check if not defined javascript code example

Example 1: js if not undefined

if (typeof myVar !== "undefined") {
    console.log("myVar is DEFINED");
}

Example 2: checking if var is not defined js

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

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

if (query){
   doStuff();
}