JavaScript check if variable exists (is defined/initialized)

Question

Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.))

Answer 1

To contribute to the debate, if I know the variable should be a string or an object I always prefer if (!variable), so checking if its falsy. This can bring to more clean code so that, for example:

if (typeof data !== "undefined" && typeof data.url === "undefined") {
    var message = 'Error receiving response';
    if (typeof data.error !== "undefined") {
        message = data.error;
    } else if (typeof data.message !== "undefined") {
        message = data.message;
    }
    alert(message); 
}

..could be reduced to:

if (data && !data.url) {
  var message = data.error || data.message || 'Error receiving response';
  alert(message)
}