Question

In javascript whats the difference between
undefined == variable and variable == undefined are both of them same? and how different will it be if i do undefined === variable or typeof variable == 'undefined'?

Can someone help me out

Answer 1

Don’t use undefined to test for an undefined variable, use the typeof operator instead!

undefined isn’t a javascript keyword, it’s just a variable name. If someone writes var undefined = true globally anywhere into the code, all your comparisons will act unexpected.

You should consider using something like JSLINT or JSHINT to avoid these type of errors in your javascript code.

Apart from that, I would always write the compared parameter first, as that’s the way I read it.
That is why If the variable foo is undefined than should be written as if (typeof foo === "undefined")

I don’t remember the name for this pattern, but I’m quite sure there is one 🙂