Question

For detect undefined variable which have been declared in javascript there at least two different ways.

var bar; // declared variable

if (typeof bar !== "undefined") {
     // make something with bar
}

if (bar) {
     // make something with bar
}

My Questions:

1) Are the two ways equal in all the aspects?

2) If not what is the best?

Answer 1

if (bar) { will return false for many cases other than an undefined variable.

For instance, if bar is 0 then if(bar) will evaluate to false. bar having a value of false will also evaluate to false.