$content='Talk about your understanding of closures

But the cookie is not indispensable: cookie 's role is to interact with the server as HTTP part of the specification exists, and Web Storage simply to the local "store" raw data

Browser support in addition to IE7 and below are not supported, other standard browsers fully support (ie and FF need to run a web server in), it is worth mentioning that IE always good things, such as IE7, IE6 in the userData fact is javascript local storage solutions. By simple code package can be unified to all the browsers support Web storage .

localStorage and sessionStorage have the same method of operation, for example setItem, getItem and removeItem etc.

cookie and session distinction:

1, cookie data is stored on the customer's browser, session data on the server.

2, cookie is not very safe, others can be analyzed at a local store and COOKIE COOKIE cheat

should be used taking into account the security session.

3, session will be within a certain time saved on the server. When accessing the increase, it would be more occupy the performance of your server

consider to reduce server performance, you should use COOKIE.

4, the data stored in a single cookie can not exceed 4K, many browsers are limited to a maximum of one site to save 20 cookie.

5, so personal recommendations:

the login information and other important information is stored as SESSION

additional information, if necessary, it can be placed in COOKIE

CSS-related issues

display: none and visibility: hidden difference?

display: none to hide the corresponding elements in the document layout is no longer space allocated to it, it is the element of each side will be closed,

on when he never existed.

visibility: hidden hides the corresponding elements, but still retain the original layout of the space in the document.

The difference in the link andimport CSS is?

(1) link belong to HTML tags, and CSSimport is provided;

(2) the page is loaded, link will also be loaded, but referencesimport CSS will wait until the page is finished loading reloading;

(3) Import only to recognize in IE5 or more, while link is HTML tag, no compatibility issues;

style right (4) linkimport way of weight above the weight of heavy.

Similarities and Differences between absolute and float attributes: position

A: common:

inline element set `float` and` absolute` property allows elements from the document flow, and can set its width and height.

B: Different point:

float will still occupy the position, position overrides other elements in the document flow.

Introduce the box-sizing property?

box-sizing property is primarily used to control element analytical model box model. The default value is content-Box .

content-Box : Let element to maintain the standard W3C box model. Width / height of the element / height is determined by the border + padding + content width, set width / height attribute refers to the content portion of the width / height

border-Box : Let element to maintain the traditional box model IE (IE6 and IE6 ~ 7 the following versions of quirks mode). Set width / height attribute refers to the border + padding + content

Under standard browser, according to W3C box model specification analysis, once the border or modify the elements within distance, it will affect the size of the box element, you have to recalculate the box the size of the elements, thus affecting the layout of the entire page.

CSS selectors are there? Which attributes can be inherited? Priority algorithm calculated? Added CSS3 pseudo-classes are those?

1.id selector (# myid)

2. Class selector (.myclassname)

3. Tag selectors (div, h1, P)

4. adjacent selector (h1 + P)

5. child selector (ul> li )

6. descendant selectors (li a)

7. wildcard selector (*)

8. attribute selector (a [rel = "external"])

9. pseudo-class selectors (a: hover, li: nth-child)

Inheritable style: font-size font-family color, text-indent;

Non Inherited Styles: border padding margin width height;

Priority under the principle of proximity, with weights whichever the case recently defined style;

Loading last loaded positioning styles to prevail;

Priority:

! Important> ID> class> tag

Important priority higher than inline, inline but higher than the id

Added CSS3 pseudo-class examples:

p: first-of-type select each <p> element belongs to the first element of its parent <p> element.

p: last-of-type select each <p> element belongs to its parent last <p> element.

p: only-of-type select each <p> element belongs to its parent element only <p> elements.

p: only-child selects each <p> element belongs to its parent element only child element.

p: nth-child (2) Select each <p> element belongs to the parent element of the second sub-element.

: Enabled: disabled control form control disabled.

: Checked radio button or checkbox is selected.

position values, relative and absolute are positioned with respect to whom?

absolute

generate absolute element positioned relative to the recent level is not static positioned parent element to be positioned.

fixed (old IE does not support)

to generate absolute element positioned relative to the browser window positioning.

relative

generating element relative positioning, relative to its position in the general stream positioning.

static default. No positioning element occurs in the normal stream

CSS3 What's new?

