Declare variable in if and use outside

Question

Consider the following code:

var factory = function(someCondition) {
    var para1 = 'some value';
    if(someCondition)
        var para2 = 'some other value';

    return new MyClass(para1, para2);
}

I know that this code is perfectly legal, even though para2 is declared inside the if but used outside.

My question is: Is this - although legal - considered bad style by major Javascript style guides? If so, by which and what are the proposed alternatives?

Just to be clear: I know about hoisting and the fact that variables are not block scoped but function scoped.

Answer 1

Technically the variable is "hoisted", the code will execute like this:

var para1 = 'some value';
var para2;
if(someCondition)
    para2 = 'some other value';

All var declarations inside a scope declare variables in this scope before code is executed. As such, it doesn't make much of a difference in practice. However, I'd consider it good style to write the code explicitly as above to make it obvious what variables exist in the scope. "Hiding" variable declarations inside conditions can lead to confusion.

Answer 2

That should not be a bad style, this could be a bit confusing.

Any way javascript will do it like bellow because of hoisting, So better write also like bellow.

var factory = function(someCondition) {
    var para1 = 'some value';
    var para2; //declare it here
    if(someCondition)
        para2 = 'some other value'; //assign the value here

    return new MyClass(para1, para2);
}

The ES6 version introducing a new keyword let which gives variables block scoping.