What does void 0 mean

Question

Reading through the Backbone js source code  I saw this   validObj attr    void 0    What is void 0  What is the purpose of using it here

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void is a reserved JavaScript keyword  It evaluates the expression and always returns undefined

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What does void 0 mean  void MDN  is a prefix keyword that takes one argument and always returns undefined  Examples void 0 void  0  void  quot hello quot  void  new Date      all will return undefined  What s the point of that  It seems pretty useless  doesn t it  If it always returns undefined  what s wrong with just using undefined itself  In a perfect world we would be able to safely just use undefined  it s much simpler and easier to understand than void 0  But in case you ve never noticed before  this isn t a perfect world  especially when it comes to Javascript  The problem with using undefined was that undefined is not a reserved word  it is actually a property of the global object  wtfjs    That is  undefined is a permissible variable name  so you could assign a new value to it at your own caprice  alert undefined     alerts  quot undefined quot  var undefined    quot new value quot   alert undefined     alerts  quot new value quot   Note  This is no longer a problem in any environment that supports ECMAScript 5 or newer  i e  in practice everywhere but IE 8   which defines the undefined property of the global object as read-only  so it is only possible to shadow the variable in your own local scope   However  this information is still useful for backwards-compatibility purposes  alert window hasOwnProperty  undefined        alerts  quot true quot  alert window undefined      alerts  quot undefined quot  alert undefined     window undefined      alerts  quot true quot  var undefined    quot new value quot   alert undefined      alerts  quot new value quot  alert undefined     window undefined      alerts  quot false quot   void  on the other hand  cannot be overidden  void 0 will always return undefined  undefined  on the other hand  can be whatever Mr  Javascript decides he wants it to be  Why void 0  specifically  Why should we use void 0  What s so special about 0  Couldn t we just as easily use 1  or 42  or 1000000 or  quot Hello  world  quot   And the answer is  yes  we could  and it would work just as well  The only benefit of passing in 0 instead of some other argument is that 0 is short and idiomatic  Why is this still relevant  Although undefined can generally be trusted in modern JavaScript environments  there is one trivial advantage of void 0  it s shorter  The difference is not enough to worry about when writing code but it can add up enough over large code bases that most code minifiers replace undefined with void 0 to reduce the number of bytes sent to the browser

User · Answer

void 0 returns undefined and can not be overwritten while undefined can be overwritten    var undefined    HAHA

[javascript] What does `void 0` mean?

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