how oto check if a variable is undefined in js code example

Example 1: javascript check if boolean is undefined

if (typeof myFlag !== "undefined") {
  // comes here whether myFlag is true or false but not defined
}

Example 2: 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).
}