By Saad Mousliki
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: https://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= {"<": "<", ">": ">","&": "&", """: """};
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
- Code Academy JavaScript tracks: https://www.codecademy.com/tracks/javascript
- Eloquent JavaScript by Marjin Haverbeke: https://eloquentjavascript.net/
- Advanced JavaScript by John Resig: https://ejohn.org/apps/learn/
Conclusion
I know that there are many other javascript 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.
References
In this article I have used my own code snippets. Some of the snippets are inspired from other articles and forums:
- JavaScript Performance Best Practices (CC)
- Google Code JavaScript tips
- StackOverFlow’s javascript tips and tricks
- TimeOut for XHR
23 – Use the map() functio* method […]
Thanks Dimitri, We’ll update it ASAP
#8 the array has a syntax error
// ;
Thanks William, We’ll update it ASAP
Nice article, good set of tips.
Your tipps are very suggestive, without explaining the cases where they apply. Some suggestions will do some complete different flow, for example the first tip: You do not explain wha’s the difference when you ‘forget’ the variable declaration, so the tip is useless and somehow dangerous.
Same as with tip 40: The two variants you mention have a complete different logic. The one which you say to avoid executes the catch every loop interval, the second breakes the loop when the error occours. It depends on what’s your target which form you shoud use.
And last but not least you should recognize your own tips in your example: In tip 37 you don’t do what you insist in tip 2 (use === instead of ==) This applies also to >== <== etc. of course.
Thanks Thomas, I’m a developer and I know that when a developer need a quick list of snippets that is verified/tested and approved, he will not spend his time reading a long explanation and test it again, he need some things which could copy/paste and use it quickly in his code.
Regards
Saad, I’m a developer too and have moved to web from windows applications recently. Sorry to say but I agree with Thomas. I understand you have put a lot of effort in writing this article but would definitely appreciate if it was more elaborate. I would rather understand the concept before “copy/pasting” your code. Hope to see a much more elaborate article. Thanks
Thanks Murthy, I think that I’ll update the article with some links to JsPerf tests, and other articles too, hope that I find time for that …
Saad, Thanks for the great tips. I actually appreciate the brevity. Hopefully I can work out the rest for myself if I need to.
On #40 I’d say the advice should be that exception handling is expensive so it should be used only for exception handling, not error handling. Probably the only time I would use exception handling in an inner loop is in some kind of batch process where you don’t want some unanticipated error in data or code to abort the entire run. Once an exception has been logged however, you should either fix the code (obviously) or code to anticipate what is now a known error and handle it instead of throwing an exception. In my experience some developers get the concept confused and think that coding for exceptions is expensive. Not so – it’s only expensive if it actually has to handle an exception.
Saad, I second your motion that these snippets don’t need to have a wall of text elaborating on appropriate use cases. Afterall, your title states ‘Tips and Tricks’.
-Great job! And Thanks!!
Since there are no strict comparison operators in JavaScript, 37 is fine.
There *are* strict comparisons in JS, see #2.
Sounds like T Maul
What on earth is the benefit of #27 – Comma operator??
IMHO it could be considered as a trick that should be known by a JavaScript developer, you could see it somewhere and don’t understand it 😉
…errrr… a trick to achieve what?
in that case you should include all the golfing tricks too
I’m sorry, but to me that’s utterly pointless
Check https://javascriptweblog.wordpress.com/2011/04/04/the-javascript-comma-operator/ for use cases.
Example:
if (expression) { doSomething(); return something; } else { doSomethingElse(); return somethingElse; }
vs
return expression ? (doSomething(), something) : (doSomethingElse(), somethingElse);
Thanks atmin for this illustration 😉
Just as a reference, doesn’t #22 implement #2 either.
