JavaScript null check

Question

I ve come across the following code   function test data        if  data    null  amp  amp  data     undefined               some code here           I m somewhat new to JavaScript  but  from other questions I ve been reading here  I m under the impression that this code does not make much sense      In particular  this answer states that      You ll get an error if you access an undefined variable in any context other than typeof     Update  The  quote of the  answer above may be misleading  It should say   an undeclared variable    instead of   an undefined variable     As I found out  in the answers by Ryan    maerics  and nwellnhof  even when no arguments are provided to a function  its variables for the arguments are always declared  This fact also proves wrong the first item in the list below     From my understanding  the following scenarios may be experienced    The function was called with no arguments  thus making data an undefined variable  and raising an error on data    null  The function was called specifically with null  or undefined   as its argument  in which case data    null already protects the inner code  rendering  amp  amp  data     undefined useless  The function was called with a non-null argument  in which case it will trivially pass both data    null and data     undefined    Q  Is my understanding correct     I ve tried the following  in Firefox s console   --  15 31 31 057  false    null  15 31 31 061  true --  15 31 37 985  false     undefined  15 31 37 989  true --  15 32 59 934  null    null  15 32 59 937  false --  15 33 05 221  undefined    null  15 33 05 225  false --  15 35 12 231        null  15 35 12 235  true --  15 35 19 214         undefined  15 35 19 218  true   I can t figure out a case where the data     undefined after data    null might be of any use

User · Answer

In your case use data  null  which is true ONLY for null and undefined - on second picture focus on rows columns null-undefined    x000D   x000D  function test data    x000D      if  data    null    x000D          console log  Data     data   x000D        x000D    x000D   x000D  test                the data undefined x000D  test null           the data null x000D  test undefined      the data undefined x000D   x000D  test 0    x000D  test false    x000D  test  something    x000D   x000D   x000D    Here you have all  src    if        its negation             its negation

User · Answer

An    undefined variable    is different from the value undefined   An undefined variable   var a  alert b      ReferenceError  b is not defined   A variable with the value undefined   var a  alert a      Alerts    undefined      When a function takes an argument  that argument is always declared even if its value is undefined  and so there won   t be any error  You are right about    null followed by     undefined being useless  though

User · Answer

The simple way to do your test is     function  data        if  data       check if null  undefined  empty                some code here

User · Answer

var a  alert a     Value is undefined  var b    Volvo    alert b     Value is Volvo  var c   null  alert c     Value is null

User · Answer

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   amp  amp  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

User · Answer

I think  testing variables for values you do not expect is not a good idea in general  Because the test as your you can consider as writing a blacklist of forbidden values  But what if you forget to list all the forbidden values  Someone  even you  can crack your code with passing an unexpected value  So a more appropriate approach is something like whitelisting - testing variables only for the expected values  not unexpected  For example  if you expect the data value to be a string  instead of this    function  data      if  data    null  amp  amp  data     undefined           some code here        but what if data     false         or data        - empty string          do something like this   function  data      if  typeof data      string   amp  amp  data length           consume string here  it is here for sure        cleaner  it is obvious what type you expect        safer  less error prone due to implicit coercion

User · Answer

Q  The function was called with no arguments  thus making data an undefined variable  and raising an error on data    null   A  Yes  data will be set to undefined  See section 10 5 Declaration Binding Instantiation of the spec  But accessing an undefined value does not raise an error  You re probably confusing this with accessing an undeclared variable in strict mode which does raise an error   Q  The function was called specifically with null  or undefined   as its argument  in which case data    null already protects the inner code  rendering  amp  amp  data     undefined useless   Q  The function was called with a non-null argument  in which case it will trivially pass both data    null and data     undefined   A  Correct  Note that the following tests are equivalent   data    null data    undefined data     null  amp  amp  data     undefined   See section 11 9 3 The Abstract Equality Comparison Algorithm and section 11 9 6 The Strict Equality Comparison Algorithm of the spec

User · Answer

In JavaScript  null is a special singleton object which is helpful for signaling  no value    You can test for it by comparison and  as usual in JavaScript  it s a good practice to use the     operator to avoid confusing type coercion   var a   null  alert a     null      true   As  rynah mentions   undefined  is a bit confusing in JavaScript   However  it s always safe to test if the typeof x  is the string  undefined   even if  x  is not a declared variable   alert typeof x       undefined       true   Also  variables can have the  undefined value  if they are not initialized   var y  alert typeof y       undefined       true   Putting it all together  your check should look like this   if   typeof data       undefined    amp  amp   data     null                However  since the variable  data  is always defined since it is a formal function parameter  using the  typeof  operator is unnecessary and you can safely compare directly with the  undefined value    function data      if   data     undefined   amp  amp   data     null                  This snippet amounts to saying  if the function was called with an argument which is defined and is not null

[javascript] JavaScript null check

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