if(negetive number) is true? Is something wrong with js?

Question

Is something wrong with js?

if("hello".indexOf("world")) { // I forgot to add > -1 here
    console.log("hello world");
}

Basically if(-1) is true. How is this possible? It took me a whole day to fix this. Is there a list available where these kind of things are listed? Or tools available to catch things like these.

What language are you coming from where negative numbers are false? — david, Apr 07 '14 at 06:52
Yes. The 'normal' or 'regular' thing is any real number (negative or positive) is true. Except zero, zero is always false. Although the best way is to look in the language specification to be sure. — marcoslhc, Apr 18 '14 at 03:31

thefourtheye · Accepted Answer · 2014-10-15T13:18:52.877

As per ECMA 5.1 Standard Specifications, the following table is used to determine the truthyness of an expression

+-----------------------------------------------------------------------+
| Argument Type | Result                                                |
|:--------------|------------------------------------------------------:|
| Undefined     | false                                                 |
|---------------|-------------------------------------------------------|
| Null          | false                                                 |
|---------------|-------------------------------------------------------|
| Boolean       | The result equals the input argument (no conversion). |
|---------------|-------------------------------------------------------|
| Number        | The result is false if the argument is +0, −0, or NaN;|
|               | otherwise the result is true.                         |
|---------------|-------------------------------------------------------|
| String        | The result is false if the argument is the empty      |
|               | String (its length is zero); otherwise the result is  |
|               | true.                                                 |
|---------------|-------------------------------------------------------|
| Object        | true                                                  |
+-----------------------------------------------------------------------+

Mosho · Answer 2 · 2014-04-07T06:44:37.603

8

The only number that is "falsey" (and would therefore evaluate to false and not pass an 'if' statement) is 0. The rest are "truthy", even negative ones.

You can test this in the console with !!-1. That means converting the value to the Boolean opposite, and repeat once. The first ! on -1 returns false and the second returns true. This is the most common way to convert an expression to its Boolean equivalent.

edited Apr 07 '14 at 06:44

answered Apr 07 '14 at 06:31

Mosho

7,099
3
34
51

Great explained. I didn't know that you can get boolean value of any object with "!!". – SteamFire Apr 08 '14 at 11:42

score 3 · Answer 3 · answered Apr 07 '14 at 06:37

You can see Truthy and Falsy Values here

The following values are always falsy:

false

0 (zero)

"" (empty string)

null

undefined

NaN (a special Number value meaning Not-a-Number!)

All other values are truthy, including "0" (zero in quotes), "false" (false in quotes), empty functions, empty arrays, and empty objects.

score 0 · Answer 4 · answered Apr 07 '14 at 06:38

As it was mentioned only 0 (considering numbers) is equivalent to zero. But yes there is list of things which are equal to false in javascript and those are:

false
null
undefined
the empty string ""
the number 0
the number NaN

everything else when comapred to false returns false. e.g. -1 == false -> false

if(negetive number) is true? Is something wrong with js?

4 Answers4

Linked