Javascript is a very flexible language, I made a compilation of some edge cases that you may have encountered while programming. The main goal is to point out some interesting specific behaviors.


1] var result = [10] + 1;
The Array doesn't have a toNumber method, instead it has a toString method: [10] + 1 becomes "10" + 1.
String has a greater priority over Number with the + operator: "10" + 1 becomes "10" + "1" = "101"
2] var result = ['a', 'b', 'c'] + "";
The Array.toString method concatenates all it's elements (converted to string) with the "," separator.
3] var result = 'a' + 5;
In Javascript, String has a greater priority than Number for the + operator, so 5 is converted to string before being concatenated.
In PHP, the + operator is only the addition so 'a' is converted to number (gives 0) before being added.
In C, a char and an integer are of the same type. 'a' is first converted to it's Ascii value (gives 65) before being added.


4] var result = 3.75 | 0;
|0 is a fast way to do a Math.floor for positive integers.
5] var result = 65 / 'a';
/ is only defined for Numbers so 'a' is first converted to a number (gives 0).
A division by 0 does not throw an error but results the object NaN (Not a Number).


6] var ob = {"10": 1};
ob[10] = 2;
ob[[1, 0]] = 3;
var result = ob["10"] + ob[10] + ob[[1, 0]];
Objects keys are strings. If you don't provide a string, it will convert the key to string.
"10" and 10 gives "10"
[1, 0] gives "1,0"
7] var $ = {"": String};
var result = !!$[([])]();
$ is a valid variable name.
Parenthesis inside the key part are useless: array[(1+2)] is the same thing as array[1+2].
Objects key are first converted to string. [].toString() is "".
It is possible to have the empty string "" as an object key.
$[""] is the String object. String(val) returns val.toString(). String() returns "".
! is the negation operator. "" == false, so !"" == true, !!"" == false
!!expr is a fast way to typecast an expression to a boolean.


8] var result = (' \t\r\n ' == 0);
Unlike PHP, strings wrapped around simple quote ' are also parsed. '\t' == "\t"
When compared to a number value, a string will be converted to a number. If it contains only spacing characters it will be converted to 0. If a string cannot be parsed as a number it will return NaN (Note that NaN != NaN).
9] var a = new String("123");
var b = "123";
var result = (a === b);
When creating an object with the new operator, the result type is always "object". The type of "123" is "string" so the type does not match for ===.
10] var a = {key: 1};
var b = {key: 1};
var result = (a == b);
Object comparison is only done with the pointers behind the objects.

If you liked this article, you might be interested in my Twitter feed as well.

Related Posts

  • August 29, 2011 Javascript: Improve Cache Performance: Reduce Lookups (2)
    In my Binary Decision Diagram Library, the performance bottleneck was the uniqueness cache. By reducing the number of cache lookup, it is possible to greatly improve the performances. Common pattern In order to test if the key is already in the cache, the usual pattern is to use key […]
  • October 5, 2011 Javascript Presentation (1)
    The talk is over. Check out the Slides & Video. For several months now I've been surveying my friends and teachers at EPITA and I came to the conclusion that they have absolutly no idea what Javascript really is. In order to help them discover a language that is getting a lot of […]
  • October 28, 2011 JSPP – Morph C++ Into Javascript (Paper) (0)
    6 months ago, I wrote the blog post "JSPP - Morph C++ into Javascript". My supervisor at the LRDE (R&D Lab of EPITA), Didier Verna, found it interesting and told me that it could be worth a publication. With his great help, I've written my first article. We have submitted it to two […]
  • August 23, 2011 Javascript – Hook Technique (5)
    Let's go back 5 years ago during the World of Warcraft beta. I was working on Cosmos UI, a projects that aimed to improve the World of Warcraft interface. As interface modification was not officially supported by Blizzard, we went ahead and directly modify the game files written in […]
  • February 20, 2010 Javascript – Slug (3)
    A slug is a way to represent a title with a limited charset (only lowercase letter and dash) to be inserted in the url. Even if it is a common function there is no good enough implentation when you Google for it. Here are the features I needed: No multiple dashes. ---- is converted […]