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Javascript provides two very handy operators, in and delete. Consider this code fragment: var obj = { foo : 'quux', bar : 'baz', doSomething : function () { // do something... } }; So, we’ve created an object, and then some other things happen. Later in the code, we want to remove the “foo” property, so we do this: obj.foo = undefined; Still later, we only want to do something if the foo property has not been unset, so we check it: if ( obj.foo !== undefined ) { // do something } That wo...