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

You declare JavaScript variables with the var keyword: var carname;

After the declaration, the variable is empty (it has no value).

To assign a value to the variable, use the equal sign var carname="Volvo";

In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input. Variable declared without a value will have the value undefined.

The variable carname will have the value undefined after the execution of the following statement: var carname;

var hoisting

In JavaScript, a variable can be declared after being used.

 bla = 2
 var bla;
  // ...

  // is implicitly understood as:

 var bla;
 bla = 2;

For that reason, it is recommended to always declare variable at the top of functions. Otherwise, it may lead to confusing cases

When declaring a variable without assigning a value to it, there still needs to be some memory available for it, otherwise you cannot make a reference to the variable later in the program. I don't think it's a noticeable amount of memory being used and won't make a difference.