Question

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.