https://eloquentjavascript.net/12_language.html#i_Y9ZDMshYCQ

Eloquent JavaScript 1204 Fixing scope.js

specialForms.set = (args, scope) => {
  if (args.length !== 2 || args[0].type !== "word") {
  	throw new Error("Incorrect use of set.");
  }

  const value = evaluate(args[1], scope);
  
  
  for(let outerScope = scope; scope; outerScope = Object.getPrototypeOf(outerScope)) {
    if (Object.prototype.hasOwnProperty.call(outerScope, args[0].name)) {
      // Found the binding at the level
      outerScope[args[0].name] = value;
      return value;
    }
    
    // Binding not found until the topScope
    throw new ReferenceError("Binding not exist.");
};

run(`
do(define(x, 4),
   define(setx, fun(val, set(x, val))),
   setx(50),
   print(x))
`);
// → 50
run(`set(quux, true)`);
// → Some kind of ReferenceError