Monkey Raptor

This already has an answer on Stack Overflow. It's using typeof operator.
As such: if (typeof your_variable === "undefined") { // Do fallback method } else { // Do your default method }

Chrome developer console screenshot

In this tests, there's an undeclared variable called rey

The recommended method

On JSLint, this method throws error message.

This message:

Unexpected 'typeof'. Use '===' to compare directly with undefined.

To fix that, simply declare the gonna-be-initialized-later variable as a var. Then compare that variable directly with the undefined property using triple equals (===). Or using Boolean negation.

Like so: var your_variable; if (your_variable === undefined) { // or use Boolean negation: !your_variable // Do fallback method } else { // Do your default method }

On Chrome console:

Links

• Discussion on Stack Overflow.
• The JavaScript quality tool: JSLint.
• The undefined primitive on MDN.