CSS3 rounded corners (border-radius), shadow (box-shadow),

add special effects to the text (text-shadow,), a linear gradient (gradient), rotation (transform)

transform: Rotate (9deg) Scale (0.85,0.90) translate (0px, -30px) skew (-9deg, 0deg); // rotation, scaling, positioning, tilt

adds more CSS selectors more background rgba

unique introduced in CSS3 pseudo-elements :: selection.

Media queries , multi-column layout

border-image

XML and JSON difference?

(1) The data volume terms.

XML JSON relative terms, the volume of data is small, the speed of delivery faster.

(2) aspects of data exchange.

JSON and JavaScript interaction more convenient and easier to parse processing, better data interaction.

(3) The data described aspects.

JSON description of data than XML poor.

(4) The transmission speed.

JSON's speed is much faster than XML.

BFC norms of understanding?

BFC, block-level formatting context, a BFC created a new box is an independent layout, style, child elements inside the box will not affect the outside elements. In the same BFC two adjoining block-level box in the vertical direction (the direction of a relationship and layout) of margin will collapse from happening.

(A concept W3C CSS 2.1 specification, which determines how the elements in the layout of its contents, and its relationship with other elements and interactions.)

Under explain CSS sprites, and how you want to use it in a page or site.

CSS Sprites in fact, the number of pages in the background image into an image file, and then use the CSS "background-image", "background- repeat", "background-position" a combination of background positioning, background-position can be used Digital can accurately locate the position of the background image. This can reduce the cost of many pictures request because the request takes a relatively long; although requests can be complicated, but there are limits, are six general browser. For the future, there is no need to do so, and because of `http2`.

html part

Talk about your understanding of the semantic?

1, removed or lost when allowing page style showing clear structure

2, is conducive to SEO: search engines and to establish good communication helps crawlers to crawl more useful information: reptiles rely on labels to determine the context and the individual right keyword weight;

3, easy to resolve other devices (such as a screen reader, blind readers, mobile devices) in meaningful ways to render web pages;

4, easy to team development and maintenance, more readable semantic, is under Important Trends step right of the page, follow the W3C standards team follow this standard, the difference can be reduced.

Doctype role? Strict mode and mixed mode how to distinguish? They mean?

(1), <! DOCTYPE> declaration in the document in the front, in the <html> tag before. Tells the browser to render the document in which mode.

(2) strict mode layout and JS mode of operation is based on the highest standards of the browser supports running.

(3), in promiscuous mode, page backward compatible with liberal display. Simulate the behavior of older browsers do not work to prevent the site.

(4), DOCTYPE does not exist or incorrectly formatted document will lead to promiscuous mode rendering.

Do you know how many document types Doctype?

The label can be declared DTD three types, respectively strict version, interim version and an HTML-based documentation framework.

HTML 4.01 specifies three document types: Strict, Transitional, and Frameset.

XHTML 1.0 specifies three XML document types: Strict, Transitional, and Frameset.

Standards (standard) mode (that is, in strict rendering mode) complies with the latest standards for rendering Web pages, while Quirks

(inclusive) mode (that is, loose rendering mode or compatibility mode) is used to render conventional web browser designed.

HTML and XHTML-- What is the difference between the two

Difference:

1. All tags must have a corresponding end tag

element and attribute names 2. All tags must be lowercase

3. All the XML tags must be properly nested

4. All attributes must be quoted. " "enclosed

5. Put all the <and & special symbols with the coded representation

6. assign a value to all of the properties

in the comment 7. Do not manipulation "-"

8. The image must have caption

Common compatibility problems?

* Png24 bit image appears on iE6 browser background, the solution is made ​​PNG8. It can refer to a script processing.

* The browser's default margin and padding. The solution is to add a global * {margin: 0; padding: 0;} to unify.

* IE6 bilateral Distance bug: After the block attribute labels float, there are rampant margin circumstances, in ie6 show larger margin than the settings.

