How can I check for “undefined” in JavaScript? [duplicate]

Answer 1

On the contrary of @Thomas Eding answer:

If I forget to declare myVar in my code, then I'll get myVar is not defined.

Let's take a real example:

I've a variable name, but I am not sure if it is declared somewhere or not.

Then @Anurag's answer will help:

var myVariableToCheck = 'myVar';
if (window[myVariableToCheck] === undefined)
    console.log("Not declared or declared, but undefined.");

// Or you can check it directly 
if (window['myVar'] === undefined) 
    console.log("Not declared or declared, but undefined.");