src/third_party/mozjs-45/js/src/jit-test/tests/basic/testDecElem2.js

var obj = {s: ""};
var name = "s";
var a = [];
for (var i = 0; i <= 13; i++) {
    a[i] = 'x';
    if (i > 8)
        a[i] = --obj[name];  // first recording changes obj.s from string to number
}
assertEq(a.join(','), Array(10).join('x,') + '-1,-2,-3,-4,-5');
assertEq(obj.s, -5);