I don't understand how this works. I've seen brainfuck before, but not truly understood it. How is it possible to open up a new tab, paste the encoded form, and have working JavaScript?
Brainfuck is an unrelated, separate programming language.
With this I was able to get the letter 'a' by starting with their initial definition `(false+"")[1]`, then continually substituting
(false+'')[1]
(![]+'')[1]
(![]+([]+[]))[1]
(![]+([]+[]))[+!(+[])]
All of the above lines when evaluated result in 'a'.
1. [] => An array object
2. []["flat"] => The built in flat function on the Array object
3. []["flat"]["constructor"] => The constructor of any JS function is the Function constructor, which can take JavaScript, as a string, as its sole parameter. Thus, to create a function that executes any CODE_AS_STRING:
4. []["flat"]["constructor"](CODE_AS_STRING) => gives you the Function object, and finally:
5. []["flat"]["constructor"](CODE_AS_STRING)() => executes it
Thus, all that's left is to encode the strings "flat", "constructor" and CODE_AS_STRING using the patterns described early in that file.
There are some tasks which lend themselves to the way the language is constructed, and can be done with few instructions.
Reversing a string, for example, is just ,[[<]<+>>[>],]<[.<] which is quite short compared to most "practical" programs in this language.
And a destructive y = y + x is just [->+<] (destructive because this also sets x to zero)