Is there a way to check for both null and undefined

Question

Since TypeScript is strongly-typed  simply using if       to check for null and undefined doesn t sound right    Does TypeScript have any dedicated function or syntax sugar for this

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If you want to pass tslint without setting strict-boolean-expressions to allow-null-union or allow-undefined-union  you need to use isNullOrUndefined from node s util module or roll your own      tslint disable no-null-keyword export const isNullOrUndefined      lt T gt  obj  T   null   undefined   obj is null   undefined   gt        return typeof obj      undefined     obj     null          tslint enable no-null-keyword   Not exactly syntactic sugar but useful when your tslint rules are strict

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In TypeScript 3 7 we have now Optional chaining  and Nullish Coalescing to check null and undefined in the same time  example  let x   foo  bar baz     this code will check if foo is defined otherwise it will return undefined old way   if foo    null  amp  amp  foo    undefined       x   foo bar baz        this  let x    foo     null    foo     undefined    undefined   foo bar     if  foo  amp  amp  foo bar  amp  amp  foo bar baz              With optional chaining will be  let x   foo  bar     if  foo  bar  baz              another new feature is Nullish Coalescing  example  let x   foo    bar       return foo if it s not null or undefined otherwise calculate bar  old way  let x    foo     null  amp  amp  foo     undefined    foo   bar     BONUS

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Late to join this thread but I find this JavaScript hack very handy in checking whether a value is undefined    if typeof something       undefined          Yes this is undefined

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you can use   if x     undefined

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A faster and shorter notation for null checks can be    value    null    UNDEFINED    value   This line is equivalent to    if value    null           console log  UNDEFINED     else       console log value      Especially when you have a lot of null check it is a nice short notation

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I think this answer needs an update  check the edit history for the old answer   Basically  you have three deferent cases null  undefined  and undeclared  see the snippet below      bad-file ts console log message    You ll get an error says that variable message is undefined  aka undeclared   of course  the Typescript compiler shouldn t let you do that but REALLY nothing can prevent you      evil-file ts     ts-gnore console log message    The compiler will be happy to just compile the code above  So  if you re sure that all variables are declared you can simply do that  if   message    null            do something with the message     the code above will check for null and undefined  BUT in case the message variable may be undeclared  for safety   you may consider the following code  if   typeof message       undefined   amp  amp  message     null            message variable is more than safe to be used      Note  the order here typeof message       undefined   amp  amp  message     null is very important you have to check for the undefined state first atherwise it will be just the same as message    null  thanks  Jaider

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if  value         will evaluate to true if value is not    null undefined NaN empty string    0 false   typescript includes javascript rules

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UPDATE  Sept 4  2020  You can now use the    operator to validate null and undefined  quot values quot  and set a default value  For example  const foo   null  const bar   foo     exampleValue   console log bar      This will print  exampleValue  due to the value condition of the foo constant  in this case  a null value   As a verbose way  if you want to compare null and undefined values ONLY  use the following example code for reference  const incomingValue   string   undefined  const somethingToCompare   string   incomingValue     If the line above is not declared  TypeScript will return an excepion  if  somethingToCompare     undefined    null       console log  Incoming value is    somethingToCompare        If incomingValue is not declared  TypeScript should return an exception  If this is declared but not defined  the console log   will return  quot Incoming value is  undefined quot   Note we are not using the strict equals operator  The  quot correct quot  way  check the other answers for details   if the incomingValue is not a boolean type  just evaluate if its value is true  this will be evaluated according to the constant variable type  A true string have to be defined explicitly as string using the      assignation  If not  it will be evaluated as false  Let s check this case using the same context  const incomingValue   string   undefined  const somethingToCompare0   string    Trumpet   const somethingToCompare1   string   incomingValue   if  somethingToCompare0      console log  somethingToCompare0 is    somethingToCompare0        Will return  quot somethingToCompare0 is  Trumpet quot        Now  we will evaluate the second constant if  somethingToCompare1      console log  somethingToCompare1 is    somethingToCompare1        Launched if incomingValue is defined   else     console log  somethingToCompare1 is    somethingToCompare1        Launched if incomingValue is undefined  Will return  quot somethingToCompare1 is  undefined quot

