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Web Storage API는 브라우저에서 쿠키를 사용하는 것보다 훨씬 직관적으로 key/value 데이터를 안전하게 저장할 수 있는 메커니즘을 제공합니다. 이 아티클은 이 간단한 기술을 어떻게 사용하는 지 설명합니다.

기본 컨셉

Storage 객체는 단순한 key-value 저장소이며, 이는 객체와 비슷합니다. 하지만 이 데이터들은 페이지 로딩에도 온전하게 유지됩니다. key와 그 value는 항상 문자열입니다. (만약 정수로 키를 사용할 경우 이는 자동으로 string으로 변경됩니다, 자바스크립트 객체의 동작방식을 생각해보세요) 객체를 사용하듯이 쉽게 값에 접근할 수 있으며, 이 때 Storage.getItem()과 Storage.setItem() 메서드를 사용합니다. 아래 세 줄은 colorSetting 엔트리를 가져오는 똑같은 방법입니다.

localStorage.colorSetting = '#a4509b';
localStorage['colorSetting'] = '#a4509b';
localStorage.setItem('colorSetting', '#a4509b');

노트: 일반 객체를 key-value 저장소로 사용할 때 pitfalls과 관련된 사항을 막기 위해 Web Storage API(setItem, getItemremoveItemkeylength)를 사용하는 걸 권장합니다.

Web Storage는 두 메커니즘을 가지고 있습니다.

  • sessionStorage는 페이지의 세션이 유지되는동안 사용할 수 있는 각 origin별로 별도의 스토리지를 관리합니다. (페이지 리로딩 및 복원을 포함한, 브라우저가 열려있는 한 최대한 긴 시간동안)
  • localStorage도 같은 일을 하지만, 브라우저가 닫히거나 다시 열리더라도 유지합니다.

이 메커니즘들은 Window.sessionStorage와 Window.localStorage 속성(좀 더 정확히 말하자면, 지원하는 브라우저에서 Window 객체는 localStorage 및 sessionStorage 속성 사용이 중단되는 WindowLocalStorageWindowSessionStorage로 구현됩니다)으로 사용 가능합니다. 이 중 하나를 호출하면 데이터를 설정, 검색 및 제거할 수 있는 Storage 객체의 인스턴스가 생성됩니다. 각 Storage 객체는 각 origin 별 sessionStorage 나 localStorage로 사용됩니다. 동작도 제각기 동작합니다.

예를 들면, 문서에서 localStorage를 호출하면 Storage 객체를 반환합니다. 문서에서 sessionStorage를 호출하면 다른 Storage 객체를 반환합니다. 둘 다 같은 방법으로 조작할 수 있지만, 서로 다릅니다.

Feature-detecting localStorage

To be able to use localStorage, we should first verify that it is supported and available in the current browsing session.

Testing for availability

Browsers that support localStorage will have a property on the window object named localStorage. However, for various reasons, just asserting that property exists may throw exceptions. If it does exist, that is still no guarantee that localStorage is actually available, as various browsers offer settings that disable localStorage. So a browser may support localStorage, but not make it available to the scripts on the page. One example of that is Safari, which in Private Browsing mode gives us an empty localStorage object with a quota of zero, effectively making it unusable. However, we might still get a legitimate QuotaExceededError, which only means that we've used up all available storage space, but storage is actually available. Our feature detect should take these scenarios into account. 

Here is a function that detects whether localStorage is both supported and available:

function storageAvailable(type) {
    try {
        var storage = window[type],
            x = '__storage_test__';
        storage.setItem(x, x);
        return true;
    catch(e) {
        return e instanceof DOMException && (
            // everything except Firefox
            e.code === 22 ||
            // Firefox
            e.code === 1014 ||
            // test name field too, because code might not be present
            // everything except Firefox
   === 'QuotaExceededError' ||
            // Firefox
            // acknowledge QuotaExceededError only if there's something already stored
            storage.length !== 0;

And here is how you would use it:

if (storageAvailable('localStorage')) {
	// Yippee! We can use localStorage awesomeness
else {
	// Too bad, no localStorage for us

You can test for sessionStorage instead by calling storageAvailable('sessionStorage')

See here for a brief history of feature-detecting localStorage.

