javascript if undefined
if (typeof myVar !== "undefined") {
console.log("myVar is DEFINED");
}if (typeof myVariable === 'undefined'){
//myVariable is undefined
}if(typeof comment === 'undefined') {
alert('Variable "comment" is undefined.');
} else if(comment === null){
alert('Variable "comment" is null.');
}let foo = undefined
//will return true
typeof foo === 'undefined'//detecting undefined object property
if (typeof person.last_name === "undefined") {
console.log("person last_name is undefined");
}
//detecting undefined variable
if (typeof myVar === "undefined") {
console.log("myVar is undefined");
}var x; //variable to change
var y; //higher priority variable
var z = 12; //lower priority variable
x = !(y === undefined) ? y : z;
console.log(x); //12