超实用的JavaScript技巧及最佳实践

 

1.使用逻辑符号&&或者||进行条件判断

var foo = 10;  
foo == 10 && doSomething(); // is the same thing as if (foo == 10) doSomething(); 
foo == 5 || doSomething(); // is the same thing as if (foo != 5) doSomething();

 AND也可以用来设置函数参数的默认值

Function doSomething(arg1){ 
    Arg1 = arg1 || 10; // arg1 will have 10 as a default value if it’s not already set
}

 2.使用map()方法来遍历数组

var squares = [1,2,3,4].map(function (val) {  
    return val * val;  
}); 
// squares will be equal to [1, 4, 9, 16] 

3.舍入小数位数 

var num =2.443242342;
num = num.toFixed(4);  // num will be equal to 2.4432

 4.浮点数问题

0.1 + 0.2 === 0.3 // is false 
9007199254740992 + 1 // is equal to 9007199254740992  
9007199254740992 + 2 // is equal to 9007199254740994

 0.1+0.2等于0.30000000000000004，为什么会发生这种情况？根据IEEE754标准，你需要知道的是所有JavaScript数字在64位二进制内的都表示浮点数。开发者可以使用toFixed()和toPrecision()方法来解决这个问题。

 

5.使用for-in loop检查遍历对象属性

下面这段代码主要是为了避免遍历对象属性。

for (var name in object) {  
    if (object.hasOwnProperty(name)) { 
        // do something with name                    
    }  
}

 6.逗号操作符

var a = 0; 
var b = ( a++, 99 ); 
console.log(a);  // a will be equal to 1 
console.log(b);  // b is equal to 99

 7.计算或查询缓存变量

在使用jQuery选择器的情况下，开发者可以缓存DOM元素

var navright = document.querySelector('#right'); 
var navleft = document.querySelector('#left'); 
var navup = document.querySelector('#up'); 
var navdown = document.querySelector('#down');

 8.在将参数传递到isFinite()之前进行验证

isFinite(0/0) ; // false 
isFinite("foo"); // false 
isFinite("10"); // true 
isFinite(10);   // true 
isFinite(undifined);  // false 
isFinite();   // false 
isFinite(null);  // true  !!! 

 9.在数组中避免负向索引

var numbersArray = [1,2,3,4,5]; 
var from = numbersArray.indexOf("foo") ;  // from is equal to -1 
numbersArray.splice(from,2);    // will return [5]

10.（使用JSON）序列化和反序列化

var person = {name :'Saad', age : 26, department : {ID : 15, name : "R&D"} }; 
var stringFromPerson = JSON.stringify(person); 
/* stringFromPerson is equal to "{"name":"Saad","age":26,"department":{"ID":15,"name":"R&D"}}"   */ 
var personFromString = JSON.parse(stringFromPerson);  
/* personFromString is equal to person object  */

11.避免使用eval()或Function构造函数

eval()和Function构造函数被称为脚本引擎，每次执行它们的时候都必须把源码转换成可执行的代码，这是非常昂过的操作。

var func1 = new Function(functionCode);
var func2 = eval(functionCode);

12.避免使用with()方法

如果在全局区域里使用with()插入变量，那么，万一有一个变量名字和它名字一样，就很容易混淆和重写。

13.避免在数组里使用for-in loop

而不是这样用：

var sum = 0;  
for (var i in arrayNumbers) {  
    sum += arrayNumbers[i];  
}

 这样会更好：

var sum = 0;  
for (var i = 0, len = arrayNumbers.length; i < len; i++) {  
    sum += arrayNumbers[i];  
}

 这样会更快：

for (var i = 0; i < arrayNumbers.length; i++)

 为什么？数组长度arraynNumbers在每次loop迭代时都会被重新计算

 

