typeof undefined vs null

Question

I often see JavaScript code which checks for undefined parameters etc  this way   if  typeof input      undefined            do stuff     This seems kind of wasteful  since it involves both a type lookup and a string comparison  not to mention its verbosity  It s needed because undefined could be renamed  though    My question is  How is that code any better than this approach   if  null    input           do stuff     As far as I know  you can t redefine null  so it s not going to break unexpectedly  And  because of the type-coercion of the    operator  this checks for both undefined and null    which is often exactly what you want  e g  for optional function parameters     Yet this form does not seem widespread  and it even causes JSLint to yell at you for using the evil    operator    Why is this considered bad style

User · Answer

You can also use the void operator to obtain an undefined value:

if (input !== void 0) {
    // do stuff    
}

(And yes, as noted in another answer, this will throw an error if the variable was not declared, but this case can often be ruled out either by code inspection, or by code refactoring, e.g. using window.input !== void 0 for testing global variables or adding var input.)

User · Answer

I ve actually come across if  typeof input      undefined   in this scenario where it s being used to provide default function parameters    function greet name  greeting      name    typeof name      undefined      name    Student     greeting    typeof greeting      undefined      greeting    Welcome      return    greeting    name        greet       Welcome Student  greet  James       Welcome James  greet  Richard    Howdy       Howdy Richard    ES6 provides new ways of introducing default function parameters this way   function greet name    Student   greeting    Welcome       return    greeting    name        greet       Welcome Student  greet  James       Welcome James  greet  Richard    Howdy       Howdy Richard    This is less verbose and cleaner than the first option

User · Answer

x000D   x000D  var bar   null  x000D  console log typeof bar      object      true yes  x000D    because null a datatype of object x000D   x000D  var barf    dff   x000D  console log typeof barf constructor    function x000D   x000D   x000D  console log Array isArray bar     falsss x000D   x000D   x000D  console log  bar     null   amp  amp   bar constructor     Object      false x000D   x000D  console log  bar     null   amp  amp   typeof bar      object         logs false x000D    because bar   null  bar is a object x000D   x000D   x000D  console log  bar     null   amp  amp    typeof bar      object       typeof bar      function        false x000D   x000D  console log typeof bar     typeof object     false x000D  console log typeof bar2     typeof undefined     true x000D  console log typeof bar3     typeof undefinedff     true x000D  console log typeof bar2    typeof undefined     true x000D   x000D  console log  bar     null   amp  amp   typeof bar      object    amp  amp   toString call bar        object Array        false x000D   x000D   x000D

User · Answer

x000D   x000D  function greet name  greeting    x000D    name    typeof name      undefined      name    Student   x000D    greeting    typeof greeting      undefined      greeting    Welcome   x000D   x000D    console log greeting name   x000D    x000D   x000D  greet       Welcome Student  x000D  greet  James       Welcome James  x000D  greet  Richard    Howdy       Howdy Richard  x000D   x000D    ES6 provides new ways of introducing default function parameters this way  x000D   x000D  function greet2 name    Student   greeting    Welcome     x000D      return    greeting    name     x000D  console log greeting name   x000D    x000D   x000D  greet2       Welcome Student  x000D  greet2  James       Welcome James  x000D  greet2  Richard    Howdy       Howdy Richard  x000D   x000D   x000D

User · Answer

good way   if typeof neverDeclared     undefined     no errors   But the best looking way is to check via    if typeof neverDeclared     typeof undefined    also no errors and no strings

User · Answer

x000D   x000D   function    x000D   x000D    var a  b   3  x000D    var ed   103  x000D     x000D        x000D   x000D   x000D   x000D    console log ed     ed is not defined x000D   x000D  console log  a defined       typeof a      undefined       no define x000D  console log  b defined       typeof b      undefined       yes define x000D  console log typeof b      number x000D  console log typeof 4 7        number x000D  console log b     3 x000D  console log typeof  4   7       string x000D  var e   ggg   x000D  console log typeof e      string x000D   var ty typeof b   x000D  console log ty     number x000D  console log typeof false     boolean x000D  console log typeof 1     number x000D  console log typeof 0     number x000D  console log typeof true     boolean x000D   x000D   x000D  console log typeof Math tan      function x000D  console log typeof function         function  x000D   x000D  if typeof neverDeclared     undefined     no errors x000D  if typeof neverDeclared      undefined     no errors x000D   x000D    if neverDeclared    null    showing error  x000D   x000D   x000D  console log typeof  a 1      object x000D  console log typeof null     object x000D  console log typeof JSON     object x000D  console log typeof Math     object x000D  console log typeof  a-z      object x000D  console log typeof new Date       object x000D   x000D  console log typeof afbc     undefined x000D    console log typeof new    error x000D   x000D  document write   lt br gt    oprator as math     x000D  var r 14  4   x000D  document write r   x000D   x000D  document write   lt br gt    oprator as string     x000D  var r 14  44   x000D  document write r   x000D   x000D  document write   lt br gt  Minus Operator work as mathematic     x000D  var r 64- 44   x000D  document write r   x000D   x000D   x000D  document write   lt br gt     x000D  console log typeof 4  7       returns number x000D  console log typeof 4  7       returns string x000D   x000D   x000D   x000D   x000D    x000D  Interview Question in JavaScript x000D   x000D   x000D

User · Answer

if  input    undefined            works just fine  It is of course not a null comparison  but I usually find that if I need to distinguish between undefined and null  I actually rather need to distinguish between undefined and just any false value  so  else if  input            does it   If a program redefines undefined it is really braindead anyway   The only reason I can think of was for IE4 compatibility  it did not understand the undefined keyword  which is not actually a keyword  unfortunately   but of course values could be undefined  so you had to have this   var undefined    and the comparison above would work just fine   In your second example  you probably need double parentheses to make lint happy

User · Answer

If the variable is declared  either with the var keyword  as a function argument  or as a global variable   I think the best way to do it is   if  my variable     undefined    jQuery does it  so it s good enough for me  -   Otherwise  you ll have to use typeof to avoid a ReferenceError   If you expect undefined to be redefined  you could wrap your code like this    function undefined          undefined is now what it s supposed to be         Or obtain it via the void operator   const undefined   void 0     also safe

User · Answer

typeof is safer as it allows the identifier to never have been declared before   if typeof neverDeclared      undefined      no errors  if neverDeclared     null     throws ReferenceError  neverDeclared is not defined

User · Answer

If you are really worried about undefined being redefined  you can protect against this with some helper method like this   function is undefined value       var undefined check     instantiate a new variable which gets initialized to the real undefined value    return value     undefined check      This works because when someone writes undefined    foo  he only lets the name undefined reference to a new value  but he doesn t change the actual value of undefined

User · Answer

You shouldn t really worry about undefined being renamed  If someone renames undefined  you will be in a lot more trouble than just a few if checks failing  If you really want to protect your code  wrap it in an IFFE  immediately invoked function expression  like this    function    Backbone     undefined          undefined is undefined here     jQuery  Backbone        If you re working with global variables  which is wrong already  in a browser enviroment  I d check for undefined like this   if window neverDefined     undefined          Code works     Since global variables are a part of the window object  you can simply check against undefined instead of casting to a string and comparing strings    On top of that  why are your variables not defined  I ve seen a lot of code where they check a variables existence and perform some action based on that  Not once have I seen where this approach has been correct

[javascript] typeof !== "undefined" vs. != null

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