Is there a null-coalescing Elvis operator or safe navigation operator in javascript

Question

I ll explain by example   Elvis Operator           The  Elvis operator  is a shortening   of Java s ternary operator  One   instance of where this is handy is for   returning a  sensible default  value   if an expression resolves to false or   null  A simple example might look like   this    def gender   user male    male     female     traditional ternary operator usage  def displayName   user name     Anonymous     more compact Elvis operator      Safe Navigation Operator           The Safe Navigation operator is used   to avoid a NullPointerException    Typically when you have a reference to   an object you might need to verify   that it is not null before accessing   methods or properties of the object    To avoid this  the safe navigation   operator will simply return null   instead of throwing an exception  like   so    def user   User find   admin                this might be null if  admin  does not exist def streetName   user  address  street      streetName will be null if user or user address is null - no NPE thrown

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Here s a simple elvis operator equivalent   function elvis object  path        return path   path split      reduce function  nestedObject  key            return nestedObject  amp  amp  nestedObject key          object    object      gt  var o     a    b  2    c  3     gt  elvis o     a    b  2    c  3     gt  elvis o   a       b  2     gt  elvis o   a b     2   gt  elvis o   x     undefined

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You could roll your own   function resolve objectToGetValueFrom  stringOfDotSeparatedParameters        var returnObject   objectToGetValueFrom          parameters   stringOfDotSeparatedParameters split               i          parameter       for  i   0  i  lt  parameters length  i              parameter   parameters i            returnObject   returnObject parameter            if  returnObject     undefined                break                      return returnObject       And use it like this   var result   resolve obj   a b c d         result is undefined if any one of a  b  c or d is undefined

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2020 Update JavaScript now has equivalents for both the Elvis Operator and the Safe Navigation Operator   Safe Property Access The optional chaining operator      is currently a stage 4 ECMAScript proposal  You can use it today with Babel      undefined  if either  a  or  b  are  null   undefined    a b c  otherwise  const myVariable   a  b  c   The logical AND operator   amp  amp   is the  quot old quot   more-verbose way to handle this scenario  const myVariable   a  amp  amp  a b  amp  amp  a b c   Providing a Default The nullish coalescing operator      is currently a stage 4 ECMAScript proposal  You can use it today with Babel  It allows you to set a default value if the left-hand side of the operator is a nullary value  null undefined   const myVariable   a  b  c     Some other value       Evaluates to  Some other value  const myVariable2   null     Some other value       Evaluates to    const myVariable3          Some other value    The logical OR operator      is an alternative solution with slightly different behavior  It allows you to set a default value if the left-hand side of the operator is falsy  Note that the result of myVariable3 below differs from myVariable3 above  const myVariable   a  b  c     Some other value       Evaluates to  Some other value  const myVariable2   null     Some other value       Evaluates to  Some other value  const myVariable3          Some other value

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Javascript s logical OR operator is short-circuiting and can replace your  Elvis  operator   var displayName   user name     Anonymous     However  to my knowledge there s no equivalent to your    operator

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For the former  you can use     The Javascript  logical or  operator  rather than simply returning canned true and false values  follows the rule of returning its left argument if it is true  and otherwise evaluating and returning its right argument  When you re only interested in the truth value it works out the same  but it also means that foo    bar    baz returns the leftmost one of foo  bar  or baz that contains a true value   You won t find one that can distinguish false from null  though  and 0 and empty string are false values  so avoid using the value    default construct where value can legitimately be 0 or

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This is more commonly known as a null-coalescing operator  Javascript does not have one

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i think lodash   get   can help here  as in   get user   name    and more complex tasks like   get o   a 0  b c    default-value

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This was an interesting solution for the safe navigation operator using some mixin    http   jsfiddle net avernet npcmv        Assume you have the following data structure   var companies           orbeon              cfo   Erik             cto   Alex                     Extend Underscore js     mixin             Safe navigation       attr  function obj  name    return obj    null   obj   obj name               So we can chain console log       log  function obj    console log obj                 Shortcut   cause I m lazy   var C     companies  chain          Simple case  returns Erik   C attr  orbeon   attr  cfo   log         Simple case too  no CEO in Orbeon  returns undefined   C attr  orbeon   attr  ceo   log         IBM unknown  but doesn t lead to an error  returns undefined   C attr  ibm   attr  ceo   log

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UPDATE SEP 2019  Yes  JS now supports this  Optional chaining is coming soon to v8 read more

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I read this article  https   www beyondjava net elvis-operator-aka-safe-navigation-javascript-typescript  and modified the solution using Proxies   function safe obj        return new Proxy obj            get  function target  name                const result   target name               if    result                    return  result instanceof Object   safe result    result                            return safe nullObj                        safe nullObj   safe      safe safeGet  function obj  expression        let safeObj   safe obj       let safeResult   expression safeObj        if  safeResult     safe nullObj            return undefined            return safeResult      You call it like this   safe safeGet example   x    gt  x foo woo    The result will be undefined for an expression that encounters null or undefined along its path  You could go wild and modify the Object prototype   Object prototype getSafe   function  expression        return safe safeGet this  expression       example getSafe  x    gt  x foo woo

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Jumping in very late  there s a proposal 1  for optional chaining currently at stage 2  with a babel plugin 2  available   It s not currently in any browser I am aware of    https   github com tc39 proposal-optional-chaining https   www npmjs com package  babel plugin-proposal-optional-chaining

