Cookies are small text files stored on your computer. When a web server sends a page to a browser, the connection is closed, and the server forgets everything about the user. Cookies solve the problem of remembering user information:

When a user visits a page, their name can be stored in a cookie.
The next time the user visits the page, the cookie "remembers" their name.

Cookies are saved in name-value pairs like:

username=John Doe

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When a browser requests a web page from a server, cookies associated with the page are sent with the request, allowing the server to remember user information.

Create a Cookie with JavaScript

JavaScript can create, read, and delete cookies with the document.cookie property.

To create a cookie:

document.cookie = "username=John Doe";

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To add an expiry date (in UTC time):

document.cookie = "username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC";

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To specify the path the cookie belongs to:

document.cookie = "username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC; path=/";

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Read a Cookie with JavaScript

To read a cookie:

let x = document.cookie;

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This returns all cookies as a single string: cookie1=value; cookie2=value; cookie3=value;

Change a Cookie with JavaScript

To change a cookie:

document.cookie = "username=John Smith; expires=Thu, 18 Dec 2013 12:00:00 UTC; path=/";

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The old cookie is overwritten.

Delete a Cookie with JavaScript

To delete a cookie, set its expiry date to a past date:

document.cookie = "username=; expires=Thu, 01 Jan 1970 00:00:00 UTC; path=/;";

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Specify the cookie path to ensure the correct cookie is deleted.

The Cookie String

The document.cookie property looks like a normal text string but only shows the name-value pairs. Setting a new cookie adds to document.cookie without overwriting existing cookies.

JavaScript Cookie Example

Create a cookie to store a visitor's name. The first time a visitor arrives, they will be asked to fill in their name. The name is stored in a cookie, and the next time the visitor arrives, they will see a welcome message.

Function to Set a Cookie

function setCookie(cname, cvalue, exdays) {  const d = new Date();  d.setTime(d.getTime() + (exdays * 24 * 60 * 60 * 1000));  let expires = "expires=" + d.toUTCString();  document.cookie = cname + "=" + cvalue + ";" + expires + ";path=/";}‍

Parameters:

cname: Cookie name
cvalue: Cookie value
exdays: Number of days until the cookie expires

Function to Get a Cookie

function getCookie(cname) {  let name = cname + "=";  let decodedCookie = decodeURIComponent(document.cookie);  let ca = decodedCookie.split(';');  for (let i = 0; i < ca.length; i++) {    let c = ca[i];    while (c.charAt(0) == ' ') {      c = c.substring(1);    }    if (c.indexOf(name) == 0) {      return c.substring(name.length, c.length);    }  }  return "";}

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Parameters:

cname: Cookie name to search for

Function to Check a Cookie

function checkCookie() {  let username = getCookie("username");  if (username != "") {    alert("Welcome again " + username);  } else {    username = prompt("Please enter your name:", "");    if (username != "" && username != null) {      setCookie("username", username, 365);    }  }}

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All Together

function setCookie(cname, cvalue, exdays) {
  const d = new Date();
  d.setTime(d.getTime() + (exdays * 24 * 60 * 60 * 1000));
  let expires = "expires=" + d.toUTCString();
  document.cookie = cname + "=" + cvalue + ";" + expires + ";path=/";
}

function getCookie(cname) {
  let name = cname + "=";
  let ca = document.cookie.split(';');
  for (let i = 0; i < ca.length; i++) {
    let c = ca[i];
    while (c.charAt(0) == ' ') {
      c = c.substring(1);
    }
    if (c.indexOf(name) == 0) {
      return c.substring(name.length, c.length);
    }
  }
  return "";
}

function checkCookie() {
  let user = getCookie("username");
  if (user != "") {
    alert("Welcome again " + user);
  } else {
    user = prompt("Please enter your name:", "");
    if (user != "" && user != null) {
      setCookie("username", user, 365);
    }
  }
}

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