if (typeof value !== 'undefined' && value) {

    //deal with value'

};

if( typeof myVar === 'undefined' || myVar === null ){

    // myVar is undefined or null

}

var myVar=null;

​

if(myVar === null){

    //I am null;

}

​

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

    //myVar is undefined

}

​

// simple check do the job

if (myString) {

 // comes here either myString is not null,

 // or myString is not undefined,

 // or myString is not '' (empty).

}

var myVar=null;

​

if(myVar === null){

    //I am null;

}

​

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

    //myVar is undefined

}

//check for null or undefined with nullish coalescing operator

let value = null ?? "Oops.. null or undefined";

​

console.log(value) //Oops.. null or undefined

​

value = 25 ?? "Oops.. null or undefined";

​

console.log(value) // 25

​

value = "" ?? "Oops.. null or undefined";

​

console.log(value) // ""

Use for Empty string, undefined, null, ...

//To check for a truthy value:

​

if (strValue) {

    // strValue was non-empty string, true, 42, Infinity, [], ...

}

​

//To check for a falsy value:

if (!strValue) {

    // strValue was empty string, false, 0, null, undefined, ...

}

// 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).

}