"summarize" JavaScript Learning summary-tips, methods, Details

Variable conversions

var myVar  = "3.14159", str   = "" "+ myVar,//  String type int   / /number  type float  // number type bool  // Boolean type array  //  Array type

however, the conversion date (new date (myVar)) and the regular expression (new RegExp (myVar)) must use the constructor, and a simplified form such as/pattern/flags is used when creating the regular Expression.

Rounding and converting to numerical type

//when a character variable participates in an operation, JS automatically converts it to a numeric type (if it cannot be converted, it becomes Nan)' 10.567890 ' | 0//Ten//all numerical types in JS are double-precision floating-point numbers.//As a result, JS will first convert these numeric operands to integers when performing bit operations, and then perform the operations//| is binary or, x|0 is always equal to x;//^ for xor, same as 0 xor 1, so x^0 is always equal to x;//~ is a bitwise reverse, two times after the value of course is the same' 10.567890 ' ^ 0//Results: Ten-2.23456789 | 0//-2~~2.23456789//2~~-2.23456789//-2~-2.23456789//1~2.23456789//-3

Date to Value

// the internal representation of the time of JS itself is the UNIX timestamp, which records the current unit     of time in milliseconds from 0 o'clock January 1, 1970. var d = +new//1488947616099

Class Array object to array

var arr =[].slice.call (arguments)

The following example:

function test () {  var res = [' a ', ' B '];   // Method 1  res = Res.concat ([].slice.call (arguments,0));  // 0 can be omitted to indicate interception    from the start position // Method 2   Array.prototype.push.apply (res, arguments);} Test (' C ', ' d ');  // ["a", "b", "c", "d"]

Conversion between the binaries

(int//  converts int to hex, eg + = "C"(int//  converts int to octal, eg. "+"//  converts hex to int, eg. "FF" = 255//  converts octal to int, eg. "+"

Determine if it is IE

//Edit Http://www.lai18.com//seemingly the shortest, using IE does not support the standard ECMAScript of the array in the end of the comma-ignored mechanismvarIE =!-[1,];//using the conditional annotations of IEvarie =/*@[email protected]*/false;//or conditional commentvarIe//@cc_on =1;//IE does not support vertical tabsvarie = ' \v ' = = ' V ';//Principle Ibid .varIE =!+ "\v1";

Try to use native methods

To find the largest number in a set of numbers, we might write a loop, for example:

var numbers = [3,342,23,22,124]; var max = 0;  for (var i=0;i<numbers.length;i++if(numbers[i] > max) {  =  numbers[i]; }}alert (max);

In fact, using the native method, can be more simple to achieve

var numbers = [3,342,23,22,124];numbers.sort (function(b) {return b- a}); Alert (numbers[0]);

of course, the simplest way is:

// returns 433

You can also do this now:

// Take maximum value // take the minimum value

Event delegation

A simple example: the HTML code is as Follows:

<H2>Great Web Resources</H2><ulID= "resources"> <Li><ahref= "http://opera.com/wsc">Opera WEB Standards Curriculum</a></Li> <Li><ahref= "http://sitepoint.com">SitePoint</a></Li> <Li><ahref= "http://alistapart.com">A List Apart</a></Li> <Li><ahref= "http://yuiblog.com">YUI Blog</a></Li> <Li><ahref= "http://blameitonthevoices.com">Blame it on the voices</a></Li> <Li><ahref= "http://oddlyspecific.com">Oddly specific</a></Li></ul>

The JS code is as Follows:

//Classic Event Handling Example(function(){ varresources = document.getElementById (' Resources ')); varLinks = resources.getelementsbytagname (' a '); varall =links.length; for(vari=0;i<all;i++){  //Attach a listener to each linkLinks[i].addeventlistener (' Click ', handler,false); }; functionhandler (e) {varx = e.target;//Get The link that is clickedAlert (x); E.preventdefault (); };}) ();

With event delegation, You can write more gracefully:

(functionvar resources = document.getElementById (' resources '); Resources.addeventlistener (' click ', handler,falsefunction  handler (e) {   var/  get the link tha  if(x.nodename.tolowercase () = = = ' A ') {   alert (' Event delegation: ' + x ');   E.preventdefault ();  } };}) ();

Do you know which version of JavaScript your browser supports?

varJs_ver =[];(Number.prototype.toFixed)? Js_ver.push ("1.5"):false;([].indexof&& [].foreach]? Js_ver.push ("1.6"):false;((function(){Try{[a, b] = [0,1];return true;}Catch(ex) {return false;}}) ())? Js_ver.push ("1.7"):false;([].reduce&& [].reduceright && JSON]? Js_ver.push ("1.8"):false;("". trimleft)? Js_ver.push ("1.8.1"):false; Js_ver.supports=function() {if(arguments[0])return(!! ~ this. Join (). indexOf (arguments[0] + ",") + ","); Else 　　　　 return( this[ this. length-1]);} Alert ("Latest Javascript version supported:" +js_ver.supports ()); Alert ("support for version 1.7:" + js_ver.supports ("1.7"));

Determine if a property exists

//Bad:this would cause an error in code if foo is undefinedif(foo) {dosomething ();}//good:this doesn ' t cause any errors. however, even when//Foo is set to NULL or false, the condition validates as trueif(typeofFoo! = "undefined") {dosomething ();}//better:this doesn ' t cause any errors and in addition//values NULL or false won ' t validate as Trueif(window.foo) {dosomething ();}

There are cases when we have deeper structures and need more appropriate checks:

// Ugly:we has to proof existence of every // object before we can be sure property actually exists if (window.ofoo && ofoo.obar && oFoo.oBar.baz) {dosomething ();}

In fact, the best way to detect whether a property exists is:

if inch Window) {  Console.log ("OPERA");} Else {  console.log ("not OPERA");}

Detects whether an object is an array

var obj=[];object.prototype.tostring.call (obj)= = "[Object Array]";

Passing objects to a function

functiondosomething () {//Leaves The function if nothing is passed 　　 if(!arguments[0]) {return false; }varOargs = Arguments[0] arg0= Oargs.arg0 | | "", Arg1= Oargs.arg1 | | "", Arg2= Oargs.arg2 | | 0, Arg3= Oargs.arg3 | |[], Arg4= Oargs.arg4 | |false;} DoSomething ({arg1:"foo", Arg2:5, Arg4:false});

Using labels in loops

Sometimes nested loops in loops, you may want to exit a layer of loops, before always using a flag variable to judge, now know there is a better way:

 outerloop:  for  (var  ii=0;ii<5;ii++ if  Span style= "color: #000000;" > (somethingistrue ()) { //  Breaks the　　Outer loop Iteration   outerloop; } innerloop:  for  (var  ia=0;ia< 5;ia++ if   (somethingelseistrue ()) { //  Breaks the inner loop iteration  break  　　innerloop; }}}  

Reprint Address: https://my.oschina.net/os2015/blog/465376

Summary JavaScript Learning summary-tips, methods, details (rpm)