Whether the JS variable or parameter has a value judgment

var node;

......

Determine if node has value, whether it is undefine, NULL, direct use of two!!, negation:

if (!! node) {

....

}else{

....

}

This condition will definitely return to True/false, previously did not know this usage, now write down for reference.

In similar cases, some JS libraries are packaged using the following structure:

!function ($) {  $ (function () {    })} (Window.jquery)

Also, when defining an execution function, return true/false, not undefined. Not detailed here, see also:

Http://stackoverflow.com/questions/10896749/what-does-function-function-window-jquery-do

Whether the JS variable or parameter has a value judgment