How to check undefined in Typescript

Question

I am using this code to check undefined variable but it s not working    x000D   x000D  var  uemail   localStorage getItem  useremail    x000D   x000D  if  typeof uemail      undefined   x000D    x000D      alert  undefined    x000D    x000D  else x000D    x000D      alert  defined    x000D    x000D   x000D   x000D

User · Answer

Late to the story but I think some details are overlooked?

if you use

if (uemail !== undefined) {
  //some function
}

You are, technically, comparing variable uemail with variable undefined and, as the latter is not instantiated, it will give both type and value of 'undefined' purely by default, hence the comparison returns true. But it overlooks the potential that a variable by the name of undefined may actually exist -however unlikely- and would therefore then not be of type undefined. In that case, the comparison will return false.

To be correct one would have to declare a constant of type undefined for example:

const _undefined: undefined

and then test by:

if (uemail === _undefined) {
  //some function
}

This test will return true as uemail now equals both value & type of _undefined as _undefined is now properly declared to be of type undefined.

Another way would be

if (typeof(uemail) === 'undefined') {
  //some function
}

In which case the boolean return is based on comparing the two strings on either end of the comparison. This is, from a technical point of view, NOT testing for undefined, although it achieves the same result.

User · Answer

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  amp  amp  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

NOT STRICTLY RELATED TO TYPESCRIPT  Just to add to all the above answers  we can also use the shorthand syntax   var result   uemail          This will give you the email if uemail variable has some value and it will simply return an empty string if uemail variable is undefined   This gives a nice syntax for handling undefined variables and also provide a way to use a default value in case the variable is undefined

User · Answer

Use  this  keyword to access variable  This worked for me  var  uemail   localStorage getItem  useremail     if  typeof this uemail      undefined         alert  undefined      else       alert  defined

[typescript] How to check undefined in Typescript

Examples related to typescript