toString can be set in JavaScript

A while ago, I wrote that you can use .toString() to reflect about user-defined functions in the JavaScript runtime. There’s a catch— this value can be set, making it not a reliable way to reflect the function.

Create a confusing function:

const one = () => '2';

Set it’s .toString to another function:

one.toString = () => '1';
one.toString(); // '1'

Now call the function:

one(); // 2

.toString() was a lie!