* Double ie generated from the float (IE6 bilateral distance problem: In IE6, if the element is set to float, while setting the margin-left or margin-right, margin value will double.)

#box {float: left; width: 10px; margin: 0 0 0 100px;}

Under such circumstances IE will produce 20px distance, the solution is to add --_ display in the label style float control: inline; be converted to inline property. (_ This symbol identifies only ie6)

* gradual recognition of the way from the local population gradually excluded.

First, the clever use of "\ 9" This marks the IE's browser separated out from all the circumstances.

Then, again using the "+" to IE8 and IE7, IE6 separated, so that IE8 has been independently identified.

.clearfix: after {content:; display ".": Block; height: 0; Clear: Both; visibility: hidden;}

1, additional labeling, <div style = "clear: both;"> </ div> (disadvantage: But this approach would cause additional HTML tag structure looks simple enough)

2, use after pseudo-class

content:; "."

<Script> src = "http://html5shim.googlecode.com/svn/trunk/html5.js" </ Script>

<[ENDIF]! ->

How to distinguish: DOCTYPE declaration \ new structural elements \ functional elements

iframe advantages and disadvantages?

: 1.` <iframe> `merit

shortcomings 2.` <iframe> `are:

4, do not call a function of time in the long run embedded JS, if must use, can be used to call `setTimeout`

Javascript blocking load specific ways

The script at the bottom. <Link> or on the head , and to ensure js before loading, to load the page properly displayed. <Script> tag on the </ body> ago.

A set of scripts : Because each <script> tag download blocking page resolution process, so limiting the page <script> total can also improve performance. Suitable for inline scripts and external scripts.

Non-blocking script : such as the page finishes loading, reloading js code. That is, in window.onload issued after the event to start the download code. (1) defer attributes: support IE4 and fierfox3.5 later browser (2) dynamic script elements: Document Object Model (DOM) allows you to use js dynamically create HTML almost the entire contents of the document. Code is as follows:

<Script>

var Script = document.createElement ("Script");

script.type = "text / JavaScript";

script.src = "file.js";

document.getElementsByTagName ("head") [0] .appendChild (Script );

</ Script>

This technique focuses on: Wherever start downloading, file downloading and running the amount will not block other page processing. Even in the head inside (except http link for downloading the file).

Closure-related issues?

For more information: Detailed js Closures

js event handler problem?

For details, see: JavaScript study summary (ix) event Comments

eval is doing what?

Its function is to parse the string corresponding to the JS code and run;

you should avoid using eval, unsafe, very time consuming properties (2 times, once parsed into js statement, once executed).

JavaScript prototype, the prototype chain? What are the characteristics?

* Prototype object is an ordinary object is an object that comes with implicit __proto__ property, prototypes may also have its own prototype, if the prototype of a prototype object is not null, then we call the prototype chain.

* Prototype chain used by some inheritance and shared properties of objects of the (limited) object chain.

Event mechanism event, IE and Firefox What is the difference? How to stop the bubble?

1. We operate in a web page (some operations corresponding to multiple events). For example: When we click on a button will generate an event. JavaScript can be detected behavior.

2. Event handling mechanism: IE is the event bubbling, firefox supports both event model, that is: type of event capture and bubbling type events. ;

3. ev.stopPropagation (); Note that the old method ie ev.cancelBubble = true;

ajax What is? ajax interaction model? synchronous and asynchronous difference? How to solve cross-border issues?

For details, see: JavaScript study summary (vii) Ajax and Http Status Word

1. Adoption of the asynchronous model to enhance the user experience

2. Optimize the transmission between the browser and the server, reducing unnecessary data back and forth, reducing the bandwidth

portion 3. Ajax running on the client, undertaken originally borne by the server work, reducing the server load large amount of users under.

What is the biggest characteristic 2. Ajax is.

Ajax enables dynamic does not refresh (partial refresh)

readyState property state can have five values: 0 = uninitialized 1 = Start 2 = send, 3 = reception, 4 = complete

ajax shortcomings

1, ajax do not support the browser back button.

2, AJAX security issues exposed the details of the server to interact with.

3, support for the search engines is relatively weak.

4, undermining the exception mechanism of the program.

