Set a variable if undefined in JavaScript

Question

I know that I can test for a JavaScript variable and then define it if it is undefined  but is there not some way of saying var setVariable   localStorage getItem  value      0   seems like a much clearer way  and I m pretty sure I ve seen this in other languages

User · Answer

It seems to me  that for current javascript implementations   var  result  default    possiblyUndefinedValue    is a nice way to do this  using object deconstruction

User · Answer

In our days you actually can do your approach with JS     Your variable is null    or     0  false  undefined let x   null      Set default value x   x     default value    console log x      default value  So your example WILL work  const setVariable   localStorage getItem  value      0

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It seems more logical to check typeof instead of undefined  I assume you expect a number as you set the var to 0 when undefined   var getVariable   localStorage getItem  value    var setVariable    typeof getVariable     number     getVariable   0    In this case if getVariable is not a number  string  object  whatever   setVariable is set to 0

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ES2020 Answer   With the Nullish Coalescing Operator  you can set a default value if value is null or undefined   const setVariable   localStorage getItem  value      0    However  you should be aware that the nullish coalescing operator does not return the default value for other types of falsy value such as 0 and      Do take note that browser support for the operator is limited  According to the data from caniuse  only 48 34  of browsers are supported  as of April 2020   Node js support was added in version 14

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If you re a FP  functional programming  fan  Ramda has a neat helper function for this called defaultTo    usage   const result   defaultTo 30  value   It s more useful when dealing with undefined boolean values   const result2   defaultTo false  dashboard someValue

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var setVariable    typeof localStorage getItem  value        undefined   amp  amp  localStorage getItem  value       0

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Logical nullish assignment  ES2020  solution New operators are currently being added to the browsers            and  amp  amp    This post will focus on      This checks if left side is undefined or null  short-circuiting if already defined  If not  the right-side is assigned to the left-side variable  Comparing Methods    Using     name      quot Dave quot      Previously  ES2020 name   name     quot Dave quot      Before that  not equivalent  but commonly used  name   name     quot Dave quot     name      quot Dave quot   Basic Examples let a          undefined let b   null let c   false  a     true     true b     true     true c     true     false     Equivalent to a   a    true  Object Array Examples let x     quot foo quot   let y     foo   quot fizz quot     x 0       quot bar quot       quot foo quot  x 1       quot bar quot       quot bar quot   y foo      quot buzz quot       quot fizz quot  y bar      quot buzz quot       quot buzz quot   x     Array    quot foo quot    quot bar quot    y     Object   foo   quot fizz quot   bar   quot buzz quot         Browser Support Jan 2021 - 81      Mozilla Documentation     Mozilla Documentation  amp  amp   Mozilla Documentation

User · Answer

Yes  it can do that  but strictly speaking that will assign the default value if the retrieved value is falsey  as opposed to truly undefined   It would therefore not only match undefined but also null  false  0  NaN       but not  0     If you want to set to default only if the variable is strictly undefined then the safest way is to write   var x    typeof x      undefined     your default value   x    On newer browsers it s actually safe to write   var x    x     undefined    your default value   x    but be aware that it is possible to subvert this on older browsers where it was permitted to declare a variable named undefined that has a defined value  causing the test to fail

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I needed to  set a variable if undefined  in several places  I created a function using  Alnitak answer  Hopefully it helps someone   function setDefaultVal value  defaultValue      return  value     undefined    defaultValue   value        Usage   hasPoints   setDefaultVal this hasPoints  true

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Works even if the default value is a boolean value   var setVariable      b   0    gt  b    localStorage getItem  value

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Ran into this scenario today as well where I didn t want zero to be overwritten for several values   We have a file with some common utility methods for scenarios like this  Here s what I added to handle the scenario and be flexible   function getIfNotSet value  newValue  overwriteNull  overwriteZero        if  typeof  value       undefined             return newValue        else if  value     null  amp  amp  overwriteNull     true            return newValue        else if  value     0  amp  amp  overwriteZero     true            return newValue        else           return value            It can then be called with the last two parameters being optional if I want to only set for undefined values or also overwrite null or 0 values   Here s an example of a call to it that will set the ID to -1 if the ID is undefined or null  but wont overwrite a 0 value   data ID   Util getIfNotSet data ID  -1  true

User · Answer

The 2018 ES6 answer is   return Object is x  undefined    y   x    If variable x is undefined  return variable y    otherwise if variable x is defined  return variable x

[javascript] Set a variable if undefined in JavaScript

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