What does the JSLint error body of a for in should be wrapped in an if statement mean

Question

I used JSLint on a JavaScript file of mine  It threw the error   for  ind in evtListeners          Problem at line 41 character 9  The body of a for in should be   wrapped in an if statement to filter unwanted    properties from the prototype    What does this mean

User · Answer

Vava s answer is on the mark  If you use jQuery  then the   each   function takes care of this  hence it is safer to use     each evtListeners  function index  elem           your code

User · Answer

First of all  never use a for in loop to enumerate over an array  Never  Use good old for var i   0  i lt arr length  i      The reason behind this is the following  each object in JavaScript has a special field called prototype  Everything you add to that field is going to be accessible on every object of that type  Suppose you want all arrays to have a cool new function called filter 0 that will filter zeroes out   Array prototype filter 0   function         var res           for  var i   0  i  lt  this length  i              if  this i     0                res push this i                        return res      console log  0  5  0  3  0  1  0  filter 0       prints  5 3 1    This is a standard way to extend objects and add new methods  Lots of libraries do this  However  let s look at how for in works now   var listeners     a    b    c    for  o in listeners        console log o       prints      0     1     2     filter 0   Do you see  It suddenly thinks filter 0 is another array index  Of course  it is not really a numeric index   but for in enumerates through object fields  not just numeric indexes  So we re now enumerating through every numeric index and filter 0  But filter 0 is not a field of any particular array object  every array object has this property now   Luckily  all objects have a hasOwnProperty method  which checks if this field really belongs to the object itself or if it is simply inherited from the prototype chain and thus belongs to all the objects of that type   for  o in listeners        if  listeners hasOwnProperty o            console log o              prints       0      1      2   Note  that although this code works as expected for arrays  you should never  never  use for in and for each in for arrays  Remember that for in enumerates the fields of an object  not array indexes or values   var listeners     a    b    c    listeners happy    Happy debugging    for  o in listeners        if  listeners hasOwnProperty o            console log o               prints       0      1      2      happy

User · Answer

This means that you should filter the properties of evtListeners with the hasOwnProperty method

User · Answer

Just to add on to the topic of for in for   each  I added a jsperf test case for using   each vs for in  http   jsperf com each-vs-for-in 2  Different browsers versions handle it differently  but it seems   each and straight out for in are the cheapest options performance-wise   If you re using for in to iterate through an associative array object  knowing what you re after and ignoring everything else  use   each if you use jQuery  or just for in  and then a break  once you ve reached what you know should be the last element   If you re iterating through an array to perform something with each key pair in it  should use the hasOwnProperty method if you DON T use jQuery  and use   each if you DO use jQuery   Always use for i 0 i lt o length i    if you don t need an associative array though    lol chrome performed that 97  faster than a for in or   each

User · Answer

Douglas Crockford  the author of jslint has written  and spoken  about this issue many times  There s a section on this page of his website which covers this       for Statement      A for class of statements should have   the following form   for  initialization  condition  update        statements    for  variable in object        if  filter            statements                The first form should be used with   arrays and with loops of a   predeterminable number of iterations       The second form should be used with   objects  Be aware that members that   are added to the prototype of the   object will be included in the   enumeration  It is wise to program   defensively by using the   hasOwnProperty method to distinguish   the true members of the object   for  variable in object        if  object hasOwnProperty variable             statements             Crockford also has a video series on YUI theater where he talks about this  Crockford s series of videos talks about javascript are a must see if you re even slightly serious about javascript

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Surely it s a little extreme to say        never use a for in loop to   enumerate over an array  Never  Use   good old for var i   0    i lt arr length  i         It is worth highlighting the section in the Douglas Crockford extract         The second form should be used with   objects      If you require an associative array   aka hashtable   dictionary   where keys are named instead of numerically indexed  you will have to implement this as an object  e g  var myAssocArray    key1   value1   key2   value2         In this case myAssocArray length will come up null  because this object doesn t have a  length  property   and your i  lt  myAssocArray length won t get you very far   In addition to providing greater convenience  I would expect associative arrays to offer performance advantages in many situations  as the array keys can be useful properties  i e  an array member s ID property or name   meaning you don t have to iterate through a lengthy array repeatedly evaluating if statements to find the array entry you re after   Anyway  thanks also for the explanation of the JSLint error messages  I will use the  isOwnProperty  check now when interating through my myriad associative arrays

User · Answer

all - everything in JavaScript is an object     so statements like  only use this on objects  are a bit misleading   In addition JavaScript is not strongly typed so that 1     1  is true  although 1      1  is not  Crockford is big on this    When it comes to the progromatic concept of arrays in JS  typing is important in the definition    Brenton - No need to be a terminology dictator   associative array    dictionary    hash    object   these programming concepts all apply to one structure in JS   It is name  key  index  value pairs  where the value can be any other object  strings are objects too   So  new Array   is the same as     new Object   is roughly similar to     var myarray         Creates a structure that is an array with the restriction that all indexes  aka keys  must be a whole number   It also allows for auto assigning of new indexes via  push    var myarray     one   two   three      Is indeed best dealt with via for initialization condition update    But what about   var myarray       myarray 100     foo   myarray push  bar      Try this   var myarray       i  myarray 100     foo   myarray push  bar    myarray 150     baz   myarray push  qux    alert myarray length   for i in myarray       if myarray hasOwnProperty i              alert i       myarray i              Perhaps not the best usage of an array  but just an illustration that things are not always clearcut   If you know your keys  and definitely if they are not whole numbers  your only array like structure option is the object   var i  myarray        first   john       last   doe      100  foo      150  baz     for i in myarray       if myarray hasOwnProperty i              alert i       myarray i

User · Answer

Bad   jsHint will throw a error   for  var name in item        console log item name        Good   for  var name in item      if  item hasOwnProperty name         console log item name

[javascript] What does the JSLint error 'body of a for in should be wrapped in an if statement' mean?

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