javascript check if variable undefined code example

Example 1: javascript check for undefined

if (typeof myVariable === 'undefined'){
    //myVariable is undefined
}

Example 2: test if is undefined javascript

let id;

if(typeof id === 'undefined') {
    console.log("id is undefined...");
}

Example 3: checking if var is not defined js

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

Example 4: How can I check whether a variable is defined in Node Js

if (query){
   doStuff();
}