookie’s expiration to the past.

const clearCookies = document.cookie.split(';').forEach(cookie =>
  document.cookie = cookie.replace(/^ +/, '').replace(/=.*/, `=;expires=${new Date(0).toUTCString()};path=/`)
);

Generate Random Hexadecimal Color

Combine Math.random with padEnd to produce a random hex code.

const randomHex = () => `#${Math.floor(Math.random() * 0xffffff).toString(16).padEnd(6, "0")}`;

console.log(randomHex()); // Result: #92b008

Array Deduplication

Use a Set to remove duplicate elements.

const removeDuplicates = arr => [...new Set(arr)];

console.log(removeDuplicates([1,2,3,3,4,4,5,5,6])); // Result: [1,2,3,4,5,6]

Get URL Query Parameters

Parse the query string manually or with URLSearchParams for a concise solution.

const getParameters = URL => {
  URL = JSON.parse('{"' +
    decodeURI(URL.split("?")[1])
      .replace(/"/g, '\\"')
      .replace(/&/g, '","')
      .replace(/=/g, '":"') +
    '"}');
  return JSON.stringify(URL);
};

getParameters(window.location); // Result: { search : "easy", page : 3 }

// Simpler version:
Object.fromEntries(new URLSearchParams(window.location.search));

Time Formatting

Extract hh:mm:ss from a Date object.

const timeFromDate = date => date.toTimeString().slice(0, 8);

console.log(timeFromDate(new Date(2021,0,10,17,30,0))); // Result: "17:30:00"

Check If Number Is Even

Simple modulus test.

const isEven = num => num % 2 === 0;

console.log(isEven(2)); // Result: True

Calculate Average of Numbers

Use reduce to sum values and divide by count.

const average = (...args) => args.reduce((a,b) => a + b) / args.length;

average(1,2,3,4); // Result: 2.5

Scroll Back to Top

Invoke window.scrollTo(0, 0) to jump to the top of the page.

const goToTop = () => window.scrollTo(0, 0);

goToTop();

Reverse a String

Combine split , reverse , and join .

const reverse = str => str.split('').reverse().join('');

reverse('hello world'); // Result: 'dlrow olleh'

Check If Array Is Empty

One‑liner returning a boolean.

const isNotEmpty = arr => Array.isArray(arr) && arr.length > 0;

isNotEmpty([1,2,3]); // Result: true

Get User‑Selected Text

Use the getSelection API.

const getSelectedText = () => window.getSelection().toString();

getSelectedText();

Shuffle an Array

Sort with a random comparator.

const shuffleArray = arr => arr.sort(() => 0.5 - Math.random());

console.log(shuffleArray([1,2,3,4])); // Result: [1,4,3,2] (example)

Detect Dark Mode Preference

Query the prefers-color-scheme media feature.

const isDarkMode = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches;

console.log(isDarkMode); // Result: True or False