JS check if undeclared, undefined or null

var_check.js

//check if undeclared
try{
  undeclaredVar
}
catch(e) {
  if(e.name === 'ReferenceError') {
    console.log('var is undeclared')
  }
}

// check if undefined
var undefinedVar

if (typeof undefinedVar === 'undefined') {
   console.log('var is undefined')
}

// check if null
var nullVar = null

if (nullVar === null) {
   console.log('var is null')
}