Can JavaScript Rewrite Itself? A Self‑Modifying Code Demo for Thanksgiving

Twenty‑five years ago the author started software development and met Dave, a former insurance‑company programmer who maintained the backend for a personal life‑insurance product.

What Is Self‑Modifying Code?

Self‑modifying code is code that alters its own instructions at runtime, often to reduce instruction length, improve performance, or simplify maintenance by eliminating redundancy. Historically it was used to maximize memory usage on machines with very limited RAM.

JavaScript Example

Inspired by this concept, the author wrote a JavaScript function that rewrites itself on Thanksgiving (November 23, 2017). The function checks the current date and, if it matches Thanksgiving, redefines itself to log “吃火鸡” ("eat turkey"); otherwise it logs “去工作” ("go to work").

selfModifyingCode = function(){
    var turkeyDay = new Date("November 23, 2017 01:00:00");
    var date = new Date();
    if (date.getDate() === turkeyDay.getDate()){
        selfModifyingCode = function(){
            console.log("吃火鸡");
        };
    } else {
        selfModifyingCode = function(){
            console.log("去工作");
        };
    }
    selfModifyingCode();
};
selfModifyingCode();

When the function runs, it first creates two date variables, compares them, and then overwrites itself with a simpler version that directly logs the appropriate message. Subsequent calls therefore execute the rewritten version without re‑evaluating the dates.

Conclusion

Self‑modifying code can be an effective way to handle runtime logic decisions, but it should be used only when truly beneficial, as it can make maintenance difficult. In the era of limited memory, such techniques were sometimes necessary, but modern systems usually have better alternatives.

While the presented example is simple, many more robust approaches exist for achieving the same outcome without rewriting functions at runtime.