Check if the intended javascript function exist or not.

Often it is a best practice to call function only if exists..

So this can be achieved calling our required function in:

function fExist(f){
     if(typeof f == "function"){
        return f();
     }
 }

Test:

function foo(){
    console.log("I am a function");
 }

 fExist(foo); 

Result wil be:
TRUE

because we have defined function "foo"...