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I think the following is equivalent to the safe navigation operator  although a bit longer   var streetName   user  amp  amp  user address  amp  amp  user address street    streetName will then be either the value of user address street or undefined   If you want it to default to something else you can combine with the above shortcut or to give   var streetName    user  amp  amp  user address  amp  amp  user address street      Unknown Street

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This was a problem for me for a long time  I had to come up with a solution that can be easily migrated once we get Elvis operator or something   This is what I use  works for both arrays and objects  put this in tools js file or something     this will create the object array if null Object prototype      function  prop        if  this prop      undefined          this prop    typeof prop     number                return this prop         this will just check if object array is null Object prototype     function  prop        return this prop      undefined        this prop       usage example   let student         classes             math            whatev             scores            math  9          whatev  20            loans            200             hey    sup             500          300          8000          3000000             use one underscore to test  console log student    classes     0      math console log student    classes     3         console log student    sports     3     injuries          console log student    scores      whatev       20 console log student    blabla      whatev          console log student    loans     2      500  console log student    loans     1     hey       sup console log student    loans     6     hey               use two underscores to create if null  student     loans      6   test      whatev   console log student     loans      6      test       whatev    well I know it makes the code a bit unreadable but it s a simple one liner solution and works great  I hope it helps someone

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I created a package that makes this a lot easier to use   NPM jsdig Github jsdig  You can handle simple things like and object   const world       locations        europe   Munich       usa   Indianapolis          world dig  locations    usa         gt   Indianapolis   world dig  locations    asia    japan         gt   null    or a little more complicated   const germany        gt   germany   const world    0  1    location    europe  germany      3   world dig 2   location    europe       germany  world dig 2   location    europe          germany

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I ve occasionally found the following idiom useful   a  b  c   can be rewritten as     a      b      c   This takes advantage of the fact that getting unknown attributes on an object returns undefined  rather than throwing an exception as it does on null or undefined  so we replace null and undefined with an empty object before navigating

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You can achieve roughly the same effect by saying   var displayName   user name     Anonymous

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I have a solution for that  tailor it to your own needs  an excerpt from one of my libs       elvisStructureSeparator               An Elvis operator replacement  See         http   coffeescript org  -- gt  The Existential Operator        http   fantom org doc docLang Expressions html safeInvoke               The fn parameter has a SPECIAL SYNTAX  E g         some structure  with a selector like this   value transforms to         some structure with a selector like this value  as an fn parameter                Configurable with tulebox elvisStructureSeparator                Usage examples          tulebox elvis scope   arbitrary path to a function   fnParamA  fnParamB  fnParamC          tulebox elvis this   currentNode favicon filename        elvis  function  scope  fn            tulebox dbg  tulebox elvis     scope          fn      args                 var implicitMsg        implicit value  undefined             if  arguments length  lt  2                tulebox dbg implicitMsg     1                 return undefined                        prepare args         var args      slice call arguments  2           if  scope     null    fn     null    scope     undefined    fn     undefined                 typeof fn      string                 tulebox dbg implicitMsg     2                 return undefined                           check levels         var levels   fn split tulebox elvisStructureSeparator           if  levels length  lt  1                tulebox dbg implicitMsg     3                 return undefined                     var lastLevel   scope           for  var i   0  i  lt  levels length  i                  if  lastLevel levels i       undefined                    tulebox dbg implicitMsg     4                     return undefined                            lastLevel   lastLevel levels i                          real return value         if  typeof lastLevel      function                 var ret   lastLevel apply scope  args               tulebox dbg      function value      ret               return ret            else               tulebox dbg      direct value      lastLevel               return lastLevel                     works like a charm  Enjoy the less pain

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You can use the logical  OR  operator in place of the Elvis operator   For example displayname   user name     Anonymous       But Javascript currently doesn t have the other functionality   I d recommend looking at CoffeeScript if you want an alternative syntax   It has some shorthand that is similar to what you are looking for   For example The Existential Operator  zip   lottery drawWinner    address  zipcode   Function shortcuts    - gt      equivalent to function       Sexy function calling  func  arg1   arg2     equivalent to func  arg1   arg2     There is also multiline comments and classes   Obviously you have to compile this to javascript or insert into the page as  lt script type  text coffeescript gt   but it adds a lot of functionality       Using  lt script type  text coffeescript  gt  is really only intended for development and not production

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Personally i use   function e e expr  try return eval expr   catch e  return null       and for example safe get   var a   e obj  e x y z searchedField

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Yes  there is    Optional chaining is in stage 4 and this enables you to use the user  address  street formula  If you can t wait the release  install  babel plugin-proposal-optional-chaining and you can use it  Here are my settings which works for me  or just read Nimmo s article     package json       quot name quot    quot optional-chaining-test quot      quot version quot    quot 1 0 0 quot      quot main quot    quot index js quot      quot devDependencies quot          quot  babel plugin-proposal-optional-chaining quot    quot 7 2 0 quot        quot  babel core quot    quot 7 2 0 quot        quot  babel preset-env quot    quot  7 5 5 quot                   babelrc       quot presets quot                  quot  babel preset-env quot                    quot debug quot   true                       quot plugins quot          quot  babel plugin-proposal-optional-chaining quot            index js  console log user  address  street       it works

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There is currently a draft spec   https   github com tc39 proposal-optional-chaining  https   tc39 github io proposal-optional-chaining   For now  though  I like to use lodash get object  path   defaultValue   or dlv delve obj  keypath   Update  as of Dec 23  2019       optional chaining has moved to stage 4

[javascript] Is there a null-coalescing (Elvis) operator or safe navigation operator in javascript?

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