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It doesn’t matter how many years I’ve been dealing with Javascript – it contains many little things that surprises me almost every week. For me, Javascript means a constant learning process.

In this article, I’ll provide ten small Javascript tips, mainly aimed for beginner and intermediate Javascript developers. Hopefully there’s at least one useful tip for every reader :).


1. Variables conversions 变量转换

This sounds quite obvious, but as far I’ve seen, using object constructors, like Array() or Number() for converting variables is quite common practice.

Always use primitive data types (sometimes referred as literals) for converting variables. These won’t do any extra tricks and they usually have better performance.


var myVar   = "3.14159",  
    str     = ""+ myVar,//  to string  
    int     = ~~myVar,  //  to integer  
    float   = 1*myVar,  //  to float  
    bool    = !!myVar,  /*  to boolean - any string with length 
                            and any number except 0 are true */  
    array   = [myVar];  //  to array  

Converting to dates (new Date(myVar)) and regular expressions (new RegExp(myVar)) must be done with constructors. However, always use /pattern/flags when creating regular expressions.

转换日期(new Date(myVar))和正则表达式(new RegExp(myVar))必须使用构造函数,而且创建正则表达式的时候要使用/pattern/flags的形式。

2. Converting decimals to hex or octals and vice versa 十进制转换为十六进制或者八进制,或者反过来

Are you writing separate functions for hex (or octal) conversios? Stop. This can be easily done with existing methods:


(int).toString(16); // converts int to hex, eg 12 => "C"  
(int).toString(8);  // converts int to octal, eg. 12 => "14"  
parseInt(string, 16) // converts hex to int, eg. "FF" => 255  
parseInt(string, 8) // converts octal to int, eg. "20" => 16  

3. More playing with numbers 玩转数字

In addition to previous section, here are some more small tricks with when dealing with numbers.


0xFF; // Hex declaration, returns 255  
020; // Octal declaration, returns 16  
1e3; // Exponential, same as 1 * Math.pow(10,3), returns 1000  
(1000).toExponential(); // Opposite with previous, returns 1e3  
(3.1415).toFixed(3); // Rounding the number, returns "3.142"  

4. Javascript Version Detection Javascript版本检测

Are you aware which version of Javascript your browser supports? If not, check Javascript Versions sheet from Wikipedia.

For some reason, features in Javascript version 1.7 are not widely supported. However, most browsers released within a year support features in version 1.8 (and in 1.8.1).

Note: all the versions of Internet Explorer (8 and older) supports only Javascript version 1.5.

Here’s a tiny script both for detecting the version of Javascript via feature detection. It also allows checking support for specific version of Javascript:

你知道你的浏览器支持哪一个版本的Javascript吗?如果不知道的话,去维基百科查一下Javascript版本表吧。出于某种原因,Javascript 1.7版本的某些特性是没有得到广泛的支持。不过大部分浏览器都支持了1.8版和1.8.1版的特性。(注:所有的IE浏览器(IE8或者更老的版本)只支持1.5版的Javascript)这里有一个脚本,既能通过检测特征来检测JavaScript版本,它还能检查特定的Javascript版本所支持的特性。

var JS_ver  = [];  
([].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;  
JS_ver.supports = function()  
    if (arguments[0])  
        return (!!~this.join().indexOf(arguments[0] +",") +",");  
        return (this[this.length-1]);  
alert("Latest Javascript version supported: "+ JS_ver.supports());  
alert("Support for version 1.7 : "+ JS_ver.supports("1.7"));  

5. window.name for simple session handling 使用window.name进行简单会话处理

This one is something I really like. You can assign values as a string for window.name property and it preserves the values until you close the tab or window.

Although I’m not providing any script, I strongly suggest you to take full advantage from it. For instance, it’s very useful for toggling between debugging and (perfomance) testing modes, when building a website or an application.


