how to check if var is undefined in javascript code example

Example 1: javascript if undefined

if(typeof comment === 'undefined') {

  alert('Variable "comment" is undefined.');

} else if(comment === null){

  alert('Variable "comment" is null.');

}

Example 2: js check if undefined

let foo = undefined
//will return true
typeof foo === 'undefined'

Example 3: javascript check undefined

if(myVar === null) {
	console.log("Element is undefined");
}

Example 4: javascript check if undefined or null

// simple check do the job
if (myVar) {
 // comes here either myVar is not null,
 // or myVar is not undefined,
 // or myVar is not '' (empty string).
}