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Does TypeScript has dedicated function or syntax sugar for this   TypeScript fully understands the JavaScript version which is something    null    TypeScript will correctly rule out both null and undefined with such checks    More  https   basarat gitbook io typescript recap null-undefined

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Using a juggling-check  you can test both null and undefined in one hit   if  x    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  x     null      You can try this with various values using this example   var a  number  var b  number   null   function check x  name        if  x    null            console log name        null               if  x     null            console log name         null               if  typeof x      undefined             console log name     is undefined             check a   a    check b   b      Output      a    null         a is undefined        b    null        b     null

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Usually I do the  juggling-check as Fenton already discussed   To make it more readable  you can use isNil from ramda   import   as isNil from  ramda src isNil    totalAmount   isNil totalAmount     0   totalAmount

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I always write it like this   var foo string   if  foo      foo  something           This will work fine and I think it s very readable

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Since TypeScript is a typed superset of ES6 JavaScript  And lodash are a library of javascript   Using lodash to checks if value is null or undefined can be done using   isNil       isNil value    Arguments  value      The value to check   Returns   boolean   Returns true if value is nullish  else false   Example    isNil null        gt  true    isNil void 0        gt  true    isNil NaN        gt  false   Link  Lodash Docs

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The simplest way is to use  import   isNullOrUndefined   from  util   and than  if   isNullOrUndefined foo

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I had this issue and some of the answer work just fine for JS but not for TS here is the reason     JS let couldBeNullOrUndefined  if couldBeNullOrUndefined    null      console log  null OR undefined   couldBeNullOrUndefined     else     console log  Has some value   couldBeNullOrUndefined       That is all good as JS has no Types    TS let couldBeNullOrUndefined   string   null     THIS NEEDS TO BE TYPED AS undefined    null    Type string   if couldBeNullOrUndefined     null       TS should always use strict-check   console log  null OR undefined   couldBeNullOrUndefined     else     console log  Has some value   couldBeNullOrUndefined       In TS if the variable wasn t defined with null when you try to check for that null the tslint   compiler will complain     tslint json      triple-equals   true          let couldBeNullOrUndefined   string     to fix it add   null   Types of property  couldBeNullOrUndefined  are incompatible        Type  string   null  is not assignable to type  string   undefined           Type  null  is not assignable to type  string   undefined

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For Typescript 2 x x you should do it in a following way using type guard    tl dr  function isDefined lt T gt  value  T   undefined   null   value is T     return  lt T gt value     undefined  amp  amp   lt T gt value     null        Why   In this way isDefined   will respect variable s type and the following code would know take this check in account   Example 1 - basic check   function getFoo foo  string   void            function getBar bar  string  undefined         getFoo bar     ERROR   bar  can be undefined   if  isDefined bar         getFoo bar      Ok now  typescript knows that  bar  is defined         Example 2 - types respect   function getFoo foo  string   void            function getBar bar  number   undefined      getFoo bar      ERROR   number   undefined  is not assignable to  string    if  isDefined bar         getFoo bar      ERROR   number  is not assignable to  string   but it s ok - we know it s number

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if data      it s mean  data   null undefined false

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All   The answer with the most votes  does not really work if you are working with an object  In that case  if a property is not present  the check will not work  And that was the issue in our case  see this sample   var x     name   Homer   LastName   Simpson      var y     name   Marge      var z     name   Bart    LastName  undefined     var a     name   Lisa    LastName         var hasLastNameX   x LastName    null  var hasLastNameY   y LastName    null  var hasLastNameZ   z LastName    null  var hasLastNameA   a LastName    null     alert  hasLastNameX         hasLastNameY         hasLastNameZ         hasLastNameA    var hasLastNameXX   x LastName     null  var hasLastNameYY   y LastName     null  var hasLastNameZZ   z LastName     null  var hasLastNameAA   a LastName     null   alert  hasLastNameXX         hasLastNameYY         hasLastNameZZ         hasLastNameAA     Outcome   true   false  false   true  in case of     true   true  true  true  in case of        gt  so in this sample not the correct answer   plunkr link  https   plnkr co edit BJpVHD95FhKlpHp1skUE

