Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.))  if (elem) { // or !elem   or  if (typeof(elem) !== 'undefined') {   or  if (el...

Why is null considered an object in JavaScript?  Is checking   if ( object == null )       Do something   the same as   if ( !object )       Do something   ?  And also:  What is the difference between null and undefined?...

I have a global variable in JavaScript (actually a window property, but I don't think it matters) which was already populated by a previous script but I don't want another script that will run later to see its value or that it was even defi...

I wanted to check whether the variable is defined or not. For example, the following throws a not-defined error    alert( x );   How can I catch this error?...

I am trying to access this inside my arrow function:  import myObject from '../myObjectPath'; export const myClass = Fluxxor.createStore({     initialize() {         this.list = [];         this.id = null;     },     myOutsideFunction(varia...

Recently I installed node.js on my Windows 7 machine.   On execution of JavaScript, I get an undefined message along with successful execution of the expression.   What's wrong here? I have not noticed any other side effects....

Hi I have the 3 javascript files.   jquery.js utility.js file1.js   In file1.js I have   jQuery.noConflict() jQuery(document).ready(function($) {   // .... });   I get an error 'jQuery' was used before it was defined.  and 'document'...

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I'm checking if(response[0].title !== undefined), but I get the error:     Uncaught TypeError: Cannot read property 'title' of undefined....

How do I verify the existence of an object in JavaScript?  The following works:  if (!null)    alert("GOT HERE");   But this throws an Error:  if (!maybeObject)    alert("GOT HERE");   The Error:      maybeObject is not defined....