5, it is not easy to debug.

Cross-domain: Setting proxy page on jsonp, iframe, window.name, window.postMessage, server

Depth cloning js object

function clone (Obj) {

var buf;

if (Obj instanceof Array) {

buf = []; // create an empty array

var i = Obj.length;

while (I -) {

buf [i] = clone (Obj [i]);

}

return buf;

} else if (Obj instanceof Object) {

buf = {}; // create an empty object

for (var k in Obj) {// add a new property to the object

buf [k] = clone (Obj [k]);

}

return buf;

} else {

return Obj;

}

}

The difference between AMD and CMD norms?

For more information: Detailed JavaScript Modular Development

Site reconstruction of understanding?

Reconstruction Site: without changing the external behavior of the premise, to simplify the structure, add readability, while the front end of the site to maintain consistent behavior. That is in the UI does not change the situation, to optimize the site, in the expansion while maintaining a consistent UI.

Reconstruction of the traditional web sites is usually:

table (table) layout to DIV + CSS

front end to make the site compatible with modern browsers (for irregularities in the CSS, such as IE6 valid)

for the mobile platform optimized

for ships SEO optimized

site reconstruction deeper aspects that should be considered

to reduce the coupling between codes

allow the code to maintain flexibility

in strict accordance with specification code

extensible API designed

to replace the old framework, language (eg VB)

enhance the user experience

generally speaking for optimized for speed is also included in the reconstruction of the

compressed JS, CSS, image and other front-end resources (usually be resolved by the server)

performance optimization program (such as data read and write)

using CDN to accelerate resource loading

for JS DOM optimized

HTTP server File Cache

How to obtain the UA?

<Script>

whatBrowser function () {

document.Browser.Name.value = navigator.appName;

document.Browser.Version.value = navigator.appVersion;

document.Browser.Code.value = navigator.appCodeName;

document.Browser.Agent. value = navigator.userAgent;

}

</ Script>

js array to weight

The following are three ways to go heavy array:

Array.prototype.unique1 = function () {

var N = []; // a new temporary array

for (var i = 0; i <this.length; i ++) // traverse the current array

{

// If the current array first i have saved into a temporary array, skip,

// otherwise the current term push to the temporary array inside

if (n.indexOf (this [i]) == -1) n.push (this [i]);

}

return N;

}

Array.prototype.unique2 = function ()

{

var N = {}, r = []; // N is a hash table, r is a temporary array

for (var i = 0; i <this.length; i ++) // traverse the current array

{

if (N [this [i]]) // If the hash table does not have the current item!

{

N [this [i]] = true; // stored in the hash table

r.push (this [i]); // the current term push the current array to a temporary array inside

}

}

return r;

}

Array.prototype.unique3 = function ()

{

var N = [this [0]]; // results array

for ( var i = 1; i <this.length; i ++) // iterate from the start of the second term

{

// if the current array of item i in the current array position is not the first occurrence of i,

it represents the i // entries are repeated, ignored. Otherwise, the result is stored in the array

if (this.indexOf (this [i]) == i) n.push (this [i]);

}

return N;

}

HTTP status code

After 100 Continue to continue, generally when sending post request, sent the http header the server will return this message, to confirm, specific parameter information after sending

200 OK normal return information

201 Created request was successful and the server created a new resource

202 Accepted The server has accepted the request, but not yet processed

301 Moved Permanently The requested page has been permanently moved to a new location.

302 Found temporary redirect.

303 See Other temporary redirection, and always use a GET request a new URI.

304 Not Modified since the last request, the requested page has not been modified.

Format 400 Bad Request Server can not understand the request, the client should not attempt to use the same content again initiated the request.

