Question

I have a function like so

function foo(x){
    if (typeof x === 'undefined'){
        var x = 123;
    }
}

is the var statement necessary? JSlint complains that variable x hides argument (probably b/c I am defining a variable in the scope of the if statement.

Answer 1

The var is not necessary, and in fact it is a mistake. You should use var to declare a new variable. Once the function has an argument x it is declared – whether it is passed a value or not.

By the way, in such cases when you know the variable is declared but just don’t know whether it’s been assigned a value or not, you can write x === undefined – using typeof and a string comparison is not necessary.