JSTests/microbenchmarks/polymorphic-structure.js

function foo(o) {
    return o.f;
}

var o = {f:42};
var result = 0;
for (var i = 0; i < 500; ++i) {
    for (var j = 0; j < 10000; ++j)
        result += foo(o);
    if (o.g)
        o = {f:42};
    else
        o.g = 43;
}

if (result != 210000000)
    throw "Bad result: " + result;