Question

Here i am just creating a global namespace for my application and everything works as expected

var MYAPP = MYAPP || {} ; 

but if I omit var keyword ( i know it’s not the right way), javascript throw an error “ReferenceError: MYAPP2 is not defined” .

MYAPP2 = MYAPP2 || {};

just out of curiosity can some one explain me in second case why javascript is not able to resolve the reference.

Answer 1

The first version doesn’t produce an error because Javascript’s variable hoisting makes it equivalent to this:

var MYAPP;            // declares, but leaves the value undefined / unchanged
MYAPP = MYAPP || {};  // creates, if the value was previously undefined

Importantly, the declaration part of this does not overwrite any existing declaration or assignment that was already made to the variable.

This is what allows this pattern to be used repeatedly within a single scope. MYAPP either retains the value it already had, or is initialised to an empty object.

in the second case, that declaration step is effectively omitted. If the variable has not already been declared (or otherwise exists in scope, i.e. as a property of the global object) then the error you see is generated.