check variable is defined or not in javascript code example

Example 1: javascript check if variable exists

if (typeof myVar !== 'undefined') {
    // 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();
}