The second best practice will be updated by a clear explanation, sorry for the inconvenience
#43 I think speed of
A.push(v);
and
A[A.length] = v;
depend on browser
https://jsperf.com/push-vs-length-test
I have seen the performance tests, an I have noticed that when length gain against push, it has about twice op/s, but when push gain it is not more than 20%. 😉
Thanks
If the array have some value, push will more faster.
You can try :
https://jsperf.com/push-vs-length-test/28
So it depend on some case, and primitive operations not aways faster
(See: https://www.avronp.com/nsn/nsn_stringvb/rsrcs/js/foundations/js_string.js)
In addition to Trim() check out all the vb functions (and more) both as:
******************************************************************************
functions – Trim(mystring);
methods – mystring.trim();
eg:
// .trim | Trim() | trims string on left and right
// .ltrim | LTrim() | left trim string
// .rtrim | Rtrim() | right trim string
// .left | Left() | Returns left part of string
// .right | Right() | Returns right part of string
// .nolf | Nolf() | removes linefeeds (ASCII 10)
// .nocr | Nocr() | removes carriage returns (ASCII 13)
// .nocrlf | Nocrlf() | removes carriagereturns + linefeeds (ASCII 13+10)
// .nolfcr | Nolfcr() | removes linefeeds + carriage returns (ASCII 10+13)
// .nosp | Nosp() | removes spaces
// .nosquotes | Nosquotes() | removes single quotes
// .crbr | Crbr() | replaces carriage returns
// .brcr | Brcr() | carriage returns replaces
// .lpad | Lpad() | left pads a string with a fill string to a count
// .rpad | Rpad() | right pads a string with a fill string to a count
// .flat | Flat() | on each line removes leading and trailing spaces
// .degremlin | Degremlin() | removes gremlins from a string
// | | Default gremlins: Some Non alphanumerics
// | | Optional: Parameter can contain list of gremlins
// .upperhtml | Upperhtml() | Convert HTML tags to uppercase
// .lowerhtml | Lowerhtml() | Convert HTML tags to lowercase
// .htmlattribs | Htmlattribs()| FORMAT HTML tags (quotes/case) | see: details in function
// .format | Format() | formats a string to a pattern | format can be expanded to include any formatting pattern required
// .count | Count() | counts occurrences of a pattern in a string
// .strreverse | Strreverse() | reverses a string
// .isnumber | Isnumber() | checks to see if a string is a number
// .isnotdigit | Notdigit() | checks to see if a string is not a digit
Great list. Typo in #20 – description refers to split it should be splice as correctly shown in code example.
Thanks Johan :D, yes it was my mistake, we will fixe it ASAP
Where is requestAnimationFrame? (setInterval is bad practice)
As I said in the article introduction, I’m sharing tips that are related to the language, this method will not work for example in server side SSJS, It’s related to the browsers and not yet implemented by all browsers, https://caniuse.com/#feat=requestanimationframe
On a related note, the JSON object didn’t exist before IE8. The few IE7 users left would need a polyfill.
who says setInterval() is bad practice? I had major issues (like not updating the screen for several seconds) with RAF on iOS (6 and 7) and had to abandon it. Of course, YMMV.
While SetInterval may be bad practice, requestAnimationFrame should only be used when updating something visible on screen (i.e. animating) but not for general timing purposes.
Use SetTimer repeatedly instead, as is done in #42 (although in that particular case perfect timer isn’t essential and setInterval could work!)
Hi Saad.
Great “little” compilation of tips & tricks.
I’ve stumbled over a few minor typo’s etc. And I just saw some of my bullets below have been commented on while I wrote. Passing them on anyway 🙂
#14 – Append an array to another array.
Using push this way is actually limited by max number of arguments (less than 2^17 on Chrome).
So for huge arrays this trick will throw an exception.
#20 – Don’t use delete to remove an item from array
A minor typo – “Use split instead of …” (split => splice)
#22 – Use logical AND/ OR for conditions
“The logical AND could …” => “The logical OR could …”
Also the function has fallen victim to some automatic spell check… (Uppercase Function and uppercase ‘Arg1’)
#26 – Check the properties of an object when using a for-in loop
“if (object.hasOwnProperty(name))” is necessary only on objects being new’ed and if you want to disregard prototype properties.