401 Unauthorized Unauthorized request.

403 Forbidden Forbidden.

404 Not Found can not find how to match resources with URI.

500 Internal Server Error The most common server-side error.

503 Service Unavailable server is temporarily unable to process the request (may be overloaded or maintenance).

js operations to get and set cookie

function setCookie (name, value, expires, path, domain, secure) {

var cookieText = encodeURIComponent (name) + '=' + encodeURIComponent (value);

if (expires instanceof Date) {

cookieText + = '; expires = '+ expires;

}

if (path) {

cookieText + = '; expires =' + expires;

}

if (domain) {

cookieText + = '; domain =' + domain;

}

if (secure) {

cookieText + = '; secure ';

}

document.cookie = cookieText;

}

function getCookie (name) {

}

function unsetCookie (name) {

document.cookie = name + "=; expires =" + new Date (0);

}

Talk about the three-way handshake TCP transport policy

In order to reach the target accurately at the data, TCP uses a three-way handshake protocol strategy. After using the TCP protocol to send out packets, TCP will not ignore the situation after transfer, it will be delivered to the other party to confirm the success. Handshake using TCP flags: SYN and ACK.

Data with the sender first sends a SYN packet to the other flag. After the receiver receives, SYN / ACK packet backhaul flags to show communicate with a confirmation message. Packet, representing the last, the sender return pass with ACK flag "handshake" End

If at some stage during the handshake inexplicably interrupted, TCP protocol will be in the same order to send the same packet again.

Talk about your understanding of the Promise

In accordance with Promise / A + defined, Promise has four states:

pending: the initial state, the non-fulfilled or rejected.

Fulfilled: the success of the operation.

rejected: the failed operation.

settled: Promise has been fulfilled or rejected, and not pending

In addition, Fulfilled and rejected together collectively settled .

Promise objects used for delayed (deferred) and asynchronous (asynchronous) calculations.

Promise of constructor

Construct a Promise , the most basic usage is as follows:

var = new Promise Promise (function (resolve, reject) {

if (...) {// Succeed

resolve (result);

} else {// fails

reject (Error (errMessage));

}

});

Promise instance has then method (with a then object method, commonly referred to thenable ). Its use is as follows:

promise.then (onFulfilled, onRejected)

Receives two function as an argument, one fulfilled is called when one rejected when they were called, received parameter is the future, onFulfilled correspond resolve , onRejected correspond to reject .

Javascript garbage collection method

Clear mark (mark and sweep)

This is the JavaScript garbage collection the most common way, when the variable into the execution environment, such as a function to declare a variable, the garbage collector will be marked as "into the environment", when leaving the environment variables (ending function execution) to labeled "leave the environment."

Garbage collector to run when all variables are stored in memory plus mark, and then remove the environment variables and variables (closures) is referenced by an environment variable, there are still marked after this is done is to delete variables

Reference count (reference counting)

In the low version IE memory leaks occur frequently, many times, because it uses reference counting for garbage collection. Policy reference count is the number used by the track record of each value, when you declare a variable number of citations and a reference type assigned to the variable when this value is increased by one, if the value of the variable into another is worth quoting this number by 1, when citations this value becomes zero, no variables described in use, this value can not be accessed, so its footprint can be recovered, so the garbage collector in Running time to clean out the reference number of the space occupied by a value of 0.

In IE though JavaScript objects marked clearing the way for garbage collection, but the BOM and DOM objects is through reference counting garbage, that is as far as BOM and DOM circular reference problem occurs.

Talk about performance optimization problem

Code level: Avoid using css expression, avoid the use of advanced selectors, wildcard selector. Cache use: Cache Ajax, use CDN, use external js and css file to the cache, add the Expires header, the server configuration Etag, reducing the number of requests and other DNS lookup: merge styles and scripts, using css image sprites, beyond the initial first screen Image resource demand loading, lazy loading static resources. Request Bandwidth: compressed file, open GZIP,

Mobile end performance optimization

Try to use css3 animation, open hardware acceleration. Appropriate use of touch event instead of click event. Avoid using css3 gradient shading effects. Use as few box-shadow and Gradients . box-shadow and gradients are often performance killer page

What is Etag?

Browser download components when they will be stored in the browser cache. If you need to get the same components, the browser cache time again check the components, if already expired, the browser sends a conditional GET request to the server, the server determines the cache still valid, it sends a 304 response, telling the browser can be reused cache component.

Then the server is based on what the judge cache is still valid? The answer is, there are two ways, one is ETag mentioned earlier, another is based on the Last-Modified