Interview questions and answers of the first company (I)

1. Talk about the understanding of cross domain , How to deal with cross domain ？

Simple understanding ： Browser or JavaScript Say it's for safety , Restrictions on homology strategies .

1. Treatment method 1 ：porxy agent
   Through the same domain name web The server creates a proxy , For example, the server background in Hangzhou （www.hangzhou.com/porxy-hangzhouservice.php) To call Guangzhou server （www.guangzhou.com/server.php) Service for , Then return the response result to the front end , thus , The effect of calling Hangzhou's same domain name service at the front end is the same as that of calling Shanghai's service .

2. Treatment method 2 ：JSONP（ I won't support it POST request ）
   It is used to solve the problem of cross data access of mainstream browsers .
   a.com in

<script>
	function jsonp(json){
    
	   alert(json["name"]);
	 }
</script>
<script src="http://a.com/json.js"></script>

b.com in

jsonp({
    'name':' Xiao Ming ', 'age':24});

3. Treatment method 3 ：XHR2
   H5 Provided XMLHttpRequestLevel2 Cross domain access and other new functions have been implemented .
   IE10 None of the following versions support
   Just make some small changes on the server
   header(‘Access-Control-Allow-Origin:*’);
   header(‘Access-Control-Allow-Methods:POST, GET’);

2. What is the concept of closure ？

1. A closure is a function defined inside a function
2. Closures are variables that can access functions （ Functions that can read internal variables of other functions ）
3. Even if the containing function is executed , Variables referenced by closures are not released

Three conditions ：

1. Access scope
2. Nested function
3. Called outside the scope

IIFE（ Immediate execution function ）： Satisfy 1, dissatisfaction 2 and 3.IIFE（ Immediate execution function ）： Satisfy 1, dissatisfaction 2 and 3.

3.vue The concept of life function in

1. What is? vue Life cycle ？
   answer ： Vue The process from creation to destruction of an instance , Life cycle . That is, create from the beginning 、 Initialization data 、 Compiling templates 、 mount Dom→ Rendering 、 to update → Rendering 、 Unloading and so on , We call it Vue Life cycle of .

2. vue What is the role of the life cycle ？
   answer ： It has multiple event hooks in its life cycle , Let's control the whole Vue It is easier to form good logic in the process of instance .

3. vue There are several stages in the life cycle ？
   answer ： It can be divided into 8 Stages ： Before creation / after , Before loading / after , Before updating / after , Before destruction / After destruction

4. Which hooks will be triggered in the first page load ？
   answer ： Triggered the first time a page is loaded beforeCreate, created, beforeMount, mounted These hooks

5. DOM Rendering in Which cycle has been completed ？
   answer ：DOM Rendering in mounted China has finished .

6. Briefly describe which scenarios are suitable for each cycle ？
   answer ： Some ways to use life cycle hooks ： beforecreate : You can add one here loading event , Triggered when an instance is loaded created : The events at the completion of initialization are written here , If it ends here loading event , Asynchronous requests are also suitable to be called here mounted : Mount element , Get DOM node updated : If the data is processed uniformly , Write the corresponding function here beforeDestroy : You can make a confirmation box to confirm the stop event nextTick : Immediate action after data update dom

4. To draw a triangle css style

Analyze a wave of , There are many kinds of triangles , But they are inseparable from width and height 0, Make sure an edge is colored , Then make other edges transparent transparent, The code is as follows ：

#triangle{
     /** This is an inverted triangle **/
   width:0;
   height:0,;
   border-top:100px solid red; /** The positive triangle foot is changed to bottom**/
   border-right:50px solid transparent; /** If you want a right angle, you only need one side color , One edge is transparent **/
   border-left:50px solid transparent;
}

5. The process of page loading

1. Enter the address 
2. The browser looks up the domain name  IP  Address 
3. This step includes  DNS  Specific search process , Include ： Browser cache -> System cache -> Router cache …
4. Browser direction  web  Server sends a  HTTP  request 
5. The server's permanent redirect response （ from  http://a.com  To  http://www.a.com）
6. Browser tracking redirect address 
7. Server processing request 
8. The server returns a  HTTP  Respond to 
9. Browser display  HTML
10. Browser send request get embedded in  HTML  The resource （ Such as images 、 Audio 、 video 、CSS、JS wait ）
11. Browser sends asynchronous request