JavaScript is THE web programming language. It gets interpreted by your browser and web applications like Google Mail, Google Maps and Facebook make heavy use of them. Almost always, when you see something working smoothly / interactive, you see JavaScript in action.
But JavaScript has a lot of "features" which are ... well, I don't have words for those. Just continue reading.
Most of the following content is from a StackOverflow question Strangest language feature
Weak typing
Example
console.log(3..toString());
Output
'3'
Explanation
3. is a floating point and can get converted to a string. But 3.toString() gives
SyntaxError: Unexpected token ILLEGAL
Weak typing and string concatenation
Example
console.log('5' + 3);
console.log('5' - 3);
Output
'53'
2
Explanation
JavaScript automatically converts datatypes and + is used for string concatenation and for addition.
In the first case, as the first datatype is a string and + is defined for strings as concatenation, JS converts 3 to '3' which results in the string '53'.
In the second case, - is only defined for subtraction so '5' gets converted to a number (int? float? I don't know. I guess int.)
Automatic semicolons
Example 1
function test() {
return
{
id : 1338,
title : 'This is a test'
};
}
console.log(test());
Example 2
function test() {
return
2 + 2;
}
console.log(test());
Output 1
Uncaught SyntaxError: Unexpected token :
Output 2
undefined
Explanation
JS adds a ; at every line end. Automatically. You can't prevent it.
Please note that the second output is no error! It has a (valid) return value of undefined.
Truth table
Example
console.log("------- 0 == XYZ ---------");
console.log(0 == 0);
console.log(0 == false);
console.log(0 == '');
console.log(0 == null);
console.log(0 == undefined);
console.log(0 == " \t\r\n");
console.log(0 == '0');
console.log(0 == 'false');
console.log("------- false == XYZ ---------");
console.log(false == false);
console.log(false == '');
console.log(false == null);
console.log(false == undefined);
console.log(false == " \t\r\n");
console.log(false == '0');
console.log(false == 'false');
console.log("------- '' == XYZ ---------");
console.log('' == '');
console.log('' == null);
console.log('' == undefined);
console.log('' == " \t\r\n");
console.log('' == '0');
console.log('' == 'false');
console.log("------- null == XYZ ---------");
console.log(null == null);
console.log(null == undefined);
console.log(null == " \t\r\n");
console.log(null == '0');
console.log(null == 'false');
console.log("------- undefined == XYZ ---------");
console.log(undefined == undefined);
console.log(undefined == " \t\r\n");
console.log(undefined == '0');
console.log(undefined == 'false');
console.log("------- \t\r\n == XYZ ---------");
console.log(" \t\r\n" == " \t\r\n");
console.log(" \t\r\n" == '0');
console.log(" \t\r\n" == 'false');
console.log("------- '0' == XYZ ---------");
console.log('0' == '0');
console.log('0' == 'false');
console.log("------- 'false' == XYZ ---------");
console.log('false' == 'false');
console.log("--------------------");
console.log("------- XYZ == 0 ---------");
console.log(0 == 0);
console.log(false == 0);
console.log('' == 0);
console.log(null == 0);
console.log(undefined == 0);
console.log(" \t\r\n" == 0);
console.log('0' == 0);
console.log('false' == 0);
console.log("------- XYZ == false ---------");
console.log(false == false);
console.log('' == false);
console.log(null == false);
console.log(undefined == false);
console.log(" \t\r\n" == false);
console.log('0' == false);
console.log('false' == false);
console.log("------- XYZ == '' ---------");
console.log('' == '');
console.log(null == '');
console.log(undefined == '');
console.log(" \t\r\n" == '');
console.log('0' == '');
console.log('false' == '');
console.log("------- XYZ == null ---------");
console.log(null == null);
console.log(undefined == null);
console.log(" \t\r\n" == null);
console.log('0' == null);
console.log('false' == null);
console.log("------- XYZ == undefined ---------");
console.log(undefined == undefined);
console.log(" \t\r\n" == undefined);
console.log('0' == undefined);
console.log('false' == undefined);
console.log("------- XYZ == \t\r\n ---------");
console.log(" \t\r\n" == " \t\r\n");
console.log('0' == " \t\r\n");
console.log('false' == " \t\r\n");
console.log("------- XYZ == '0' ---------");
console.log('0' == '0');
console.log('false' == '0');
console.log("------- XYZ == 'false' ---------");
console.log('false' == 'false');
Output
| == | 0 | false | '' | null | undefined | " \t\r\n" | '0' | 'false' |
|---|---|---|---|---|---|---|---|---|
| 0 | true | true | true | false | false | true | true | false |
| false | true | true | true | false | false | true | true | false |
| '' | true | true | true | false | false | false | false | false |
| null | false | false | false | true | true | false | false | false |
| undefined | false | false | false | true | true | false | false | false |
| " \t\r\n" | true | true | false | false | false | true | false | false |
| '0' | true | true | false | false | false | false | true | false |
| 'false' | false | false | false | false | false | false | false | true |
Explanation Wow. This is fucked up. I've expected that strings compared to anything that is not the string itself evaluates to false.
At least the table is symmetric and the diagonal is true. Please also note that JavaScript has a === operator.
The truth
Example
console.log(2 == [2]);
console.log(2 == [[2]]);
Output
true
true
Explanation At StackOverflow
Date
Example
var futureDate = new Date(2010,77,154);
console.log(futureDate);
console.log(futureDate.getYear());
Output
Tue Nov 01 2016 00:00:00 GMT+0100 (CET)
116
Explanation 77 months and 154 days from the 0th day of 0th month of 2010
You should use getFullYear() instead of getYear(), as the later one returns year - 1900 (Why? When is this useful?)
Integer overflow
Example
console.log(111111111111111111111);
Output
111111111111111110000
Explanation It's ok that JavaScript fails at handling this integer. I think it converts this to a float, but I'm not sure about that. But no matter what it does here, it doesn't throw an error. That's bad. How is a developer supposed to know in a big application know when something went wrong?
Function parameters
In Javascript, declaring the parameters a function accepts is only a convenience to the programmer. All variables passed through the function call are accessible by the keyword arguments. So the following would alert "world":
wzxhzdk:16
Source on StackOverflow
Global variables
Example
var a = 12;
function test() {
a = 1337;
}
test();
console.log(a);
Output
1337
Explanation
When you forget to use var inside of test() you might accidentally use a global variable.
See also
Do you know some more JavaScript WTFs?
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