1if (typeof myVar !== "undefined") {
2    console.log("myVar is DEFINED");
3}

1if (typeof myVariable === 'undefined'){
2    //myVariable is undefined
3}

1if(typeof x !== 'undefined'){
2  alert("Variable x is defined.");
3}

1let id;
2
3if(typeof id === 'undefined') {
4    console.log("id is undefined...");
5}

1if ( typeof query !== 'undefined' && query )
2{
3  //do stuff if query is defined and not null
4}
5else
6{
7
8}
9

1let foo = undefined
2//will return true
3typeof foo === 'undefined'