Why can I do 'if (a = 7) ...' but not 'if (var a = 7) ...' ?

Question

Title says it all. Why does

if (prev = this.Prev()) {
    ...
}

work but

if (var prev = this.Prev()) {
    ...
}

does not? this.Prev() is a method for a Point object which returns a reference to a previous Point if it exists, and false if it does not. I don't want to declare the variable to be global, and I don't want something verbose like:

var prev = this.Prev();
if (prev) {
    ...
}

EDIT: What's the most elegant way to do something like what I am trying?

Answer 1

This happens because the if statement expects a expression:

Syntax:

IfStatement :
    if ( Expression ) Statement else Statement 

var is a statement that's why you get a SyntaxError.

Your first example works because an assignment is a expression (AssignmentExpression)

Edit: Let me quote this part:

I don't want to declare the variable to be global

I understand your concern, an assignment made to an undeclared identifier may end up creating a property on the global object, moreover with the ECMAScript 5th Strict Mode, an undeclared assignment will cause a ReferenceError, breaking your code

Variables in JavaScript are declared before the actual code execution, all occurrences of the var statement are bound to the current Variable Object, and they are initialized with undefined, you can't really declare a variable conditionally.

Answer 2

C'est la vie. JavaScript does not let you declare variables there.

(It is a boolean expression and not a statement)

Answer 3

This is because you can't have full keyword-prefixed statements used as expressions. Some statements, like those that aren't prefixed by keywords, can be readily used as expressions without fault.

Answer 4

You say "I don't want to declare the variable to be global". However, keep in mind that JavaScript does not have block scope, but only function scope. Therefore that would not have allowed you to change the scope of the variable, even if it was possible (which is not, as @CMS pointed out).

Quoting Douglas Crockford:

JavaScript does not have block scope, so defining variables in blocks can confuse programmers who are experienced with other C family languages. Define all variables at the top of the function.