JavaScript: Checking Undefined Variable

This already has an answer on Stack Overflow. It's using typeof operator.

As such:

if (typeof yourVariable === "undefined") { // Do fallback method. } else { // Do your default method. }

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Chrome Developer Console Snapshot

In this test, there's an undeclared variable called rey.

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The Other Method

On JSLint, that method above will thrown an error message.

This message:

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

To fix that, simply declare the uninitialised variable with var (or let). Then compare that variable directly with undefined using triple equal (===).

Like so:

var yourVariable; if (yourVariable === undefined) { // Do fallback method } else { // Do your default method }

Snapshot from Chrome console:

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