spot-the-error.js

// Consider a function:
function multiply(a, b) {
    return a*b;
}

print(multiply(Math.PI, Math.exp(1)))


// We want to replace it with an "enhanced" version that does some
// stuff before invoking the original, e.g. generate debugging output
function newMultiply(a, b) {
    a = Math.round(a);
    b = Math.round(b);
    print("DEBUG: round product = " + a*b);

    // Invoke original exactly like we were invoked
    return oldMultiply.apply(this, arguments);
}

// "Patch" in the new function
var oldMultiply = multiply;
multiply = newMultiply;

// Now the same line from above:
print(multiply(Math.PI, Math.exp(1)));