typeof !== "undefined" vs. != null

typeof is safer as it allows the identifier to never have been declared before:

if(typeof neverDeclared === "undefined") // no errorsif(neverDeclared === null) // throws ReferenceError: neverDeclared is not defined

javascript coding-style

If the variable is declared (either with the var keyword, as a function argument, or as a global variable), I think the best way to do it is:

if (my_variable === undefined)

jQuery does it, so it's good enough for me :-)

Otherwise, you'll have to use typeof to avoid a ReferenceError.

If you expect undefined to be redefined, you could wrap your code like this:

(function(undefined){    // undefined is now what it's supposed to be})();

Or obtain it via the void operator:

const undefined = void 0;// also safe

javascript coding-style

good way:

if(typeof neverDeclared == "undefined") //no errors

But the best looking way is to check via :

if(typeof neverDeclared === typeof undefined) //also no errors and no strings

CodeHunter