js not defined check code example

Example 1: checking if var is not defined js

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

Example 2: not defined javascript

/* Means something was not defined for example you wanted to print */ "X"
/* but X is not defined to defined you have to make a variable for it */
X = "Something" /* Once you define it it'll print the value successfully */

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

if (query){
   doStuff();
}