remove undefined from array
//? to use Array.prototype.filter here might be obvious.
//So to remove only undefined values we could call
var data = [42, 21, undefined, 50, 40, undefined, 9];
data = data.filter(function( element ) {
return element !== undefined;
});
//If we want to filter out all the falsy values (such as 0 or null),
//we can use return !!element; instead. But we can do it slighty more elegant,
//by just passing the Boolean constructor function,
//respectively the Number constructor function to .filter:
data = data.filter( Number );
//That would do the job in this instance,
//to generally remove any falsy value,
//we would call
data = data.filter( Boolean );
//Since the Boolean() constructor returns true on truthy values
//and false on any falsy value, this is a very neat option.