14.不要向setTimeout()和setInterval()方法里传递字符串

如果在这两个方法里传递字符串，那么字符串会像eval那样重新计算，这样速度就会变慢，而不是这样使用：

setInterval('doSomethingPeriodically()', 1000);  
setTimeOut('doSomethingAfterFiveSeconds()', 5000);

 相反，应该这样用：

setInterval(doSomethingPeriodically, 1000);  
setTimeOut(doSomethingAfterFiveSeconds, 5000);

15.使用switch/case语句代替较长的if/else语句

 如果有超过2个以上的case，那么使用switch/case速度会快很多，而且代码看起来更加优雅。

16.遇到数值范围时，可以选用switch/casne

function getCategory(age) {  
    var category = "";  
    switch (true) {  
        case isNaN(age):  
            category = "not an age";  
            break;  
        case (age >= 50):  
            category = "Old";  
            break;  
        case (age <= 20):  
            category = "Baby";  
            break;  
        default:  
            category = "Young";  
            break;  
    };  
    return category;  
}  
getCategory(5);  // will return "Baby"

 17.创建一个对象，该对象的属性是一个给定的对象

 可以编写一个这样的函数，创建一个对象，该对象属性是一个给定的对象，好比这样

function clone(object) {  
    function OneShotConstructor(){}; 
    OneShotConstructor.prototype= object;  
    return new OneShotConstructor(); 
} 
clone(Array).prototype ;  // []

 18.一个HTML escaper函数

function escapeHTML(text) {  
    var replacements= {"<": "<", ">": ">","&": "&", "\"": """};                      
    return text.replace(/[<>&"]/g, function(character) {  
        return replacements[character];  
    }); 
}

 19.在一个loop里避免使用try-catch-finally

try-catch-finally在当前范围里运行时会创建一个新的变量，在执行catch时，捕获异常对象会赋值给变量。

不要这样使用：

var object = ['foo', 'bar'], i;  
for (i = 0, len = object.length; i <len; i++) {  
    try {  
        // do something that throws an exception 
    }  
    catch (e) {   
        // handle exception  
    } 
}

 应该这样使用：

var object = ['foo', 'bar'], i;  
try { 
    for (i = 0, len = object.length; i <len; i++) {  
        // do something that throws an exception 
    } 
} 
catch (e) {   
    // handle exception  
} 

 20.给XMLHttpRequests设置timeouts

如果一个XHR需要花费太长时间，你可以终止链接（例如网络问题），通过给XHR使用setTimeout()解决。

var xhr = new XMLHttpRequest (); 
xhr.onreadystatechange = function () {  
    if (this.readyState == 4) {  
        clearTimeout(timeout);  
        // do something with response data 
    }  
}  
var timeout = setTimeout( function () {  
    xhr.abort(); // call error callback  
}, 60*1000 /* timeout after a minute */ ); 
xhr.open('GET', url, true);  

xhr.send();

 此外，通常你应该完全避免同步Ajax调用。

 

21.处理WebSocket超时

一般来说，当创建一个WebSocket链接时，服务器可能在闲置30秒后链接超时，在闲置一段时间后，防火墙也可能会链接超时。

为了解决这种超时问题，你可以定期地向服务器发送空信息，在代码里添加两个函数：一个函数用来保持链接一直是活的，另一个用来取消链接是活的，使用这种方法，你将控制超时问题。

添加一个timeID……

var timerID = 0; 
function keepAlive() { 
    var timeout = 15000;  
    if (webSocket.readyState == webSocket.OPEN) {  
        webSocket.send('');  
    }  
    timerId = setTimeout(keepAlive, timeout);  
}  
function cancelKeepAlive() {  
    if (timerId) {  
        cancelTimeout(timerId);  
    }  
}

 keepAlive()方法应该添加在WebSocket链接方法onOpen()的末端，cancelKeepAlive()方法放在onClose()方法下面。

 

22.记住，最原始的操作要比函数调用快

对于简单的任务，最好使用基本操作方式来实现，而不是使用函数调用实现。

例如

var min = Math.min(a,b); 
A.push(v);

 基本操作方式：

var min = a < b ? a : b; 
A[A.length] = v;

 23. – Don’t forget var keyword when assigning a variable’s value for the first time.

