How to check undefined in Typescript

I am using this code to check undefined variable but it's not working.

var  uemail = localStorage.getItem("useremail");

if (typeof uemail === "undefined")
{
    alert('undefined');
}
else
{
    alert('defined');
}

Solution 1:

In Typescript 2 you can use Undefined type to check for undefined values. So if you declare a variable as:

let uemail : string | undefined;

Then you can check if the variable z is undefined as:

if(uemail === undefined)
{

}

Solution 2:

You can just check for truthy on this:

if(uemail) {
    console.log("I have something");
} else {
    console.log("Nothing here...");
}

Go and check out the answer from here: Is there a standard function to check for null, undefined, or blank variables in JavaScript?

Hope this helps!

Solution 3:

From Typescript 3.7 on, you can also use nullish coalescing:

let x = foo ?? bar();

Which is the equivalent for checking for null or undefined:

let x = (foo !== null && foo !== undefined) ?
    foo :
    bar();

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#nullish-coalescing

While not exactly the same, you could write your code as:

var uemail = localStorage.getItem("useremail") ?? alert('Undefined');

Solution 4:

It's because it's already null or undefined. Null or undefined does not have any type. You can check if it's is undefined first. In typescript (null == undefined) is true.

  if (uemail == undefined) {
      alert('undefined');
  } else {
      alert('defined');
  }

or

  if (uemail == null) {
      alert('undefined');
  } else {
      alert('defined');
  }