Yep, in addition the other part that helped me understand was how it encodes eval for actually executing code. Details are on this line, https://github.com/aemkei/jsfuck/blob/1f02651e98da1c13aef76d... , and details of that are:

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.