javascript test for existence of nested object key

If I have a reference to an object –

var test = {};

that will potentially (but not immediately) have nested objects, something like –

{ level1:{level2:{level3:'level3'}} };

what is the best way to test for the existence of keys in the most deeply nested objects?

This –

alert(test.level1);

returns ‘undefined’, but this –

alert(test.level1.level2.level3);

fails.

I’m currently doing…