js check if undefined
if (typeof myVar !== "undefined") {
console.log("myVar is DEFINED");
}if (typeof myVariable === 'undefined'){
//myVariable is undefined
}if(myVar === null) {
console.log("Element is undefined");
}if(typeof comment === 'undefined') {
alert('Variable "comment" is undefined.');
} else if(comment === null){
alert('Variable "comment" is null.');
}if (typeof something === "undefined") {
alert("something is undefined");
}let foo = undefined
//will return true
typeof foo === 'undefined'
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