1. var f = foo||bar , what does this line of code mean? Why do you write like this?
or short circuit
If foo is false, 0, null, undefined, empty string, take the value of bar
This is a quick way to assign
2. Briefly describe your understanding of toString and valueOf
toString is to convert any object, basic type, and string into a string, and others will rewrite toString
valueof returns the original value of the object itself
3. Describe the running process of the new operator
Implementation principle of new operator_izwell's Blog-CSDN Blog_Principle of new operator
Create a new object O, point the property of O __proto__to the prototype property of the constructorprototype
Bind the execution context of the constructor to O, and execute the constructor to get the return value S
If S is an object, the new operator returns the S object
If S is not an object, the new operator returns O
4. How to implement Javascript prototype inheritance, the difference between prototype and __proto__ of the prototype chain
Introduction to javascript prototypal inheritance_360问题
https://www.jb51.net/article/81766.htm
5. What are the methods of js lazy loading?
Several ways of JS lazy loading_meijory's blog-CSDN blog_What are the ways of js lazy loading?
js lazy loading, that is, wait until the page is loaded before loading the js file, which helps to improve the page loading speed
The main methods are defer attribute, async attribute, dynamically create DOM method, jQuery getScript method, setTimeout, and put the js file at the bottom of the file
defer：
Set the deger attribute in <script>, the browser will download it immediately and delay execution, which is suitable for external script files of HTML4.
async：
Set the async attribute in <script> to prevent the page from waiting for the script to be downloaded and executed, thereby loading other content on the page asynchronously, suitable for external script files
6. Which Javascript libraries have you used, and which Javascript plug-ins have you used?
jQuery，moment、axios、awesome、echarts、validator、css-modal、
7. Briefly introduce the front-end frameworks and differences you know
Comparison of the three mainstream web front-end frameworks - Morality first - Blog Park
Compared to other frameworks — Vue.js
Angular、React、Vue
Similarities: Both are progressive frameworks , which can realize more powerful functions by supporting different tools; both are component-based development , which can well solve the problems of division of labor and maintenance; both are one-way data flow
Vue features: lightweight, two-way data binding, instructions, plug-ins, official documents are clear and easy to learn
React features: adopt life paradigm, simulate DOM, efficient, flexible, fast, cross-browser compatible
Angular features: the module is powerful, can be customized and specified, and can be introduced into the backend (embedding, injection, testing)
8. What are the HTTP status codes? What does represent respectively mean?
HTTP learning - what are the common HTTP status codes?
1xx provisional response
2xx success
3xx redirect
4xx Client Request Error
5xx server errors
Commonly used status codes are as follows:
200 OK The request was successful
301 Resource moved, the requested resource automatically goes to the new URL, and the browser automatically jumps to the new URL
304 The server resource is consistent with the client's last request and does not need to be retransmitted
400 Bad Request The client sends a bad request
403 Not authorized
404 Not Found page missing, resource not found
500 internal server error
501 An internal server error prevented the request from being serviced
9. Please talk about the debugging tools you have used in the front-end development process
Common front-end debugging tools - short book
F12 (Element, Console, Source, Network, Performance, Memory, Applications, Security, Benchmark)
breakpoint
postman
10. Talk about the difference between cookie and session
1-cookie data is stored in the client browser, session data is stored on the server
2-cookie is insecure and can be cheated, session is safe
3-session will be saved on the server for a certain period of time. If the access is too large, it will take up server performance. At this time, cookies should be used
4- The data saved by a single cookie cannot exceed 4k, and most browsers limit a site to save up to 20 cookies
5-cookie can only save ascll string, session can access any type of data
6-The validity period of the cookie is longer (can be set), and the session is invalid when the browser is closed
7-cookie supports cross-domain access, session does not support cross-domain access
8-cookie needs browser support, and can be set to be valid for this window and sub-windows, or set to be valid for all browser windows; session can only be set to be valid for this window and sub-windows.
11. What will the following code output? Why?
var foo = 1;
function test(){
console.log(foo);
var foo = 2;
console.log(foo);
}
test();
undefined -- because the variable is promoted; if it is let, it becomes a dead zone
2 -- The local variable foo is defined
12. What will the following code output? Why?
var User = {
count: 1,
getCount: function() {
return this.count;
}
};
console.log(User.getCount());
var func = User.getCount;console.log(func());
1
undefined --this points to window, and an error will be reported in strict mode
Thirteen, look at the following code, give the output result. How to make the code output 1 2 3?
for(var i=1;i<=3;i++){
setTimeout(function(){
console.log(i);
},0);
};444
If you want to output 123, you need to replace var with let, which forms a block-level scope/closure