Question

I’m looking at the source code for this http://raphaeljs.com/australia.html example, and trying to interpret what has been done in order to create a similar project using the RaphaelJS graphics library.

I am, however, confused about part of the code. Specifically, the author uses a parameter “st” in a function inside a for loop, that has not been defined before, while the second parameter “state” has. I’m not sure what I’m missing, but can someone please explain what’s going on here? Is this a generic parameter or a call to something specific?

for (var state in aus) {
            aus[state].color = Raphael.getColor();
            (function (st, state) {
                st[0].style.cursor = "pointer";
                st[0].onmouseover = function () {
                    current && aus[current].animate({fill: "#333", stroke: "#666"}, 500) && (document.getElementById(current).style.display = "");
                    st.animate({fill: st.color, stroke: "#ccc"}, 500);
                    st.toFront();
                    R.safari();
                    document.getElementById(state).style.display = "block";
                    current = state;
                };
                st[0].onmouseout = function () {
                    st.animate({fill: "#333", stroke: "#666"}, 500);
                    st.toFront();
                    R.safari();
                };
                if (state == "nsw") {
                    st[0].onmouseover();
                }
            })(aus[state], state);
        }

Answer 1

st is a named parameter of the surrounding closure:

(function (st, state) {
    // ...
})(aus[state], state);

This is known as an Immediately-Invoked Function Expression (often called Self-Executing Block or a Temporary Scope) used to “save” a state by extracting code from the surrounding context.

In order to introduce variables from outside the closure, one can pass them to the trailing parentheses as arguments (here aus[state], state), and name them in the function’s signature (here st, state).

References

• Temporary Scope and Closures and Looping on John Resig’s Learning Advanced JavaScript interactive tutorials
• Closures on Mozilla Developer Network

Further Reading

• Immediately-Invoked Function Expression by Ben Alman, on his blog