Object literals can safely be run though using for..in.
#27 – Comma operator
I agree with your note that developers should know how to use the comma operator.
Actually, many use it in var lists and they should have explicit knowledge that there is no great difference between your example and the var list use.
#30 – Avoid negative indexes in arrays
“… passed to indexOf are …” => “… passed to splice are …”
An interresting use of out-of-bounds indeces is: “for (var i=0; myArray[i]; ++i)” since element lookup outside the range will return “undefined” (of course, this require the array to contain no other falsy items 🙂
#33 – Avoid using with() (The good part)
A good example would be
function f(object) {
var x = 2;
with (object)
++x;
return x;
}
Calling f for any object not having a ‘x’ property will make f always return 3.
Calling f({x:7}) (an object with property ‘x’ being numerical – or convertable to) will return 8.
Calling f with any object having a non-numerical ‘x’ will return 2.
Unless applied with care using “with” is best suited for code obfuscation – or job security 🙂
#34 – Avoid using for-in loop for arrays
Last line: “Why? The length of the array arrayNumbers is recalculated every time the loop iterates.”
I doubt length is recalculated. However, the object property lookup will often be slower than local variable lookup.
Having a sufficiently good optimizing interpreter no discernable difference should show.
Thanks Christian for the notes, many benchmarking tests were done in JsPerf to demonstrate those tips, and as you may know JavaScript implementation depends on (JavaScript Runtime Engines) : the browsers (client side), and also on the Server (SSJS :Server-Side JavaScript ) implementation, it’s why the results could be differents depending in the implementation, in the list, I have chosen the tips which overcome in the max of browsers and SSJS Server, I add also that many tips (#34,#26, #14, #38, #42 …etc. ) will have a native implementation in the latest JavaScript version ES6 and ES7 (work in progress)
it’s not a Bible 😉
#12 is **not** a good way to shuffle an array. In fact it really has no well-defined behavior at all. Use a Fisher-Yates shuffle.
Indeed, Array.prototype.sort isn’t the way to go. If anyone is interested in the reasoning (or if you want to see a good Javascript implementation of Fisher-Yates) check out the answers to this StackOverflow question: https://stackoverflow.com/a/962890/388639
Typo in #14, you need a colon between name and “Joe” in: var array1 = [12 , “foo” , {name: “Joe”} , -2458];
Otherwise, fantastic list.
#11 has a reference error
Yes, I have just seen it.
Thanks Debjeet
Great article!
Very useful list, thanks !
For 14, use concat
var one = [“Cecilie”, “Lone”];
var two = [“Emil”, “Tobias”, “Linus”];
var both = one.concat(two);
+1
Hi Aamir, the tip is about appending and not concatenating, the concat() method could be a good option too
thanks
Correction for #11:
var rdmstring = “”;
Should be
var rdmString = “”; // because currently when you call rdmString later on, it will cause error due to being undefined
#43 is missing the colon in your conditional operator. Not sure if someone already pointed this out. Thanks for the article!!
Also a good read on flippinawesome: https://flippinawesome.org/2013/11/25/fun-with-javascript-native-array-functions/
For 24 see https://stackoverflow.com/questions/10015027/javascript-tofixed-not-rounding. There is no easy answer.
Example 10 will not contain 0 in the numbersArray
#10 Would return [1…99] instead of the indicated [0,1,2,3 … 100]
————-
var numbersArray = [] , max = 100;
for( var i=1; numbersArray.push(i++) < max;); // numbers = [0,1,2,3 … 100]
It returns [1,2,3 … 100] and not to 99,
it will be fixed ASAP
Thanks
Actually, this is a trick and not a good practice.
