JavaScript null check -

i've come across following code:

function (data) {     if (data != null && data !== undefined) {         // code here     } } 

i'm new javascript, but, other questions i've been reading here, i'm under impression code not make sense.

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in particular, this answer states

you'll error if access undefined variable in context other typeof.

update: (quote of the) answer above may misleading. should «an undeclared variable», instead of «an undefined variable».

as found out, in answers ryan ♦, maerics, , nwellnhof, when no arguments provided function, variables arguments declared. fact proves wrong first item in list below.

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from understanding, following scenarios may experienced:

• the function called no arguments, making data undefined variable, , raising error on data != null.

• the function called null (or undefined), argument, in case data != null protects inner code, rendering && data !== undefined useless.

• the function called non-null argument, in case trivially pass both data != null and data !== undefined.

q: understanding correct?

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i've tried following, in firefox's console:

-- [15:31:31.057] false != null [15:31:31.061] true -- [15:31:37.985] false !== undefined [15:31:37.989] true -- [15:32:59.934] null != null [15:32:59.937] false -- [15:33:05.221] undefined != null [15:33:05.225] false -- [15:35:12.231] "" != null [15:35:12.235] true -- [15:35:19.214] "" !== undefined [15:35:19.218] true 

i can't figure out case data !== undefined after data != null might of use.

an “undefined variable” different value undefined.

an undefined variable:

var a; alert(b); // referenceerror: b not defined 

a variable value undefined:

var a; alert(a); // alerts “undefined” 

when function takes argument, argument declared if value undefined, , there won’t error. right != null followed !== undefined being useless, though.