Thursday, 13 October 2016

Javascript Local Storage

Javascript Local Storage

LocalStorage is used to stored web application data within the user's browser. LocalStorage is only store strings in the different keys

How to check user's browser support?
if (typeof(Storage) !== "undefined") {
    // Code for localStorage/sessionStorage.
} else {
    // Sorry! No Web Storage support..
}

How to Set Values in localStorage
localStorage.setItem('favoriteflavor','vanilla');

How to Access localStorage variable

If you read out the favoriteflavor key, you will get back “vanilla”:
var taste = localStorage.getItem('favoriteflavor');

How to remove Elements from localStorage

localStorage.removeItem('favoriteflavor');
var taste = localStorage.getItem('favoriteflavor');


How to Store Array in LocalStorage
localStorage is for key : value pairs, so what you'd probably want to do is JSON.stringify the array
and store the string in the mycars key and then you can pull it back out and JSON.parse

Example 1:
var mycars = new Array();
mycars[0] = "Saab";
mycars[1] = "Volvo";
mycars[2] = "BMW";

localStorage["mycars"] = JSON.stringify(mycars);

var cars = JSON.parse(localStorage["mycars"]);

$.each(cars, function(key, value){
    console.log(key + ' = ' + value);
});

Example 2:
var cast = {
    "Adm. Adama" : "Edward James Olmos",
    "President Roslin" : "Mary McDonnell",
    "Captain Adama" : "Jamie Bamber",
    "Gaius Baltar" : "James Callis",
    "Number Six" : "Tricia Helfer",
    "Kara Thrace" : "    Katee Sackhoff"
};

// Stores the JavaScript object as a string
localStorage.setItem("cast", JSON.stringify(cast));

// Parses the saved string into a JavaScript object again
JSON.parse(localStorage.getItem("cast"));

How to Storing image in localstorage

Example 1:
HTML

<figure>
    <img id="elephant" src="about:blank" alt="A close up of an elephant">
    <noscript>
        <img src="elephant.png" alt="A close up of an elephant">
    </noscript>  
    <figcaption>A mighty big elephant, and mighty close too!</figcaption>
</figure>

// localStorage with image
var storageFiles = JSON.parse(localStorage.getItem("storageFiles")) || {},
    elephant = document.getElementById("elephant"),
    storageFilesDate = storageFiles.date,
    date = new Date(),
    todaysDate = (date.getMonth() + 1).toString() + date.getDate().toString();

// Compare date and create localStorage if it's not existing/too old  
if (typeof storageFilesDate === "undefined" || storageFilesDate < todaysDate) {
    // Take action when the image has loaded
    elephant.addEventListener("load", function () {
        var imgCanvas = document.createElement("canvas"),
            imgContext = imgCanvas.getContext("2d");

        // Make sure canvas is as big as the picture
        imgCanvas.width = elephant.width;
        imgCanvas.height = elephant.height;

        // Draw image into canvas element
        imgContext.drawImage(elephant, 0, 0, elephant.width, elephant.height);

        // Save image as a data URL
        storageFiles.elephant = imgCanvas.toDataURL("image/png");

        // Set date for localStorage
        storageFiles.date = todaysDate;

        // Save as JSON in localStorage
        try {
            localStorage.setItem("storageFiles", JSON.stringify(storageFiles));
        }
        catch (e) {
            console.log("Storage failed: " + e);
        }
    }, false);

    // Set initial image src  
    elephant.setAttribute("src", "elephant.png");
}
else {
    // Use image from localStorage
    elephant.setAttribute("src", storageFiles.elephant);
}

Alternative Method:

bannerImage = document.getElementById('bannerImg');
imgData = getBase64Image(bannerImage);
localStorage.setItem("imgData", imgData);

Here is the function that converts the image to a Base64 sting:

function getBase64Image(img) {
    var canvas = document.createElement("canvas");
    canvas.width = img.width;
    canvas.height = img.height;

    var ctx = canvas.getContext("2d");
    ctx.drawImage(img, 0, 0);

    var dataURL = canvas.toDataURL("image/png");

    return dataURL.replace(/^data:image\/(png|jpg);base64,/, "");
}

Method used to display in image
var dataImage = localStorage.getItem('imgData');
bannerImg = document.getElementById('tableBanner');
bannerImg.src = "data:image/png;base64," + dataImage;


How to Save Form data in localStorage

When the user submits the form we are going to save their name so that we can use it later to show them a personalised message.

<form id="contactForm" action="contact.php" method="POST">
    <div class="field">
        <label for="name">Name</label>
        <input type="text" name="name" id="name">
    </div>
  <div class="field">
    <label for="email">Email</label>
    <input type="email" name="email" id="email">
  </div>
  <div class="field">
    <label for="message">Message</label>
    <textarea name="message" id="message"></textarea>
  </div>
  <div class="field">
    <input type="submit" value="send">
  </div>
</form>

window.onload = function() {

  // Check for LocalStorage support.
  if (localStorage) {

    // Add an event listener for form submissions
    document.getElementById('contactForm').addEventListener('submit', function() {
      // Get the value of the name field.
      var name = document.getElementById('name').value;

      // Save the name in localStorage.
      localStorage.setItem('name', name);
    });

  }

}

window.onload = function() {
  ...

  // Retrieve the users name.
  var name = localStorage.getItem('name');

  if (name != "undefined" || name != "null") {
    document.getElementById('welcomeMessage').innerHTML = "Hello " + name + "!";
  } else
    document.getElementById('welcomeMessage').innerHTML = "Hello!";
  }
}

Clearing the Datastore
If you want to delete all of the data in the datastore you can use the clear() function.
localStorage.clear();

key(), that can be used to retrieve the key of a data item using the index (numerical position) of the item in the datastore.

for (var i = 0; i < localStorage.length; i++) {
  console.log(localStorage.key(i))
};

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