Golfing is the act art of making source code smaller, usually by abusing the language in some way.
Start by typing in some code below (example taken from here):
First we make sure that the string has an even number of characters and add a space to the end if necessary. For each character, we obtain its code:
Then we merge adjacent chars into pairs, each forming a single hex value:
The next step is prefixing each pair with `%u`, constructing a string of escaped characters:
For a total of characters (compare with the characters of your input). Notice that template literals are abused here, taking advantage of the fact that the language doesn't complain about missing parentheses when we pass them as parameter. Also notice that your input string has to be big enough to compensate for the extra code needed to decode the resulting golfed string. That basically means your input has to be at least 96 characters long for the hack to be effective.
To understand how decoding happens, just read it from inside out. Start by escape()ing your golfed string (spaces added for readability):
You will see our pairs back. The trick now is to break them into 8-bit chars again. That's what the `replace() part does. It replaces every `%uXXYY` with `%XX%YY` in a very hacky way with a very short regular expression (again, spaces added for readability):
Then unescape() comes into play, converting those hex values back into characters:
And we're done! We have our original source code back and ready to be eval()uated :-)