js check if variable is not defined code example

Example 1: checking if var is not defined js

if(typeof x !== 'undefined'){
  alert("Variable x is defined.");
}

Example 2: javascript set variable if not defined

var x = (typeof x === 'undefined') ? your_default_value : x;

Example 3: How can I check whether a variable is defined in Node Js

if (query){
   doStuff();
}