Basic example of self-modifying function in JS (https://en.wikipedia.org/wiki/Self-modifying_code)

simple-self-modifying-program.js

window.fx_update = 'window.fx = function() { setTimeout(function() { window.fx_modified = false; window.fx = window.fx_source; }, 5000); return "Modified state of fx() 5s later"; };';

window.fx_source = function() {
    if (!window.fx_modified) {
        setTimeout(function() {
            eval(window.fx_update);
        }, 5000);
        
        window.fx_modified = true;
    }
  
    return "Initial state of fx()";
};

window.fx_modified = false;
window.fx = window.fx_source;

// Run
window.fx();
setInterval(function() { console.log(window.fx()) }, 1000);