How to check undefined in Typescript

Question

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

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Answer 1

It actually is working, but there is difference between null and undefined. You are actually assigning to uemail, which would return a value or null in case it does not exists. As per documentation.

For more information about the difference between the both of them, see this answer.

For a solution to this Garfty's answer may work, depending on what your requirement is. You may also want to have a look here.

Answer 2

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');

Answer 3

Adding this late answer to check for object.propertie that can help in some cases:

Using a juggling-check, you can test both null and undefined in one hit:

if (object.property == null) {

If you use a strict-check, it will only be true for values set to null and won't evaluate as true for undefined variables:

if (object.property === null) {

Typescript does NOT have a function to check if a variable is defined.

Update October 2020

You can now also use the nullish coallesing operator introduced in Typescript.

let neverNullOrUndefined = someValue ?? anotherValue;

Here, anotherValue will only be returned if someValue is null or undefined.