45个实用的JavaScript小贴士，技巧和最佳实践

jopen 11年前

本文中的代码片段已经在最新的谷歌Chrome30版，它使用的V8 JavaScript引擎（V83.20.17.15）进行了测试。

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

Assignment to an undeclared variable automatically results in a global variable being created. Avoid global variables.

2 – use === instead of ==

The == (or !=) operator performs an automatic type conversion if needed. The === (or !==) operator will not perform any conversion. It compares the value and the type, which could be considered faster than ==.

[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

3 – undefined, null, 0, false, NaN, '' (empty string) are all falsy.
4 – Use Semicolons for line termination

The use of semi-colons for line termination is a good practice. You won’t be warned if you forget it, because in most cases it will be inserted by the JavaScript parser. For more details about why you should use semi-colons, take a look to this artice: http://davidwalsh.name/javascript-semicolons.

5 – Create an object constructor

function Person(firstName, lastName){      this.firstName =  firstName;      this.lastName = lastName;          }      var Saad = new Person("Saad", "Mousliki");

6 – Be careful when using typeof, instanceof and constructor.

  • typeof : a JavaScript unary operator used to  return a string that represents the primitive type of a variable,  don’t forget that typeof null will return “object”, and for the majority of object types (Array, Date, and others) will return also “object”.
  • constructor : is a property of the internal prototype property, which could be overridden by code.
  • instanceof : is another JavaScript operator that check in all the prototypes chain the constructor it returns true if it’s found and false if not.
var arr = ["a", "b", "c"];  typeof arr;   // return "object"   arr  instanceof Array // true  arr.constructor();  //[]

7 – Create a Self-calling Function

This is often called a Self-Invoked Anonymous Function or Immediately Invoked Function Expression (IIFE). It is a function that executes automatically when you create it, and has the following form:

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

8 – Get a random item from an array

var items = [12, 548 , 'a' , 2 , 5478 , 'foo' , 8852, , 'Doe' , 2145 , 119];    var  randomItem = items[Math.floor(Math.random() * items.length)];

9 – Get a random number in a specific range

This code snippet can be useful when trying to generate fake data for testing purposes, such as a salary between min and max.

var x = Math.floor(Math.random() * (max - min + 1)) + min;

10 – Generate an array of numbers with numbers from 0 to max

var numbersArray = [] , max = 100;    for( var i=1; numbersArray.push(i++) < max;);  // numbers = [1,2,3 ... 100]

11 – Generate a random set of alphanumeric characters

function generateRandomAlphaNum(len) {      var rdmString = "";      for( ; rdmString.length < len; rdmString  += Math.random().toString(36).substr(2));      return  rdmString.substr(0, len);    }

12 – Shuffle an array of numbers

var numbers = [5, 458 , 120 , -215 , 228 , 400 , 122205, -85411];  numbers = numbers.sort(function(){ return Math.random() - 0.5});  /* the array numbers will be equal for example to [120, 5, 228, -215, 400, 458, -85411, 122205]  */

A better option could be to implement a random sort order by code (e.g. : Fisher-Yates shuffle), than using the native sort JavaScript function. For more details take a look to this discussion.

13 – 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, "");};

A native implementation of the trim() function is available in the recent JavaScript engines.

14 – 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] */

15 – Transform the arguments object into an array

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

16 – Verify that a given argument is a number

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

17 – 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

18 – 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);

19 – Empty an array

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

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

Use splice 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.

21 – 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].

As a bonus, if you set the array length to a higher value, the length will be changed and new items will be added with undefined as a value. The array length is not a read only property.

myArray.length = 10; // the new array length is 10   myArray[myArray.length - 1] ; // undefined

22 – Use logical AND/ OR for conditions

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();

The logical OR could also be used to set a default value for function argument.

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

23 – Use the map() function method to loop through an array’s items

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

24 – Rounding number to N decimal place

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

NOTE : the toFixed() function returns a string and not a number.

25 – Floating point problems

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

Why does this happen? 0.1 +0.2 is equal to 0.30000000000000004. What you need to know is that all JavaScript numbers are floating points represented internally in 64 bit binary according to the IEEE 754 standard. For more explanation, take a look to this blog post.

You can use toFixed() and toPrecision() to resolve this problem.

26 – Check the properties of an object when using a for-in loop

This code snippet could be useful in order to avoid iterating through the properties from the object’s prototype.

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

27 – Comma operator

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

28 – Cache variables that need calculation or querying

In the case of a jQuery selector, we could cache the DOM element.

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

29 – Verify the argument before passing it to isFinite()

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

30 – Avoid negative indexes in arrays

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

Make sure that the arguments passed to splice are not negative.

31 – Serialization and deserialization (working with JSON)

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  */

32 – Avoid the use of eval() or the Function constructor

Use of eval or the Function constructor are expensive operations as each time they are called script engine must convert source code to executable code.

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

33 – Avoid using with() (The good part)

Using with() inserts a variable at the global scope. Thus, if another variable has the same name it could cause confusion and overwrite the value.

34 – Avoid using for-in loop for arrays

Instead of using…

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

…it’s better to use…

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

As a bonus, the instantiation of i and len is executed once because it’s in the first statement of the for loop. Thsi is faster than using…

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

Why? The length of the array arrayNumbers is recalculated every time the loop iterates.

NOTE : the issue of recalculating the length in each iteration was fixed in the latest JavaScript engines.

35 – Pass functions, not strings, to setTimeout() and setInterval()

If you pass a string into setTimeout() or setInterval(), the string will be evaluated the same way as with eval, which is slow. Instead of using…

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

…use…

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

36 – Use a switch/case statement instead of a series of if/else

Using switch/case is faster when there are more than 2 cases, and it is more elegant (better organized code). Avoid using it when you have more than 10 cases.

37 – Use switch/case statement with numeric ranges

Using a switch/case statement with numeric ranges is possible with this trick.

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"

38 – Create an object whose prototype is a given object

It’s possible to write a function that creates an object whose prototype is the given argument like this…

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

39 – An HTML escaper function

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

40 – Avoid using try-catch-finally inside a loop

The try-catch-finally construct creates a new variable in the current scope at runtime each time the catch clause is executed where the caught exception object is assigned to a variable.

Instead of using…

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        }   }

…use…

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    }

41 – Set timeouts to XMLHttpRequests

You could abort the connection if an XHR takes a long time (for example, due to a network issue), by using setTimeout() with the XHR call.

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();

As a bonus, you should generally avoid synchronous XHR calls completely.

42 – Deal with WebSocket timeout

Generally when a WebSocket connection is established, a server could time out your connection after 30 seconds of inactivity. The firewall could also time out the connection after a period of inactivity.

To deal with the timeout issue you could send an empty message to the server periodically. To do this, add these two functions to your code: one to keep alive the connection and the other one to cancel the keep alive. Using this trick, you’ll control the timeout.

Add a timerID…

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);        }    }

The keepAlive() function should be added at the end of the onOpen() method of the webSocket connection and the cancelKeepAlive() at the end of the onClose() method.

43 – Keep in mind that primitive operations can be faster than function calls. Use VanillaJS.

For example, instead of using…

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

…use…

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

44 – Don’t forget to use a code beautifier when coding. Use JSLint and minification (JSMin, for example) before going live.

45 – JavaScript is awesome: Best Resources To Learn JavaScript