javascript if undefined

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

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

1if(typeof comment === 'undefined') {
2
3  alert('Variable "comment" is undefined.');
4
5} else if(comment === null){
6
7  alert('Variable "comment" is null.');
8
9}

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

1if (variable == null) {
2    // variable is either null or undefined
3}

1var x; //variable to change
2var y; //higher priority variable
3var z = 12; //lower priority variable
4x = !(y === undefined) ? y : z;
5console.log(x); //12