You've probably seen or heard about this already (it's been quite Twitter-friendly).
Our target string will be
We'll be using our six superhero characters this way: with
 we can create arrays, with the negation and addition operators
+ we can do operations on them and finally use
() to group operations.
Let's start off with a simple array:
Negating an array with
! will coerce the array into a boolean. Arrays are considered to be truthy, so negating it produces
! === false
In the expression
true to a number, resulting in the expression
2 + 1.
In the expression
2 + "2" === "22".
These conversion rules aren't that bad, but it gets quite interesting quite quickly with other types.
Fun with arrays
Adding arrays together will convert them both to strings and concatenate them.
 +  === "" + "" === ""
The same will happen when adding an array with something else, lets say, a boolean.
! +  === "false" + "" === "false"
Aha! Now we're conjured a string containing the characters we need to produce our end goal, the string
If we only had a number or four, we could extract the characters we need in the right sequence..
"false" === "s" (! + ) === "s"
Let's go looking for numbers!
Fun with arrays part II
In the previous section, we coerced an array to a boolean.
What happens if we coerce it to a number using
+ === ???
valueOf on the array, which will fail and fall back to a simple
+ === +"" == 0
Coercing a string to a number will produce the following results:
+"42" === 42 +"esg" == NaN +"" === 0
+ === +"" === 0
Aha! Now we have a number, albeit not a very useful one. However, we can keep playing the coercion game:
!0 === true
Negating zero will make the falsy value of zero into a truthy boolean. A truthy boolean as a number becomes... one!
+true === 1
Hooray! One one can become two ones... which is two. You get the point.
Using the substitutions we've learned to create numbers:
1 === +true == +(!0) ==== +(!(+)) === +!+ +!+ +!+ === 2
Putting it all together
Let's put together all the things we've learned.
! +  === "false" +!+ === 1 (! + ) + (! + ) + (! + ) + (! + ) ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ "false" "false" "false" "false" ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ s e l f
Our final expression thus becomes:
(! + )[+!++!++!+] + (! + )[+!++!++!++!+] + (! + )[+!++!+] + (! + )[+]
Easy as pie!
Thank you, Brendan Eich ❤️