Which is the better way to check undefined in Javascript?

Question

I had been checking for undefined values myself like:

if(variable !== 'undefined')

then I came across the following code:

if(typeof variable1 !== 'undefined' || typeof variable2 !== 'undefined')

I followed the same convention knowing it's safer when variable goes undeclared. While in my code reviews, it was pointed I could simple write:

if(variable1 || variable2)

Which of these is the most standard way to check undefined?

Are you talking about _undeclared_ variables or the value `undefined`? Because these approaches aren't interchangeable if you're asking about the former — Xetera, Jan 03 '20 at 05:59
never pass undefined variables to your functions by using functional programming versus procedural programming. — mike510a, Jan 03 '20 at 06:01
`if(variable !== 'undefined')` will *never* work, because `'undefined'` the string is not the same as the primitive value `undefined`. `if(variable1)` works if the variable, if defined, will always be truthy — CertainPerformance, Jan 03 '20 at 06:03

score 2 · Accepted Answer · answered Jan 03 '20 at 06:00

There are various use case as following

if (variable) is standard way to check truthiness of any variable in javascript. You can examples of what values will be truthy on Truthy | MDN Web Docs. Also, Falsy | MDN Docs
The cases where you explicitly would check for undefined is when a variable has been declared but not assigned value or explicitly assigned undefined. In that case use if (variable !== undefined).
If you are receiving response from an API which might consist of stringified value of undefined, which you are sure of, then only do the check if (variable !== 'undefined')

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