What's the difference between variable definition and declaration in JavaScript?

Question

Is this a variable definition or declaration? And why?

var x;

..and is the memory reserved for x after this st

Answer 1

> var x;
undefined
> x
undefined // now it looks like x is defined to the value undefined
> y
ReferenceError: y is not defined

Although it is usually said that Javascript is an interpreted language, but there is also a compilation step that happens very fast just before the interpreter runs. The job of this compilation step is to create scope chains, where variables are declared(no read/write operation here, just simple name-keeping) in their respective scopes. These variables will point to some memory location but value in it will be undefined until some execution is carried out by the interpreter.

> Compiler run:

When compiler sees var x;, it will simply book-keep this variable in its respective scope.

The next x; and y; are simply ignored in the compilation step as they are execution statements.

> Interpreter run:

When interpreter sees var x;, it will skip this as there is no read/write operation here.

Now when interpreter sees x;(execution statement), "x" will already be declared in the scope, and it will hold value "undefined", which is what you get on the console.

But when interpreter sees y; similarly, there has been no previous declaration or name-keeping for it in the compilation step, and thus we get the ReferenceError as expected.

Hope someone finds this comment useful.