Проверка JavaScript для неопределенного
if (typeof myVariable === 'undefined'){
//myVariable is undefined
}
Grepper
if (typeof myVariable === 'undefined'){
//myVariable is undefined
}if (typeof myVar !== "undefined") {
console.log("myVar is DEFINED");
}let myVar;
if (myVar === undefined){}
//!! Note: myVar == undefined would also check wether myVar is null
//alternatively
if (typeof myVar === 'undefined'){ }if(typeof comment === 'undefined') {
alert('Variable "comment" is undefined.');
} else if(comment === null){
alert('Variable "comment" is null.');
}let id;
if(typeof id === 'undefined') {
console.log("id is undefined...");
}if (typeof something != "undefined") {
// ...
}