Summary of H5 localStorage usage

This time I will bring you a summary of the use of localStorage in H5. What are the precautions when using localStorage? Here are practical cases, let’s take a look.

1. What is localStorage, sessionStorage

In HTML5, a new localStorage feature has been added, this The feature is mainly used as local storage, which solves the problem of insufficient cookie storage space (the storage space of each cookie in the cookie is 4k). Generally, browsers support a size of 5M in localStorage. This is different in different browsers. localStorage will be different.

2. Advantages and limitations of localStorage

Advantages of localStorage

1. LocalStorage expansion Overcoming the 4K limit of cookies

2. LocalStorage will be able to store the first requested data directly locally. This is equivalent to a 5M database for the front-end page. Compared with cookies, it can save bandwidth. However, this is only supported in higher version browsers.

Limitations of localStorage

1. The sizes of browsers are not uniform, and in IE8 or above Only the IE version supports the attribute of localStorage

2. Currently, all browsers limit the value type of localStorage to the string type. This is required for our daily common JSON object types. Some conversions

3. LocalStorage is not readable in the browser's privacy mode

4. LocalStorage essentially reads the string. If stored If there is a lot of content, it will consume memory space and cause the page to become stuck.

5. LocalStorage cannot be crawled by crawlers.

The only difference between localStorage and sessionStorage is that localStorage is a permanent storage, while sessionStorage When the session ends, the key-value pairs in sessionStorage will be cleared

Here we use localStorage to analyze

3. The use of localStorage

LocalStorage browser support:

A special statement here is that if you are using IE browser, then UserData is required As storage, the main explanation here is the content of localStorage, so userData will not be explained too much. In the blogger's personal opinion, there is no need to learn the use of UserData, because the current IE6/IE7 is obsolete. position, and many page development today will involve emerging technologies such as Html5\CSS3, so we generally will not make it compatible when using the above

First of all, when using localStorage, we need to judge Whether the browser supports the localStorage attribute

if(！window.localStorage){
            alert("浏览器支持localstorage");
            return false;
        }else{
            //主逻辑业务
        }

There are three ways to write localStorage. Here we will introduce them one by one

if(！window.localStorage){
            alert("浏览器支持localstorage");
            return false;
        }else{
            var storage=window.localStorage;
            //写入a字段
            storage["a"]=1;
            //写入b字段
            storage.a=1;
            //写入c字段
            storage.setItem("c",3);
            console.log(typeof storage["a"]);
            console.log(typeof storage["b"]);
            console.log(typeof storage["c"]);
        }

The results after running are as follows:

It should be noted here that the use of localStorage also follows the same-origin policy, so different websites cannot directly share the same localStorage

The final result printed on the console is:

I don’t know if readers have noticed that what was just stored was of type int, but it was printed as type of string. This is related to the characteristics of localStorage itself. LocalStorage only supports storage of string type.

Reading of localStorage

if(!window.localStorage){
            alert("浏览器支持localstorage");
        }else{
            var storage=window.localStorage;
            //写入a字段
            storage["a"]=1;
            //写入b字段
            storage.a=1;
            //写入c字段
            storage.setItem("c",3);
            console.log(typeof storage["a"]);
            console.log(typeof storage["b"]);
            console.log(typeof storage["c"]);
            //第一种方法读取
            var a=storage.a;
            console.log(a);
            //第二种方法读取
            var b=storage["b"];
            console.log(b);
            //第三种方法读取
            var c=storage.getItem("c");
            console.log(c);
        }

There are three ways to read localStorage, among which the two officially recommended methods are getItem\setItem to save it. Take, don't ask me why, because I don't know this either

I said before that localStorage is equivalent to a front-end database. The database mainly consists of four steps of adding, deleting, checking and modifying. The reading here and writing are equivalent to the two steps of adding and checking.

Let’s talk about the two steps of deleting and modifying localStorage.

It is easier to understand this step. The idea is Just like re-changing the value of a global variable, here we will take an example to briefly explain

if(!window.localStorage){
            alert("浏览器支持localstorage");
        }else{
            var storage=window.localStorage;
            //写入a字段
            storage["a"]=1;
            //写入b字段
            storage.b=1;
            //写入c字段
            storage.setItem("c",3);
            console.log(storage.a);
            // console.log(typeof storage["a"]);
            // console.log(typeof storage["b"]);
            // console.log(typeof storage["c"]);
            /*分割线*/
            storage.a=4;
            console.log(storage.a);
        }

On the console, we can see that the a key has been changed to 4

localStorage的删除

1、将localStorage的所有内容清除

var storage=window.localStorage;
            storage.a=1;
            storage.setItem("c",3);
            console.log(storage);
            storage.clear();
            console.log(storage);

2、 将localStorage中的某个键值对删除

var storage=window.localStorage;
            storage.a=1;
            storage.setItem("c",3);
            console.log(storage);
            storage.removeItem("a");
            console.log(storage.a);

控制台查看结果

localStorage的键获取

var storage=window.localStorage;
            storage.a=1;
            storage.setItem("c",3);
            for(var i=0;i<storage.length;i++){
                var key=storage.key(i);
                console.log(key);
            }

使用key()方法，向其中出入索引即可获取对应的键

四、localStorage其他注意事项

 一般我们会将JSON存入localStorage中，但是在localStorage会自动将localStorage转换成为字符串形式

这个时候我们可以使用JSON.stringify()这个方法，来将JSON转换成为JSON字符串

示例：

if(!window.localStorage){
            alert("浏览器支持localstorage");
        }else{
            var storage=window.localStorage;
            var data={
                name:'xiecanyong',
                sex:'man',
                hobby:'program'
            };
            var d=JSON.stringify(data);
            storage.setItem("data",d);
            console.log(storage.data);
        }

读取之后要将JSON字符串转换成为JSON对象，使用JSON.parse()方法

var storage=window.localStorage;
            var data={
                name:'xiecanyong',
                sex:'man',
                hobby:'program'
            };
            var d=JSON.stringify(data);
            storage.setItem("data",d);
            //将JSON字符串转换成为JSON对象输出
            var json=storage.getItem("data");
            var jsonObj=JSON.parse(json);
            console.log(typeof jsonObj);

打印出来是Object对象

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