if(myVar === null) { console.log("Element is undefined"); }
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.'); }
//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"); }
let foo = undefined //will return true typeof foo === 'undefined'
OAS3