if value is undefined javascript code example

Example 1: javascript if undefined

if(typeof comment === 'undefined') {

  alert('Variable "comment" is undefined.');

} else if(comment === null){

  alert('Variable "comment" is null.');

}

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
}

Example 4: check if variable is undefined or null jquery

if (variable == null) {
    // variable is either null or undefined
}

Example 5: javascript check if undefined or null

// simple check do the job
if (myVar) {
 // comes here either myVar is not null,
 // or myVar is not undefined,
 // or myVar is not '' (empty string).
}

Example 6: How can I check whether a variable is defined in Node Js

if (query){
   doStuff();
}