A simple example

To illustrate some typical web storage usage, we have created a simple example, imaginatively called Web Storage Demo. The landing page provides controls that can be used to customize the color, font, and decorative image:

When you choose different options, the page is instantly updated; in addition, your choices are stored in localStorage,  so that when you leave the page and load it again later on, your choices are remembered.

We have also provided an event output page — if you load this page in another tab, then make changes to your choices in the landing page, you'll see the updated storage information outputted as a StorageEvent is fired.

Note: As well as viewing the example pages live using the above links, you can also check out the source code.

Testing whether your storage has been populated

To start with on main.js, we will test whether the storage object has already been populated (i.e., the page was previously accessed):

if(!localStorage.getItem('bgcolor')) {
} else {

The Storage.getItem() method is used to get a data item from storage; in this case we are testing to see whether the bgcolor item exists; if not, we run populateStorage() to add the existing customization values to the storage. If there are already values there, we run setStyles() to update the page styling with the stored values.

Note: You could also use Storage.length to test whether the storage object is empty or not.

Getting values from storage

As noted above, values can be retrieved from storage using Storage.getItem(). This takes the key of the data item as an argument, and returns the data value. For example:

function setStyles() {
  var currentColor = localStorage.getItem('bgcolor');
  var currentFont = localStorage.getItem('font');
  var currentImage = localStorage.getItem('image');

  document.getElementById('bgcolor').value = currentColor;
  document.getElementById('font').value = currentFont;
  document.getElementById('image').value = currentImage; = '#' + currentColor; = currentFont;
  imgElem.setAttribute('src', currentImage);

Here, the first three lines grab the values from local storage. Next, we set the values displayed in the form elements to those values, so that they keep in sync when you reload the page. Finally, we update the styles/decorative image on the page, so your customization options come up again on reload.

Setting values in storage

Storage.setItem() is used both to create new data items, and (if the data item already exists) update existing values. This takes two arguments — the key of the data item to create/modify, and the value to store in it.

function populateStorage() {
  localStorage.setItem('bgcolor', document.getElementById('bgcolor').value);
  localStorage.setItem('font', document.getElementById('font').value);
  localStorage.setItem('image', document.getElementById('image').value);


The populateStorage() function sets three items in local storage — the background color, font, and image path. It then runs the setStyles() function to update the page styles, etc.

We've also included an onchange handler on each form element, so that the data and styling is updated whenever a form value is changed:

bgcolorForm.onchange = populateStorage;
fontForm.onchange = populateStorage;
imageForm.onchange = populateStorage;

Responding to storage changes with the StorageEvent

The StorageEvent is fired whenever a change is made to the Storage object. This won't work on the same page that is making the changes — it is really a way for other pages on the domain using the storage to sync any changes that are made. Pages on other domains can't access the same storage objects.

On the events page (see events.js) the only JavaScript is as follows:

window.addEventListener('storage', function(e) {  
  document.querySelector('.my-key').textContent = e.key;
  document.querySelector('.my-old').textContent = e.oldValue;
  document.querySelector('.my-new').textContent = e.newValue;
  document.querySelector('.my-url').textContent = e.url;
  document.querySelector('.my-storage').textContent = e.storageArea;

Here we add an event listener to the window object that fires when the Storage object associated with the current origin is changed. As you can see above, the event object associated with this event has a number of properties containing useful information — the key of the data that changed, the old value before the change, the new value after that change, the URL of the document that changed the storage, and the storage object itself.

Deleting data records

Web Storage also provides a couple of simple methods to remove data. We don't use these in our demo, but they are very simple to add to your project:

  • Storage.removeItem() takes a single argument — the key of the data item you want to remove — and removes it from the storage object for that domain.
  • Storage.clear() takes no arguments, and simply empties the entire storage object for that domain.


Specification Status Comment
HTML Living Standard Living Standard  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
localStorage 4 3.5 8 10.50 4
sessionStorage 5 2 8 10.50 4
Feature Android Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support 2.1 ? 8 11 iOS 3.2

All browsers have varying capacity levels for both localStorage and sessionStorage. Here is a detailed rundown of all the storage capacities for various browsers.

Note: since iOS 5.1, Safari Mobile stores localStorage data in the cache folder, which is subject to occasional clean up, at the behest of the OS, typically if space is short.

See also

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