6. Testing existence of property 判断属性是否存在

This issue can be approached at least from two directions. Either we check whether property exists or we check the type of property. But always avoid these small mistakes:


// BAD: This will cause an error in code when foo is undefined  
if (foo) {  
// GOOD: This doesn't cause any errors. However, even when  
// foo is set to NULL or false, the condition validates as true  
if (typeof foo != "undefined") {  
// BETTER: This doesn't cause any errors and in addition  
// values NULL or false won't validate as true  
if (window.foo) {  

However, there may be situations, when we have deeper structure and proper checking would look like this:


// UGLY: we have to proof existence of every  
// object before we can be sure property actually exists  
if (window.oFoo && oFoo.oBar && oFoo.oBar.baz) {  

7. Passing arguments for function 给函数传递参数

When function has both required and optional parameters (arguments), eventually we may end up with functions and function calls looking like this:


function doSomething(arg0, arg1, arg2, arg3, arg4) {  
doSomething('', 'foo', 5, [], false);  

It’s always easier to pass only one object instead of several arguments:


function doSomething() {  
    // Leaves the function if nothing is passed  
    if (!arguments[0]) {  
        return false;  
    var oArgs   = arguments[0]  
        arg0    = oArgs.arg0 || "",  
        arg1    = oArgs.arg1 || "",  
        arg2    = oArgs.arg2 || 0,  
        arg3    = oArgs.arg3 || [],  
        arg4    = oArgs.arg4 || false;  
    arg1    : "foo",  
    arg2    : 5,  
    arg4    : false  

This is only a rough example of passing an object as an argument. For instance, we could declare an object with name of the variable as keys and default values as properties (and/or data types).


8. Using document.createDocumentFragment()

You may need to dynamically append multiple elements into document. However, appending them directly into document will fire redrawing of whole view every time, which causes perfomance penalty. Instead, you should use document fragments, which are appended only once after completion:


function createList() {  
    var aLI = ["first item", "second item", "third item",  
        "fourth item", "fith item"];  
    // Creates the fragment  
    var oFrag   = document.createDocumentFragment();  
    while (aLI.length) {  
        var oLI = document.createElement("li");  
        // Removes the first item from array and appends it  
        // as a text node to LI element  

9. Passing a function for replace() method 为replace()方法传递一个函数

There are situations when you want to replace specific parts of the string with specific values. The best way of doing this would be passing a separate function for method String.replace().

Following example is a rough implementation of making a more verbose output from a single deal in online poker:


var sFlop   = "Flop: [Ah] [Ks] [7c]";  
var aValues = {"A":"Ace","K":"King",7:"Seven"};  
var aSuits  = {"h":"Hearts","s":"Spades",  
sFlop   = sFlop.replace(/\[\w+\]/gi, function(match) {  
    match   = match.replace(match[2], aSuits[match[2]]);  
    match   = match.replace(match[1], aValues[match[1]] +" of ");  
    return match;  
// string sFlop now contains:  
// "Flop: [Ace of Hearts] [King of Spades] [Seven of Clubs]"  

10. Labeling of loops (iterations) 循环中标签的使用

Sometimes, you may have iterations inside iterations and you may want to exit between looping. This can be done by labeling:


for (var iI=0;iI<5;iI++) {  
    if (somethingIsTrue()) {  
        // Breaks the outer loop iteration  
        break outerloop;  
    for (var iA=0;iA<5;iA++) {  
        if (somethingElseIsTrue()) {  
            // Breaks the inner loop iteration  
            break innerloop;  


Go ahead and comment! Did you learn anything new? Do you have good tips to share? I'm always delighted for sharing information about all the little details in Javascript.

And if you want to familiarize with Javascript irregularities, I suggest you visiting at wtfjs :).





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《软件随想录:程序员部落酋长Joel谈软件》 Joel Spolsky (作者), 阮一峰 (译者)

《软件随想录:程序员部落酋长Joel谈软件》是一部关于软件技术、人才、创业和企业管理的随想文集,作者以诙谐幽默的笔触将自己在软件行业的亲身感悟娓娓道来,观点新颖独特,内容简洁实用。全书分为 36讲,每一讲都是一个独立的专题。《软件随想录:程序员部落酋长Joel谈软件》从不同侧面满足了软件开发人员、设计人员、管理人员及从事软件相关工作的人员的学习与工作需要。