Reading too fast, one would think it would return [1 … 99] but it will return [1 … 100] because the push method is an instruction that is irremediably evaluated, whatever the result of the conditional for-loop parameter is.
100 is a value that is pushed in-extremis. After that, the test returns false, but it is already too late.
You are not tricking the interpreter, but the reader. And that’s not good.
The conditional segment should stay as clear as possible of any misinterpretation.
I like your blog article. But it lacks of reminders about “good tricks versus good practices”.
In good practice, devs should avoid the ternary operator as often as possible. They are good practice in limited use at the beginning of methods to set local variables and such. Not in the core implementations. And should not be used to branch code logic. Use the good old if (){} else {}. And choose to prefer not go against JSLint advices.
If you really need other devs to be unable to read you, or to read you correctly, use an obfuscator !
Ternary branching are also hard to debug with breakpoints. That’s a minor point.
A[A.length] = v; // is not a good practice either. But it depends entirely of your scope structure or your browser or your real array end. Some would prefer stacking objects anyway. And not caring about the indices. Never assume your array would never be trimmed down or spliced. You may one day be surprised your array length is shorter than expected.
Keep up the good work in compiling those tricks anyway. They make good cheat sheets though.
Technically, #24 returns “2.4432”, it is a string, not a number. So if you were to add another number to n after the toFixed, it would simply add it to the end as in string concatenation.
var n = 2.414233;
n = n.toFixed(4);
console.log(typeof n); // string
Nice article man !! Thanks.
What happened to tip #3?
Nice catch. My fault. Fixed.
Nice article. Flipping Awesome.
Crockford’s “The Good Parts” recommends against using the new operator in tip #5 (under the “Bad Parts” – “new” and “The Constructor Invocation Pattern” sections) and also against the ++ increment operator in tip #34 (under the “Bad Parts” – “++” section). Stefanov’s “JavaScript Patterns” book suggests using the += increment operator instead of ++ (e.g., i += 1 instead of i++) in the “Essentials – for Loops” section. “The Good Parts” gives several recommended alternates to the new operator in “Chapter 5. Inheritance”. Thanks for all of the tips, it’s always nice to review articles like this for reinforcement and finding new gems!
nice article ! but with some typo witch will easily mislead new fish … like the sugguestion for remove a item from a array with slice or delete . typoed the slice to splite ….
These are very handy tips. Thanks for sharing!
#4 – Not very persuasive about semicolons. It’s “good practice” yes, but why? Well, it will bite you if you write a return value on the next line after the keyword return, such as
function greeting () {
return
“hello world”
}
This function will actually return undefined because a semicolon will get inserted after return. AFAIK this is the main reason for enforcing the semicolon rule, although the more effective rule would be “begin your return value on the same line as the keyword return”. If people only did that, they could be as sloppy as they liked with semicolons.
Your advice to use jslint is admirable. I wish you had used it on your own examples! e.g. you didn’t use the single var pattern in the longer snippets, and it really can be a source of errors to declare vars anywhere but at the top of the function because of hoisting etc. I am sure you know this, as will many readers.
Otherwise, thanks for a good article. Some useful tricks.
Bugfixes/Improvements:
– It’s hard (and silly) to remember the tip without understanding why it’s important, please add missing explanations.
– syntax highlighting!
– #6 – this is a good example of a tip that misses proper explanation
– #7 `})(10,20)` – missing semicolon (see point #4)
– #13 extending build-in objects is controversial
– #14 `{name “Joe”}` -> `{name “Joe”}`
– #16 linking to the author of this code would be nice ( https://stackoverflow.com/questions/18082/validate-numbers-in-javascript-isnumeric )
– #19 and #21 are the same thing
– #22 `Function doSomething(arg1){ ` -> `function doSomething(arg1){ `, `Arg1 = arg1 || 10;` -> `arg1 = arg1 || 10;`
– #22 It’s worth mentioning that last example is not entirely true. If arg1 will be set to e.g. 0 or false it will be replaced with default value.
