if object is undefined javascript code example

Example 1: how to check if object is undefined in javascript

if (typeof something === "undefined") {
    alert("something is undefined");
}

Example 2: test if is undefined javascript

let id;

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

Example 3: javascript check if boolean is undefined

if (typeof myFlag !== "undefined") {
  // comes here whether myFlag is true or false but not defined
}