Question

How expensive are local variables (var v), global variables (window.v) and cross-global variables (parent.v) in JavaScript, in the major browsers? Has anyone performed any good tests on this one?

Answer 1

Ignoring interpreter/parser pros and cons, all that matters is how much the runtime has to look at parts of the scope chain.

Consider this simple example:

foo = 42;         // implicitly window.foo; cost: 2
var bar = 3;      // cost: 1

function woot(a) {
    a;            // cost: 1
    bar;          // cost: 2
    foo;          // cost: 3

    var other = 9;// cost: 1

    other;        // cost: 1

    a.something;  // cost: 2
    foo.win.fail.arg.func(a.something).thing = 0; // cost: 8 + 2
}

woot(bar);        // cost: 2 + 1

Remember that functions share the same namespace as variables (I think) and act like variables with regards to resolution.

There is no additional cost to using foo['bar'] instead of foo.bar, except perhaps optimizations by the Javscript engine (though if you’re optimizing the latter it should be easy to optimize the former if the contents are literal).