– #27 interesting, but not useful
– #30 “Make sure that the arguments passed to indexOf are not negative.” -> “Make sure that the arguments passed to splice are not negative.”? I’m not 100% sure what you meant here.
– #34 “The length of the array arrayNumbers is recalculated every time the loop iterates.” – it’s no longer true for modern JS engines
– #35 `setTimeOut` -> `setTimeout`
– #36 “Avoid using it when you have more than 10 cases.” – why?
– #39 link to the source: https://gist.github.com/leommoore/4710081/
– #41 “synchronous Ajax” -> “synchronous XHR”. A in Ajax stands for “asynchronous”.
– #42 `var min = a `var min = a < b ? a : b; `
– #45 this resource is locked and outdated (it's from 2011)
Thanks Konrad for this good review, As I said the list is open to the readers suggestions and improvments, I’ll check this list and others, after that update the article ;).
For #45, have you could share a list of good resources if you have !
Re: 13 – A String Trim Function: JavaScript does have a string trim function: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/Trim
You wrote «slice» instead of «splice». Also JS does have a ‘trim’ function, check out this: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/Trim
– otherwise this is a great write-up!
#22 in the first example doSomething() always return something, so in the first case the variable will take the returned value, even undefined. In the case from comments it won’t.
Nice article. You wrote you didnt want explanations. They or links for live examples or external explanations would be useful.
Regards
#10 should be:
var max = 100, numbersArray = new Array(max); // faster
for( var i=1; numbersArray.push(i++) < max;); // numbers = [0,1,2,3 … 100]
If you use jQuery you could do:
var numbersArray = jQuery.map(new Array(100), function(el, i) { return i; }); // numbers = [0,1,2,3 … 100]
I know for a fact, that the D3 library has changes all its Array initialization to use use new Array([array length]) solely for performance reasons.
This code :
var max = 100, numbersArray = new Array(max); // faster
will create an array of 100 undefined items, As I said in the top, the tips are not related to any frameworks (JQuery, D3 …etc), only native JavaScript code 😉
Thanks for sharing
Thanks dotnetCarpenter, I’ll update the comment the created array strats from 1 to 100, and not from 0.
#19 If the array contain references to DOM nodes or titanium proxy objects you’ll get a memory leak in older IE and Appcelerator.
Instead you have to explicitly null the references before emptying the array.
var myArray = [12 , objReference, 1000 ];
myArray.forEach(explNull); // forEach is part of ecmaScript5
myArray.length = 0; // myArray will be equal to [].
function explNull(item) {
item = null; // reference is freed for the garbage collector
}
In ecmaScript3 (IE8 and before) you can polyfil it with:
if(!Array.prototype.forEach) {
Array.prototype. forEach = function forEach(fn, context) { // named functions are easier to debug
for(var i = 0, len = this.length; i < len; ++i)
fn.call(context, this[i], i, this);
};
}
#43 has a typo. “var min = a < b ? a b;" should be "var min = a < b ? a : b;"
Nice list. I didn't know about the trick in #37. Very nice
var z = 1; // ONE
function fn(x, y){ console.log(‘@args:’, x, y, z); return x + y + z; }
fn(( z && (z++, 4) ), 6); // returns 12
var z = 0; // ZERO
function fn(x, y){ console.log(‘@args:’, x, y, z); return x + y + z; }
fn(( z && (z++, 4) ), 6); // returns 6
Kinda neat-o!
Cody, Thanks for this snippet 🙂
Tip #14 looks good but array1.concat(array2) is the natural way of joining 2 arrays into one in JavaScript
Tip #16
A little tricky, silly implementation I learnt from Lea Verou
function isNumber(n) {
return n === +n;
}
29 – isFinite(und*i*fined); // false
23 – map() should never be an automatic choice for looping over an array. Function call overhead on every iteration. Only use map() if you don’t care about performance.
