There are some things we don’t know about how the ! operator works in Javascript.
Basics
The ! is a logical operator that will convert a value to its opposite boolean. Since JavaScript will coerce values, it will “convert” a value to its truthy/falsey form and return the opposite boolean value.
When we perform the ! operation on a number other than 0 it, returns false. This is because all numbers other than 0 are truthy.
// evaluates to false
!1
!-10
!1.2
The ! operation on 0, null, undefined, NaN returns true since they are falsey.
//evaluates to true
!0
!NaN
!null
!undefined
If we perform ! on undefined variables then it returns true.
var a ;
!a // true
Similarly, if we perform ! on a variable which has some value or points to some a reference then it returns false.
var a = [];
!a; // false
The weird part
When we do:
! + [] // then it is evaluated to true
The reason is, the + operator only works on strings and numbers, so if we perform a binary operation on an array, then the array is converted to a string. So the detailed view is:
! + []
is converted internally into
! + String([])
! + "" //it evaluates true because ! on empty string returns true
So now let’s try another large problem (which I found on Twitter, the solution also available there 😃)
(! + [] + [] + [] + ![])
! + [] --> true
true + [] --> "true"
"true" +[] --> "true"
"true" + ![] -- >"truefalse"
If you want to learn more about these concepts, you should study JavaScript type coercion and how truthy/falsey values work in JS.
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