javascript - Interpret a cumbersome conditional expression -

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last night looking angularjs snippet manage vertical tabs menu , found expression:

if(+!!variablename)   dosomething();

sorry, can't understand mean +!!variablename how can resolve conditional expression?

this 1 way of saying if number if not null or undefined

if(+!!variablename)   dosomething();

if variablename 0, empty (""), null or undefined, translate false or else true.

for example, if

var variablename = null;

now, translate (+!!null) -> (+!true) -> (+false) -> false

similarly, if 

var variablename = "anything";

this translate (+!!"anything") -> (+!false) -> (+true) -> true

+ in case has no effect on outcome, can safely removed.

in fact, whole conditional expression can replaced by

if(variablename)   dosomething();

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