36 – Use a switch/case statement instead of a series of if/else
please see test https://jsperf.com/if-vs-switch-statement
switch more slow than if statement
look and second test https://jsperf.com/if-vs-switch-statement2
43 – Keep in mind that primitive operations can be faster than function calls. Use VanillaJS.
Keep in mind that examples should be tested https://jsperf.com/vanilla-vs-math-min/2 =)
Thanks I’ll check the given jsperf tests 😉
Great reference for when you need something quick. Thanks for putting it together.
One typo… You’re missing a : on…
var min = a < b ? a : b;
A[A.length] = v;
In tip 34 ‘ Avoid using for-in loop for arrays’ you state that one should use a for loop instead of a for-in loop.
Of course this is true and your exampled alternative is indeed a lot better. But how about this one:
for(var i=myArray.length; i–;){
// Do something under the awareness that we are looping back.
}
This actually is an even faster way to loop through the array since we do not have to check the value of i against the length of the array. This is because i will become 0 (falsy) at one point and the loop will end. Maybe it’s a minor performance gain but i like it.
What do you think about this method?
for (var i = myArray.length, item; item = myArray[–i];){
// do something with item
}
Notice: array should not contain values interpreted like false (empty string, undefined, zero…)
can you say the reason about 37 ?
In 42, you have used `cancelTimeout,` which is not a JS function AFAIK. What you probably want is `clearTimeout`
Thanks, I just see it, It was my mastake, it’ll be updated ASAP 😉
It’s awesome man!! Thanks for article!
#3 I would add an exemple.
This is good practice to not be lazy and to write for example: “if ( val === null) {}” instead of “if ( !val ) {}”.
As you stated: undefined, 0, “”, null, NaN and false, are false.
But you don’t want to know if this is false, in that case, you would want to know why, or specifically what is.
Is this because this is binary false, because it’s null or undefined ?
So write it.
Of course, “if ( !val ){}” stays valid and can be used if you know what your are doing.
Very nice….
Hi! I found this article really useful! Can i translate it, mentionioning the source and the actual author, and put it in my blog? https://www.paitadesign.com/blog ? Thanks!
Hi Giacomo,
Ok, I hope that it could be helpful for people who doesn’t understand english 😉
Thanks! As soon as i can i will start, once i end the post i will notify you.
very useful list.
Thanks.
Good Tips & Tricks. Waiting for some more . . .
Great list! See few more: https://zinoui.com/blog/javascript-best-practices
KeepAlive should probably have an else statement that calls CancelkeepAlive if the websocket is unavailable 🙂
Great list.
P.s Ignore the haters. If they weren’t ignorant and/or pedantic they could have filtered the list and learn a thing or two.
ES5 shim
5 No. No, really. Don’t do it.
https://ericleads.com/2012/09/stop-using-constructor-functions-in-javascript/
https://ericleads.com/2013/01/javascript-constructor-functions-vs-factory-functions/
https://medium.com/javascript-scene/the-two-pillars-of-javascript-ee6f3281e7f3
7 Don’t do this with modules anymore. Instead, use node-style modules or ES6 modules (the latter with ES6 transpiler).
13 Use ES5 shim instead.
14 Use Array.prototype.concat() instead.
15 Can be safely replaced with [].slice.call(arguments). This is not needed in ES6. Don’t worry about perf. I’ve tested it heavily. It won’t slow you down or cause a blip with memory.
17 Use ES5
Array.isArray()
instead. Never useinstanceof
. It’s easy to forget that it breaks across execution contexts.18 Cool trick! =)
19 Cleaner approach: myArray = [];
20 Be careful about mutating arrays. If other parts of the code are touching the array, it could cause bugs. Copy the array if it’s feasible.
21 As with 20, you may want to use Array.prototype.slice() and create a truncated copy instead of mutating an existing array.
