As you know, JavaScript is the
number one programming language in the world, the language of the web, of
mobile hybrid apps (like PhoneGap or Appcelerator),
of the server side (like NodeJS or Wakanda)
and has many other implementations. It’s also the starting point for many new
developers to the world of programming, as it can be used to display a simple
alert in the web browser but also to control a robot (using nodebot,
or nodruino).
The developers who master JavaScript and write organized and performant code
have become the most sought after in the job market.
In this article, I’ll share a set of
JavaScript tips, tricks and best practices that should be known by all
JavaScript developers regardless of their browser/engine or the SSJS (Server
Side JavaScript) interpreter.
Note that the code snippets in this
article have been tested in the latest Google Chrome version 30, which uses the
V8 JavaScript Engine (V8 3.20.17.15).
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 Stringobject.
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= {"<":
"<", ">": ">","&":
"&", "\"": """};
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
…use…
var object = ['foo', 'bar'],
i;
try {
for (i = 0, len = object.length; i
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.
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.
I know that there are many other
tips, tricks and best practices, so if you have any ones to add or if you have
any feedback or corrections to the ones that I have shared, please adda
comment.
In this article I have used my own
code snippets. Some of the snippets are inspired from other articles and
forums:
