1if( typeof myVar === 'undefined' || myVar === null ){
2    // myVar is undefined or null
3}

1if (variable === null) { //Executes only if variable is null but not undefined
2  //Code here
3}
4
5if (variable == null) { //Executes if variable is null OR undefined
6  //Code here
7}

1if (typeof value !== 'undefined' && value) {
2    //deal with value'
3};

1var myVar=null;
2
3if(myVar === null){
4    //I am null;
5}
6
7if (typeof myVar === 'undefined'){
8    //myVar is undefined
9}
10

1var myVar=null;
2
3if(myVar === null){
4    //I am null;
5}
6
7if (typeof myVar === 'undefined'){
8    //myVar is undefined
9}