 

 24. – use === instead of ==

[10] === 10    // is false
[10]  == 10    // is true
'10' == 10     // is true
'10' === 10    // is false
 []   == 0     // is true
 [] ===  0     // is false
 '' == false   // is true but true == "a" is false
 '' ===   false // is false 

 25. – undefined, null, 0, false, NaN, '' (empty string) are all false.

 

 26. – Create a Self-calling Function

(function(){
    // some private code that will be executed automatically
})();  
(function(a,b){
    var result = a+b;
    return result;
})(10,20)

 27. – A string trim function

The classic trim function of Java, C#, PHP and many other language that remove whitespace from a string doesn’t exist in JavaScript, so we could add it to the String object.

String.prototype.trim = function(){return this.replace(/^\s+|\s+$/g, "");};

 28. – Append an array to another array

var array1 = [12 , "foo" , {name "Joe"} , -2458];

var array2 = ["Doe" , 555 , 100];
Array.prototype.push.apply(array1, array2);
/* array1 will be equal to  [12 , "foo" , {name "Joe"} , -2458 , "Doe" , 555 , 100] */

 29. – Transform the arguments object into an array

var argArray = Array.prototype.slice.call(arguments);

 30. – Verify that a given argument is a number

function isNumber(n){
    return !isNaN(parseFloat(n)) && isFinite(n);
}

 31. – Verify that a given argument is an array

function isArray(obj){
    return Object.prototype.toString.call(obj) === '[object Array]' ;
}

 Note that if the toString() method is overridden, you will not get the expected result using this trick.

Or use…

Array.isArray(obj); // its a new Array method

 You could also use instanceof if you are not working with multiple frames. However, if you have many contexts, you will get a wrong result.

var myFrame = document.createElement('iframe');
document.body.appendChild(myFrame);

var myArray = window.frames[window.frames.length-1].Array;
var arr = new myArray(a,b,10); // [a,b,10]  

// instanceof will not work correctly, myArray loses his constructor 
// constructor is not shared between frames
arr instanceof Array; // false

 32. – Get the max or the min in an array of numbers

var  numbers = [5, 458 , 120 , -215 , 228 , 400 , 122205, -85411]; 
var maxInNumbers = Math.max.apply(Math, numbers); 
var minInNumbers = Math.min.apply(Math, numbers);

 33. – Empty an array

var myArray = [12 , 222 , 1000 ];  
myArray.length = 0; // myArray will be equal to [].

 34. – Don’t use delete to remove an item from array

Use split instead of using delete to delete an item from an array. Using delete replaces the item with undefined instead of the removing it from the array.

Instead of… >

var items = [12, 548 ,'a' , 2 , 5478 , 'foo' , 8852, , 'Doe' ,2154 , 119 ]; 
items.length; // return 11 
delete items[3]; // return true 
items.length; // return 11 
/* items will be equal to [12, 548, "a", undefined × 1, 5478, "foo", 8852, undefined × 1, "Doe", 2154,       119]   */

 Use…

var items = [12, 548 ,'a' , 2 , 5478 , 'foo' , 8852, , 'Doe' ,2154 , 119 ]; 
items.length; // return 11 
items.splice(3,1) ; 
items.length; // return 10 
/* items will be equal to [12, 548, "a", 5478, "foo", 8852, undefined × 1, "Doe", 2154,       119]   */

 The delete method should be used to delete an object property.

 35. – Truncate an array using length

Like the previous example of emptying an array, we truncate it using the length property.

var myArray = [12 , 222 , 1000 , 124 , 98 , 10 ];  
myArray.length = 4; // myArray will be equal to [12 , 222 , 1000 , 124].