Javascript code semantics

Question

if(myVar = img.parent('a').length > 0){
    var Y = 1;
}else{
    var Y = 2;
}

When I run this code myVar (being announced for the first time) takes the value of img.parent('a').length > 0 and becomes either false or true depending on the case.

First Question:
Is this a correct way of defining myVar?

Second Question:
Am I defining Y for the second time? Is my second 'var' excess?
i.e. should i just write Y = 2;

Answer 1

First question: IMO, using an assignment on the condition of the if statement, might cause confusion, also if myVar is not declared previously with the var statement it might become a global variable.

Second question: No, you aren't re-declaring Y a second time, actually Y is defined before any assignment, it is hoisted to the top of its enclosing scope.

This is how actually var behaves in your code:

var Y; // declared and initialized with `undefined`

if (myVar = img.parent('a').length > 0) {
  Y = 1; // assignment
} else {
  Y = 2; // assignment
}

You can observe this behavior with the following example:

var Y = 'foo';
(function () {
  alert(Y); //alerts `undefined`
  var Y;
})();

As you see, the alert is before the var declaration in that function, but since the var statement is hoisted, the Y variable from this new scope is set up before the execution, when the Variable Instantiation process takes place.

The most straight forward approach, would be to declare and assign myVar:

var Y, myVar = img.parent('a').length > 0;

if (myVar) {
  Y = 1;
} else {
  Y = 2;
}
// Or Y = myVar ? 1 : 2;

Or even shorter, in a single var statement:

var myVar = img.parent('a').length > 0,
    Y = myVar ? 1 : 2;
//...