Why avoid increment and decrement -- operators in JavaScript

Question

One of the tips for jslint tool is         and --   The     increment  and --  decrement     operators have been known to contribute to bad code by   encouraging excessive trickiness  They   are second only to faulty architecture   in enabling to viruses and other   security menaces  There is a plusplus   option that prohibits the use of these   operators    I know that PHP constructs like  foo  bar    has may easily result with off-by-one errors  but I couldn t figure out a better way to control the loop than a while  a  lt  10   do      foo    a      or for  var i 0  i lt 10  i         foo        Is the jslint highlighting them because there are some similar languages that lack the      and  --  syntax or handle it differently  or are there other rationales for avoiding      and  --  that I might be missing

User · Answer

As mentioned in some of the existing answers (which annoyingly I'm unable to comment on), the problem is that x++ ++x evaluate to different values (before vs after the increment), which is not obvious and can be very confusing - if that value is used. cdmckay suggests quite wisely to allow use of increment operator, but only in a way that the returned value is not used, e.g. on its own line. I would also include the standard use within a for loop (but only in the third statement, whose return value is not used). I can't think of another example. Having been "burnt" myself, I would recommend the same guideline for other languages as well.

I disagree with the claim that this over-strictness is due to a lot of JS programmers being inexperienced. This is the exact kind of writing typical of "overly-clever" programmers, and I'm sure it's much more common in more traditional languages and with JS developers who have a background in such languages.

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My 2cents is that they should be avoided in two cases   1  When you have a variable that is used in many rows and you increase decrease it on the first statement that uses it  or last  or  even worse  in the middle       It s Java  but applies to Js too vi   list get     i    vi1   list get   i   1   out println    Processing values      vi          vi1   if   i  lt  list size    - 1         In examples like this  you can easily miss that the variable is auto-incremented decremented or even remove the first statement  In other words  use it only in very short blocks or where the variable appears in the block on just a couple of close statements    2  In case of multiple    and -- about the same variable in the same statement  It s very hard to remember what happens in cases like this   result       x - --x     x      Exams and professional tests asks about examples like above and indeed I ve stumbled upon this question while looking for documentation about one of them  but in real life one shouldn t be forced to think so much about a single line of code

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I m frankly confused by that advice  Part of me wonders if it has more to do with a lack of experience  perceived or actual  with javascript coders    I can see how someone just  hacking  away at some sample code could make an innocent mistake with    and --  but I don t see why an experienced professional would avoid them

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The  pre  and  post  nature of increment and decrement operators can tend to be confusing for those who are not familiar with them  that s one way in which they can be tricky

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If you read JavaScript The Good Parts  you ll see that Crockford s replacement for i   in a for loop is i  1  not i i 1   That s pretty clean and readable  and is less likely to morph into something  tricky    Crockford made disallowing autoincrement and autodecrement an option in jsLint  You choose whether to follow the advice or not   My own personal rule is to not do anything combined with autoincrement or autodecrement   I ve learned from years of experience in C that I don t get buffer overruns  or array index out of bounds  if I keep use of it simple  But I ve discovered that I do get buffer overruns if I fall into the  excessively tricky  practice of doing other things in the same statement   So  for my own rules  the use of i   as the increment in a for loop is fine

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In a loop it s harmless  but in an assignment statement it can lead to unexpected results   var x   5  var y   x       y is now 5 and x is 6 var z     x     z is now 7 and x is 7   Whitespace between the variable and the operator can lead to unexpected results as well   a   b   c   1  a      b --   c  console log  a    a   b    b   c    c    In a closure  unexpected results can be an issue as well   var foobar   function i  var count   count    i  return function   return count       baz   foobar 1   baz      1 baz      2   var alphabeta   function i  var count   count    i  return function   return   count     omega   alphabeta 1   omega      2 omega      3   And it triggers automatic semicolon insertion after a newline   var foo   1  bar   2  baz   3  alpha   4  beta   5  delta   alpha   beta    delta is 4  alpha is 4  beta is 6      preincrement postincrement confusion can produce off-by-one errors that are extremely difficult to diagnose  Fortunately  they are also complete unnecessary  There are better ways to add 1 to a variable    References   JSLint Help  Increment and Decrement Operators

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I think programmers should be competent in the language they are using  use it clearly  and use it well  I don t think they should artificially cripple the language they are using  I speak from experience  I once worked literally next door to a Cobol shop where they didn t use ELSE  because it was too complicated   Reductio ad absurdam

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The most important rationale for avoiding    or -- is that the operators return values and cause side effects at the same time  making it harder to reason about the code    For efficiency s sake  I prefer      i when not using the return value  no temporary  i   when using the return value  no pipeline stall    I am a fan of Mr  Crockford  but in this case I have to disagree    i is 25  less text to parse than i  1 and arguably clearer

