蛋疼....

root@test:~# node test.js
100
300
true
100
true
100

root@test:~# cat test.js 
function Test(){
}
Test.prototype.v = 100;

var t = new Test();
console.log(t.v);
t.v = 300;

console.log(t.v);
console.log(delete t.v);
console.log(t.v);
console.log(delete t.v);
console.log(t.v);

delete operator

If a property with the same name exists on the object's prototype chain, then, after deletion, the object will use the property from the prototype chain (in other words, delete only has an effect on own properties).