How to handle 'undefined' in JavaScript [duplicate]

Answer 1

In JavaScript, the following values will cause the if condition to fail and not execute its statement: null, undefined, false, NaN, the number 0, and the empty string ''.

Assuming that the variable jsVar is a boolean and that we want to call the proceed() method when jsVar is true, we can do the following check.

if (jsVar && jsVar == true)
    proceed();

The above code snippet first check that jsVar has been defined and then checks that its value is true. The if condition will be satisfied only if both the conditions are met.

If jsVar is not a boolean then we can substitute the appropriate check in place of jsVar == true in the code above.

Answer 2

You can check the fact with

if (typeof(jsVar) == 'undefined') {
  ...
}

Answer 3

As is often the case with JavaScript, there are multiple ways to do this:

typeof foo !== 'undefined'
window.foo !== undefined
'foo' in window

The first two should be equivalent (as long as foo isn't shadowed by a local variable), whereas the last one will return true if the global varible is defined, but not initialized (or explicitly set to undefined).