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In my experience    i or i   has never caused confusion other than when first learning about how the operator works  It is essential for the most basic for loops and while loops that are taught by any highschool or college course taught in languages where you can use the operator  I personally find doing something like what is below to look and read better than something with a   being on a separate line    x000D   x000D  while   a  lt  10    x000D      array a      val x000D    x000D   x000D   x000D    In the end it is a style preference and not anything more  what is more important is that when you do this in your code you stay consistent so that others working on the same code can follow and not have to process the same functionality in different ways   Also  Crockford seems to use i- 1  which I find to be harder to read than --i or i--

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Is Fortran a C-like language   It has neither    nor --   Here is how you write a loop        integer i  n  sum        sum   0       do 10 i   1  n          sum   sum   i          write       i     i          write       sum     sum   10  continue   The index element i is incremented by the language rules each time through the loop   If you want to increment by something other than 1  count backwards by two for instance  the syntax is             integer i        do 20 i   10  1  -2          write       i     i   20  continue   Is Python C-like   It uses range and list comprehensions and other syntaxes to bypass the need for incrementing an index   print range 10 1 -2    prints  10 8 6 4 2   x x for x in range 1 10     returns  1 4 9 16         So based on this rudimentary exploration of exactly two alternatives  language designers may avoid    and -- by anticipating use cases and providing an alternate syntax   Are Fortran and Python notably less of a bug magnet than procedural languages which have    and --   I have no evidence   I claim that Fortran and Python are C-like because I have never met someone fluent in C who could not with 90  accuracy guess correctly the intent of non-obfuscated Fortran or Python

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Another example  more simple than some others with simple return of incremented value   function testIncrement1 x        return x       function testIncrement2 x        return   x     function testIncrement3 x        return x    1     console log testIncrement1 0       0 console log testIncrement2 0       1 console log testIncrement3 0       1   As you can see  no post-increment decrement should be used at return statement  if you want this operator to influence the result  But return doesn t  catch  post-increment decrement operators   function closureIncrementTest         var x   0       function postIncrementX             return x               var y   postIncrementX         console log x      1

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Consider the following code      int a 10       a 0    0      a 1    0      a 2    0      a 3    0      int i   0      a i      i        a i      i        a i      i      since i   gets evaluated twice the output is  from vs2005 debugger        0  0   int      1  0   int      2  2   int      3  0   int      4  4   int   Now consider the following code        int a 10       a 0    0      a 1    0      a 2    0      a 3    0      int i   0      a   i      i      a   i      i      a   i      i    Notice that the output is the same  Now you might think that   i and i   are the same  They are not       0  0   int      1  0   int      2  2   int      3  0   int      4  4   int   Finally consider this code      int a 10       a 0    0      a 1    0      a 2    0      a 3    0      int i   0      a   i    i        a   i    i        a   i    i      The output is now          0  0   int      1  1   int      2  0   int      3  3   int      4  0   int      5  5   int   So they are not the same  mixing both result in not so intuitive behavior  I think that for loops are ok with     but watch out when you have multiple    symbols on the same line or same instruction

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My view is to always use    and -- by themselves on a single line  as in   i    array i    foo    instead of  array   i    foo    Anything beyond that can be confusing to some programmers and is just not worth it in my view   For loops are an exception  as the use of the increment operator is idiomatic and thus always clear

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I ve been watching Douglas Crockford s video on this and his explanation for not using increment and decrement is that   It has been used in the past in other languages to break the bounds of arrays and cause all manners of badness and That it is more confusing and inexperienced JS developers don t know exactly what it does     Firstly arrays in JavaScript are dynamically sized and so  forgive me if I m wrong  it is not possible to break the bounds of an array and access data that shouldn t be accessed using this method in JavaScript    Secondly  should we avoid things that are complicated  surely the problem is not that we have this facility but the problem is that there are developers out there that claim to do JavaScript but don t know how these operators work   It is simple enough  value    give me the current value and after the expression add one to it    value  increment the value before giving me it   Expressions like a         b  are simple to work out if you just remember the above   var a   1  b   1  c  c   a         b     c   1   2   3      a   2  equals two after the expression is finished      b   2    I suppose you ve just got to remember who has to read through the code  if you have a team that knows JS inside out then you don t need to worry  If not then comment it  write it differently  etc  Do what you got to do  I don t think increment and decrement is inherently bad or bug generating  or vulnerability creating  maybe just less readable depending on your audience   Btw  I think Douglas Crockford is a legend anyway  but I think he s caused a lot of scare over an operator that didn t deserve it   I live to be proven wrong though

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There is a history in C of doing things like   while   a      b       to copy a string  perhaps this is the source of the excessive trickery he is referring to     And there s always the question of what    i   i      or   i   i       i    actually do   It s defined in some languages  and in other s there s no guarantee what will happen   Those examples aside  I don t think there s anything more idiomatic than a for loop that uses    to increment   In some cases you could get away with a foreach loop  or a while loop that checked a different condtion   But contorting your code to try and avoid using incrementing is ridiculous

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In my view   Explicit is always better than implicit   Because at some point  you may got confused with this increments statement y    x       y  A good programmer always makes his or her code more readable

[javascript] Why avoid increment ("++") and decrement ("--") operators in JavaScript?

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