You can only override a known function by setting another function in its place (e.g., you can't intercept ALL function calls):

(function () {
    // An anonymous function wrapper helps you keep oldSomeFunction private
    var oldSomeFunction = someFunction;

    someFunction = function () {
        alert("intercepted!");
        oldSomeFunction();
    }
})();

You can only override a known function by setting another function in its place (e.g., you can't intercept ALL function calls):

(function () {
    // An anonymous function wrapper helps you keep oldSomeFunction private
    var oldSomeFunction = someFunction;

    someFunction = function () {
        alert("intercepted!");
        oldSomeFunction();
    }
})();

Note that, if someFunction was already aliased/referenced by another script before it was changed by this code, those references would still point to the original function not be overridden by the replacement function.