JavaScript null check

Question

I\'ve come across the following code:

function test(data) {
    if (data != null && data !== undefined) {
        // some code here
    }
}
         


        

Answer 1

typeof foo === "undefined" is different from foo === undefined, never confuse them. typeof foo === "undefined" is what you really need. Also, use !== in place of !=

So the statement can be written as

function (data) {
  if (typeof data !== "undefined" && data !== null) {
    // some code here
  }
}

Edit:

You can not use foo === undefined for undeclared variables.

var t1;

if(typeof t1 === "undefined")
{
  alert("cp1");
}

if(t1 === undefined)
{
  alert("cp2");
}

if(typeof t2 === "undefined")
{
  alert("cp3");
}

if(t2 === undefined) // fails as t2 is never declared
{
  alert("cp4");
}