22 Code readability is much more important than code writeability.
if
reads easier. For defaults / overrides pattern, it’s safer to use named paramaters and_.extend({}, defaults, options)
or$.extend({}, defaults, options)
, instead.36 Try using an action object, instead. See https://ericleads.com/2012/12/switch-case-considered-harmful/
38 This is standard in ES5 as
Object.create()
. Use that, instead.39 This is unsafe. Use a security library, instead. See WASP recommendations, https://github.com/mapbox/sanitize-caja, https://github.com/punkave/sanitize-html and remember to penetration test: IronWASP https://blog.ironwasp.org/
43 You’re optimizing the wrong things. https://ericleads.com/2013/04/youre-optimizing-the-wrong-things/
Additional suggestions: The biggest and most common mistakes I see JS developers making is that they often miss two of JavaScript’s most important features: prototypal OO and functional programming. See “JavaScript Training Sucks”: https://medium.com/javascript-scene/javascript-training-sucks-284b53666245
I notice you got a lot of other suggestions. Some of them good ones. I’d love to see version 2 of this article after you’ve had time to absorb all these comments. 😉
I like this article and translate it into Chinese: https://chensd.com/2015-01/45-useful-javascript-tips-tricks-and-best-practices.html
great post!!!
Thanks, it seems that this article should be updated, I’ll try to find time for a new version 😉
A type error:
Thsi
should beThis
.The correct anwesr, of course, is given by Internet Explorer (4).Nope, the ECMA 262-5 spec indicates that Firefox’s behavior is correct.The anwesrs are different because Firefox assumes you’re an idiot. Seriously… Firefox (and other browsers) tend to ignore trailing commas – which in this case incorrectly chops off an “undefined” array value. […] Unfortunately, that’s a terrible idea… and it’s no wonder why JavaScript developers (until very recently) were looked down upon in the software industry. This low bar for correct syntax made JS developers look like a sad bunch of mediocre software wannabes. Funny story, trailing commas in C# are no-ops, so are they in Python or Ruby. Or pretty much any sane language really. I do not believe I’ve ever seen a case were a trailing comma was intended as create a new empty slot .And of course going both against the spec and against the behavior of every other browser means IE most definitely isn’t right.
It seems that two slashes were lost in the regex of the trim function ( \s+ instead of s+)
It shoud be:
String.prototype.trim = function(){return this.replace(/^\s+|\s+$/g, “”);};
Thank you for these tricks!
great post
As previously mentioned, #19 could have been simply myArray = [];
Yeah, sorry. I went back and tested evenrthiyg and it seems to be good now(weird how that works). I had noticed the highlighting thing before, but when I read that you fixed legacy issues, I guess I assumed you were talking about that. In any case, I was only able to find one situation in which the parameter insight didn’t work correctly, and that was when I called a constructor inside a function, though I’m not sure if it works for all variations between the two.foo(Type(1,2,3));The parameter insight always shows the outermost set of parameters, ie the ones from foo().
Wow, I barely looked at this and I already love it, thanks a lot for this post!
This list is absolutely awesome! Thank you. I have a solid understanding of JS and all I need now is a good bag of tricks and tools. And this list provides just that.
Great write up! really helpful.Another helpful article that might interest viewers https://ow.ly/XCZq309vD4F
Great article. Bookmarked.
10 – Generate an array of numbers with numbers from 0 to max
******************************************************************************
const max = 100;
const numbersArray = […Array(max).keys()].map( n => n);
Great article. I found a more succinct alternative for #10 and a fix for the missing 0 at the beginning of the array.
var max = 10, numbersArray = […Array(max+1).keys()];
Best!
Excellent article, Thank you so much!
Must note, if you have calculations in code, use Math library.
case (age >= 50):
category = “Old”;
noooooo
If anyone comes here and reads this article, it is FILLED with incorrect statements. go search kyle simpson, you do not know javascript. The author needs to go read his books.
This is an extremely bad article and very misleading.
Nice post. I am impressed by the quality of information on this website. Thanks for sharing this post.