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I did different tests on the typescript playground   http   www typescriptlang org play   let a  let b   null  let c       var output        if  a    null  output     a is null or undefined n   if  b    null  output     b is null or undefined n   if  c    null  output     c is null or undefined n   if  a    null  output     a is defined n   if  b    null  output     b is defined n   if  c    null  output     c is defined n   if  a  output     a is defined  2nd method  n   if  b  output     b is defined  2nd method  n   if  c  output     c is defined  2nd method  n    console log output     gives   a is null or undefined b is null or undefined c is defined   so    checking if  a    null  is right to know if a is null or undefined checking if  a    null  is right to know if a is defined checking if  a  is wrong to know if a is defined

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careful if you re using local storage  you can end up with the string undefined rather than the value undefined   localStorage setItem  mykey  JSON stringify undefined    localStorage getItem  mykey        undefined  true   People may find this useful  https   github com angular components blob master src cdk coercion boolean-property spec ts          license    Copyright Google LLC All Rights Reserved        Use of this source code is governed by an MIT-style license that can be    found in the LICENSE file at https   angular io license          Coerces a data-bound value  typically a string  to a boolean     export function coerceBooleanProperty value  any   boolean     return value    null  amp  amp     value        false      import  coerceBooleanProperty  from    boolean-property    describe  coerceBooleanProperty        gt       it  should coerce undefined to false        gt        expect coerceBooleanProperty undefined   toBe false            it  should coerce null to false        gt        expect coerceBooleanProperty null   toBe false            it  should coerce the empty string to true        gt        expect coerceBooleanProperty      toBe true            it  should coerce zero to true        gt        expect coerceBooleanProperty 0   toBe true            it  should coerce the string  false  to false        gt        expect coerceBooleanProperty  false    toBe false            it  should coerce the boolean false to false        gt        expect coerceBooleanProperty false   toBe false            it  should coerce the boolean true to true        gt        expect coerceBooleanProperty true   toBe true            it  should coerce the string  true  to true        gt        expect coerceBooleanProperty  true    toBe true            it  should coerce an arbitrary string to true        gt        expect coerceBooleanProperty  pink    toBe true            it  should coerce an object to true        gt        expect coerceBooleanProperty      toBe true            it  should coerce an array to true        gt        expect coerceBooleanProperty      toBe true

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You may want to try  if   someValue    with      Explanation  The first   will turn your expression into a boolean value   Then  someValue is true if someValue is falsy and false if someValue is truthy  This might be confusing   By adding another    the expression is now true if someValue is truthy and false if someValue is falsy  which is much easier to manage   Discussion  Now  why do I bother myself with if    someValue  when something like if  someValue  would have give me the same result   Because   someValue is precisely a boolean expression  whereas someValue could be absolutely anything  This kind of expression will now alow to write functions  and God we need those  like   isSomeValueDefined    boolean     return   someValue     instead of   isSomeValueDefined    boolean     if someValue        return true       return false     I hope it helps

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If you are using TypeScript  it is a better approach to let the compiler check for nulls and undefineds  or the possibility thereof   rather than checking for them at run-time   If you do want to check at run-time  then as many answers indicate  just use value    null    Use the compile option strictNullChecks to tell the compiler to choke on possible null or undefined values  If you set this option  and then there is a situation where you do want to allow null and undefined  you can define the type as Type   null   undefined

[typescript] Is there a way to check for both `null` and `undefined`?

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