Assign variable in if condition statement good practice or not

Question

I moved one years ago from classic OO languages such like Java to JavaScript  The following code is definitely not recommended  or even not correct  in Java   if dayNumber   getClickedDayNumber dayInfo         alert  day number found       dayNumber     function getClickedDayNumber dayInfo        dayNumber   dayInfo indexOf  fc-day        if dayNumber    -1    substring found                 normally any calendar month consists of  40  days  so this will definitely pick up its day number          return parseInt dayInfo substring dayNumber 6  dayNumber 8              else return false      Basically I just found out that I can assign a variable to a value in an if condition statement  and immediately check the assigned value as if it is boolean   For a safer bet  I usually separate that into two lines of code  assign first then check the variable  but now that I found this  I am just wondering whether is it good practice or not in the eyes of experienced JavaScript developers

User · Answer

It s not good practice  You soon will get confused about it  It looks similiar to a common error  misuse     and      operators   You should break it into 2 lines of codes  It not only helps to make the code clearer  but also easy to refactor in the future  Imagine that you change the IF condition  You may accidently remove the line and your variable no longer get the value assigned to it

User · Answer

I see no proof that it is not good practice  Yes  it may look like a mistake but that is easily remedied by judicious commenting   Take for instance   if  x   processorIntensiveFunction         declaration inside if intended     alert x       Why should that function be allowed to run a 2nd time with   alert processorIntensiveFunction       Because the first version LOOKS bad   I cannot agree with that logic

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You can do this in Java too  And no  it s not a good practice       And use the     in Javascript for typed equality  Read Crockford s The Good Parts book on JS

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You can do assignments within if statements in Java as well  A good example would be reading something in and writing it out   http   www exampledepot com egs java io CopyFile html l new  The code      Copies src file to dst file     If the dst file does not exist  it is created void copy File src  File dst  throws IOException        InputStream in   new FileInputStream src       OutputStream out   new FileOutputStream dst           Transfer bytes from in to out     byte   buf   new byte 1024       int len      while   len   in read buf    gt  0            out write buf  0  len             in close        out close

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I wouldn t recommend it  The problem is  it looks like a common error where you try to compare values  but use a single   instead of    or      For example  when you see this   if  value   someFunction                  you don t know if that s what they meant to do  or if they intended to write this   if  value    someFunction                  If you really want to do the assignment in place  I would recommend doing an explicit comparison as well   if   value   someFunction         lt whatever truthy value you are expecting gt

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There is one case when you do it  with while-loops  When reading files  you usualy do like this   void readFile String pathToFile           Create a FileInputStream object     FileInputStream fileIn   null      try              Create the FileInputStream         fileIn   new FileInputStream pathToFile              Create a variable to store the current line s text in         String currentLine             While the file has lines left  read the next line             store it in the variable and do whatever is in the loop         while  currentLine   in readLine       null                   Print out the current line in the console                 you can do whatever you want with the line  this is just an example              System out println currentLine                   catch IOException e               Handle exception       finally           try                  Close the FileInputStream             fileIn close              catch IOException e                   Handle exception                     Look at the while-loop at line 9  There  a new line is read and stored in a variable  and then the content of the loop is ran  I know this isn t an if-statement  but I guess a while loop can be included in your question as well   The reason to this is that when using a FileInputStream  every time you call FileInputStream readLine    it reads the next line in the file  so if you would have called it from the loop with just fileIn readLine      null without assigning the variable  instead of calling  currentLine   fileIn readLine       null  and then called it from inside of the loop too  you would only get every second line     Hope you understand  and good luck

User · Answer

If you were to refer to Martin Fowlers book Refactoring improving the design of existing code   Then there are several cases where it would be good practice eg  long complex conditionals to use a function or method call to assert your case    quot Motivation One of the most common areas of complexity in a program lies in complex conditional logic  As you write code to test conditions and to do various things depending on various conditions  you quickly end up with a pretty long method  Length of a method is in itself a  factor that makes it harder to read  but conditions increase the difficulty  The problem usually lies in the fact that the code  both in the condition checks and in the actions  tells you what happens but can easily obscure why it happens  As with any large block of code  you can make your intention clearer by decomposing it and replacing chunks of code with a method call named after the intention of that block of code   gt  With conditions you can receive further benefit by doing this for the conditional part and each of the alternatives  This way you highlight the condition and make it clearly what you  gt  are branching on  You also highlight the reason for the branching  quot   And yes his answer is also valid for Java implementations  It does not assign the conditional function to a variable though in the examples

User · Answer

you could do something like so   if  value      sic    some function       use value value

User · Answer

I would consider this more of an old-school C style  it is not really good practice in JavaScript so you should avoid it

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I came here from golang  where it s common to see something like  if  err    doSomething    err    nil        return nil  err     In which err is scoped to that if block only   As such  here s what I m doing in es6  which seems pretty ugly  but doesn t make my rather strict eslint rules whinge  and achieves the same       const err   doSomething     if  err    null        return  null  err          The extra braces define a new  uh   lexical scope    Which means I can use const  and err isn t available to the outer block

User · Answer

I did it many times  To bypass the JavaScript warning  I add two parens   if   result   get something           You should avoid it  if you really want to use it  write a comment above it saying what you are doing

[javascript] Assign variable in if condition statement, good practice or not?

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