how to check if something is undefined in javascript code example

Example 1: js if not undefined

if (typeof myVar !== "undefined") {
    console.log("myVar is DEFINED");
}

Example 2: javascript check for undefined

if (typeof myVariable === 'undefined'){
    //myVariable is undefined
}

Example 3: javascript if undefined

if(typeof comment === 'undefined') {

  alert('Variable "comment" is undefined.');

} else if(comment === null){

  alert('Variable "comment" is null.');

}

Example 4: js check if undefined

let foo = undefined
//will return true
typeof foo === 'undefined'

Example 5: nodejs check if variable is undefined

if ( typeof query !== 'undefined' && query )
{
  //do stuff if query is defined and not null
}
else
{

}

Example 6: undefined javascript check

if(undefinedElement === null) {
	console.log("Element is undefined");
}