Querying for non existent property doesn't always throw error

Question

Why when executing this piece of code, only checking for thisThrowsError actually throws error, but querying for non existent property on the prototype object or on this context doesn't?

function Test()
{

    if(this.someVar){}
    if(Test.prototype.someVar){}  
    if(thisThrowsError){}
 }

var test = new Test();

Answer 1

请参阅： 为什么Javascript中的未定义变量有时会评估为false，有时会抛出未捕获的ReferenceError？